Master Cylinder Rebuild or Replace?

[justin brady1]

Looks like my master cylinder is on the way out, I've got an intermittent spongy pedal which I assume means an internal leak past the piston on the
master. (other opinions?)

What's the preference on these? Rebuild or replace with new?
Anyone got a line on the part number for the new ones?
Is there a direct swap available?
Seems like I recall reading something about fabbing up a bracket?

--
Justin Brady
http://www.thegmcrv.com/
1976 Palm Beach 455

 

[sgltrac]

Could also be your supply vacuum to the booster or the booster itself if
pumping the pedal brings it back to life or if the pedal slowly drops after
braking pressure applied and held.

Sully
Bellevue wa

On Tue, Apr 16, 2019 at 6:13 AM Justin Brady via Gmclist <

> Looks like my master cylinder is on the way out, I've got an intermittent
> spongy pedal which I assume means an internal leak past the piston on the
> master. (other opinions?)
>
> What's the preference on these? Rebuild or replace with new?
> Anyone got a line on the part number for the new ones?
> Is there a direct swap available?
> Seems like I recall reading something about fabbing up a bracket?
>
>
> --
> Justin Brady
> http://www.thegmcrv.com/
> 1976 Palm Beach 455
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

 

[justin brady1]

Thanks Sully,
Any way to really tell?

Basically every few presses it's soft, releasing and repressing usually gets it to work OK but not 100%. I've been noticing them getting worse lately
so I rebuilt the rear, bled, and changed pads in the front which helped but I still felt it was a little off. Then on the way home from our last trip
I started having to double pump the brakes to get them working so my logic is saying it was the master leaking a bit all along and it's just now
getting bad enough to full leak past the piston.

I hadn't really considered the booster. Hmm.
--
Justin Brady
http://www.thegmcrv.com/
1976 Palm Beach 455

 

[sgltrac]

There are others here who are far more knowledgeable on brakes that I am
sure will chime in soon. I am not sure that a failing master would give you
intermittent issues. That sounds more like a booster. There is a check
valve on the booster where the vacuum line attaches off the engine. I would
check the hose from the motor to the booster for cracks/dryness/looseness.
The check valve has a rubber grommet that seals it to the booster and the
check valve itself could be faulty (it is basically a one way valve so I
think you can test it with a suck n blow test) you replace the rubber brake
hoses at the calipers/wheel cylinders?

Sully
Bellevue wa

On Tue, Apr 16, 2019 at 6:35 AM Justin Brady via Gmclist <

> Thanks Sully,
> Any way to really tell?
>
> Basically every few presses it's soft, releasing and repressing usually
> gets it to work OK but not 100%. I've been noticing them getting worse
> lately
> so I rebuilt the rear, bled, and changed pads in the front which helped
> but I still felt it was a little off. Then on the way home from our last
> trip
> I started having to double pump the brakes to get them working so my logic
> is saying it was the master leaking a bit all along and it's just now
> getting bad enough to full leak past the piston.
>
> I hadn't really considered the booster. Hmm.
> --
> Justin Brady
> http://www.thegmcrv.com/
> 1976 Palm Beach 455
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

 

[Jim Hupy]

Master cylinders are a relatively easy swap, if you have a pressure
bleeder.
Pump 'n hold, not so much. If that is how you bled your brakes after
you repaired them, you have air in the system.
Perhaps power bleeding might fix that if the master cylinder is good.
BUT, it likely is not, due to its location, it is exposed to rain water
leaking through the hatch cover, and has an affinity for absorbing water,
particularly if frequent fluid changes are not performed. Water and brake
fluid are not happy when mixed. Buy the highest quality USA made master
cylinder you can find, follow the bench bleeding procedures, and do your
due diligence on brake inspections. Brakes are nothing to screw around
with. It means your lives if they fail.
Jim Hupy
Salem, Or
78 GMC ROYALE 403

On Tue, Apr 16, 2019, 7:45 AM Todd Sullivan via Gmclist <

> There are others here who are far more knowledgeable on brakes that I am
> sure will chime in soon. I am not sure that a failing master would give you
> intermittent issues. That sounds more like a booster. There is a check
> valve on the booster where the vacuum line attaches off the engine. I would
> check the hose from the motor to the booster for cracks/dryness/looseness.
> The check valve has a rubber grommet that seals it to the booster and the
> check valve itself could be faulty (it is basically a one way valve so I
> think you can test it with a suck n blow test) you replace the rubber brake
> hoses at the calipers/wheel cylinders?
>
> Sully
> Bellevue wa
>
> On Tue, Apr 16, 2019 at 6:35 AM Justin Brady via Gmclist <

>
> > Thanks Sully,
> > Any way to really tell?
> >
> > Basically every few presses it's soft, releasing and repressing usually
> > gets it to work OK but not 100%. I've been noticing them getting worse
> > lately
> > so I rebuilt the rear, bled, and changed pads in the front which helped
> > but I still felt it was a little off. Then on the way home from our last
> > trip
> > I started having to double pump the brakes to get them working so my
> logic
> > is saying it was the master leaking a bit all along and it's just now
> > getting bad enough to full leak past the piston.
> >
> > I hadn't really considered the booster. Hmm.
> > --
> > Justin Brady
> > http://www.thegmcrv.com/
> > 1976 Palm Beach 455
> >
> > _______________________________________________
> > GMCnet mailing list
> > Unsubscribe or Change List Options:
> > http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
> >
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

 

[justin brady1]

Thanks Jim,
I use a vacuum bleeder for bleeding. It seems to do the job, but I'm open to better options if there are any!
Agreed on the brakes, not something to cheap out on for sure!

> Master cylinders are a relatively easy swap, if you have a pressure
> bleeder.
> Pump 'n hold, not so much. If that is how you bled your brakes after
> you repaired them, you have air in the system.
> Perhaps power bleeding might fix that if the master cylinder is good.
> BUT, it likely is not, due to its location, it is exposed to rain water
> leaking through the hatch cover, and has an affinity for absorbing water,
> particularly if frequent fluid changes are not performed. Water and brake
> fluid are not happy when mixed. Buy the highest quality USA made master
> cylinder you can find, follow the bench bleeding procedures, and do your
> due diligence on brake inspections. Brakes are nothing to screw around
> with. It means your lives if they fail.
> Jim Hupy
> Salem, Or
> 78 GMC ROYALE 403
>
> On Tue, Apr 16, 2019, 7:45 AM Todd Sullivan via Gmclist <

>
> > There are others here who are far more knowledgeable on brakes that I am
> > sure will chime in soon. I am not sure that a failing master would give you
> > intermittent issues. That sounds more like a booster. There is a check
> > valve on the booster where the vacuum line attaches off the engine. I would
> > check the hose from the motor to the booster for cracks/dryness/looseness.
> > The check valve has a rubber grommet that seals it to the booster and the
> > check valve itself could be faulty (it is basically a one way valve so I
> > think you can test it with a suck n blow test) you replace the rubber brake
> > hoses at the calipers/wheel cylinders?
> >
> > Sully
> > Bellevue wa
> >

> >
> >> Thanks Sully,
> >> Any way to really tell?
> >>
> >> Basically every few presses it's soft, releasing and repressing usually
> >> gets it to work OK but not 100%. I've been noticing them getting worse
> >> lately
> >> so I rebuilt the rear, bled, and changed pads in the front which helped
> >> but I still felt it was a little off. Then on the way home from our last
> >> trip
> >> I started having to double pump the brakes to get them working so my
> > logic
> >> is saying it was the master leaking a bit all along and it's just now
> >> getting bad enough to full leak past the piston.
> >>
> >> I hadn't really considered the booster. Hmm.
> >> --
> >> Justin Brady
> >> http://www.thegmcrv.com/
> >> 1976 Palm Beach 455
> >>
> >> _______________________________________________
> >> GMCnet mailing list
> >> Unsubscribe or Change List Options:
> >> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
> >>
> > _______________________________________________
> > GMCnet mailing list
> > Unsubscribe or Change List Options:
> > http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
> >
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org

--
Justin Brady
http://www.thegmcrv.com/
1976 Palm Beach 455

 

[bdub1]

I'll loan you my Hupy Power bleeder adapter if you want to borrow it. All
you need then is a small pressure sprayer and a quart or so of fluid. Cost
will be $7.55 both ways in a USPS small priority mail box.

On Tue, Apr 16, 2019 at 10:13 AM Justin Brady via Gmclist <

> Thanks Jim,
> I use a vacuum bleeder for bleeding. It seems to do the job, but I'm open
> to better options if there are any!
> Agreed on the brakes, not something to cheap out on for sure!
>

> > Master cylinders are a relatively easy swap, if you have a pressure
> > bleeder.
> > Pump 'n hold, not so much. If that is how you bled your brakes after
> > you repaired them, you have air in the system.
> > Perhaps power bleeding might fix that if the master cylinder is
> good.
> > BUT, it likely is not, due to its location, it is exposed to rain water
> > leaking through the hatch cover, and has an affinity for absorbing water,
> > particularly if frequent fluid changes are not performed. Water and brake
> > fluid are not happy when mixed. Buy the highest quality USA made master
> > cylinder you can find, follow the bench bleeding procedures, and do your
> > due diligence on brake inspections. Brakes are nothing to screw around
> > with. It means your lives if they fail.
> > Jim Hupy
> > Salem, Or
> > 78 GMC ROYALE 403
> >
> > On Tue, Apr 16, 2019, 7:45 AM Todd Sullivan via Gmclist <

> >
> > > There are others here who are far more knowledgeable on brakes that I
> am
> > > sure will chime in soon. I am not sure that a failing master would
> give you
> > > intermittent issues. That sounds more like a booster. There is a check
> > > valve on the booster where the vacuum line attaches off the engine. I
> would
> > > check the hose from the motor to the booster for
> cracks/dryness/looseness.
> > > The check valve has a rubber grommet that seals it to the booster and
> the
> > > check valve itself could be faulty (it is basically a one way valve
> so I
> > > think you can test it with a suck n blow test) you replace the rubber
> brake
> > > hoses at the calipers/wheel cylinders?
> > >
> > > Sully
> > > Bellevue wa
> > >
> > > On Tue, Apr 16, 2019 at 6:35 AM Justin Brady via Gmclist

> > >
> > >> Thanks Sully,
> > >> Any way to really tell?
> > >>
> > >> Basically every few presses it's soft, releasing and repressing
> usually
> > >> gets it to work OK but not 100%. I've been noticing them getting
> worse
> > >> lately
> > >> so I rebuilt the rear, bled, and changed pads in the front which
> helped
> > >> but I still felt it was a little off. Then on the way home from our
> last
> > >> trip
> > >> I started having to double pump the brakes to get them working so my
> > > logic
> > >> is saying it was the master leaking a bit all along and it's just now
> > >> getting bad enough to full leak past the piston.
> > >>
> > >> I hadn't really considered the booster. Hmm.
> > >> --
> > >> Justin Brady
> > >> http://www.thegmcrv.com/
> > >> 1976 Palm Beach 455
> > >>
> > >> _______________________________________________
> > >> GMCnet mailing list
> > >> Unsubscribe or Change List Options:
> > >> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
> > >>
> > > _______________________________________________
> > > GMCnet mailing list
> > > Unsubscribe or Change List Options:
> > > http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
> > >
> > _______________________________________________
> > GMCnet mailing list
> > Unsubscribe or Change List Options:
> > http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
>
> --
> Justin Brady
> http://www.thegmcrv.com/
> 1976 Palm Beach 455
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

 

[bdub1]

But, knowing your skills, you can probably whip one up from scraps for less
money.
http://www.gmcmotorhomemarketplace.com/jhupy/

> I'll loan you my Hupy Power bleeder adapter if you want to borrow it. All
> you need then is a small pressure sprayer and a quart or so of fluid. Cost
> will be $7.55 both ways in a USPS small priority mail box.
>
> On Tue, Apr 16, 2019 at 10:13 AM Justin Brady via Gmclist <

>
>> Thanks Jim,
>> I use a vacuum bleeder for bleeding. It seems to do the job, but I'm open
>> to better options if there are any!
>> Agreed on the brakes, not something to cheap out on for sure!
>>

>> > Master cylinders are a relatively easy swap, if you have a pressure
>> > bleeder.
>> > Pump 'n hold, not so much. If that is how you bled your brakes
>> after
>> > you repaired them, you have air in the system.
>> > Perhaps power bleeding might fix that if the master cylinder is
>> good.
>> > BUT, it likely is not, due to its location, it is exposed to rain water
>> > leaking through the hatch cover, and has an affinity for absorbing
>> water,
>> > particularly if frequent fluid changes are not performed. Water and
>> brake
>> > fluid are not happy when mixed. Buy the highest quality USA made master
>> > cylinder you can find, follow the bench bleeding procedures, and do your
>> > due diligence on brake inspections. Brakes are nothing to screw around
>> > with. It means your lives if they fail.
>> > Jim Hupy
>> > Salem, Or
>> > 78 GMC ROYALE 403
>> >
>> > On Tue, Apr 16, 2019, 7:45 AM Todd Sullivan via Gmclist <

>> >
>> > > There are others here who are far more knowledgeable on brakes that
>> I am
>> > > sure will chime in soon. I am not sure that a failing master would
>> give you
>> > > intermittent issues. That sounds more like a booster. There is a
>> check
>> > > valve on the booster where the vacuum line attaches off the engine.
>> I would
>> > > check the hose from the motor to the booster for
>> cracks/dryness/looseness.
>> > > The check valve has a rubber grommet that seals it to the booster
>> and the
>> > > check valve itself could be faulty (it is basically a one way valve
>> so I
>> > > think you can test it with a suck n blow test) you replace the
>> rubber brake
>> > > hoses at the calipers/wheel cylinders?
>> > >
>> > > Sully
>> > > Bellevue wa
>> > >
>> > > On Tue, Apr 16, 2019 at 6:35 AM Justin Brady via Gmclist

>> > >
>> > >> Thanks Sully,
>> > >> Any way to really tell?
>> > >>
>> > >> Basically every few presses it's soft, releasing and repressing
>> usually
>> > >> gets it to work OK but not 100%. I've been noticing them getting
>> worse
>> > >> lately
>> > >> so I rebuilt the rear, bled, and changed pads in the front which
>> helped
>> > >> but I still felt it was a little off. Then on the way home from our
>> last
>> > >> trip
>> > >> I started having to double pump the brakes to get them working so my
>> > > logic
>> > >> is saying it was the master leaking a bit all along and it's just
>> now
>> > >> getting bad enough to full leak past the piston.
>> > >>
>> > >> I hadn't really considered the booster. Hmm.
>> > >> --
>> > >> Justin Brady
>> > >> http://www.thegmcrv.com/
>> > >> 1976 Palm Beach 455
>> > >>
>> > >> _______________________________________________
>> > >> GMCnet mailing list
>> > >> Unsubscribe or Change List Options:
>> > >> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>> > >>
>> > > _______________________________________________
>> > > GMCnet mailing list
>> > > Unsubscribe or Change List Options:
>> > > http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>> > >
>> > _______________________________________________
>> > GMCnet mailing list
>> > Unsubscribe or Change List Options:
>> > http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>>
>>
>> --
>> Justin Brady
>> http://www.thegmcrv.com/
>> 1976 Palm Beach 455
>>
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>>
>

 

[LQQKatJon]

Justin,

I will second or third recommend Jim's pressure bleeder. What a beautiful thing it is.

As for your master cylinder problem, Call up Dave Lenzi and have him send you a master cylinder- then you know for sure you have the best. He
gets a supply that I believe is american made, then he machines off where the nuts sit just a little bit. You can get a ACDelco, a Raybestos(that
is what i have installed a couple of), but the new casting is thicker now then they used to be, and when you install the nuts, you will see they are
not catching every thread. I have never heard of one popping off, and I know many running that way(including me). but that does not mean that it
could not happen, and Dave's price to provide both the best master cylinder and to do the machining is not astronomical.

The last master cylinder I swapped out on a 77 eleganza, was done in an apartment parking lot, and took me maybe a 1/2 hour including bleeding. I
have swapped a few over the years, it is one of the easist things to do on a GMC. Lines, and two bolts on the bracket that goes up, and the two nuts
that hold the cylinder to the booster. with Jim's bleeder, it is 5 minutes of setup, and maybe 5 minutes of bleeding, then clean up. start
spraying the brake fittings on the master cylinder now.

that all depends on if your bleeders do not break off. If that happens you need more work then a master cylinder. That is a rare occurrence, if
you use the right tools, and the brakes cylinders are in some sort of OK shape.

--
Jon Roche
75 palm beach
St. Cloud, MN
http://lqqkatjon.blogspot.com/

 

[johnny]

I have vacuum bled - cos I had the pump - and pressure bled - cos Zhookoff lent me his bleeder. The pressure bleeder is the least worst way. If you
have a LOT of patience and nothing to do for a day or so, you can gravity bleed them if you take the proportioning valve to the out of the line
position.

--johnny
--
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
"I forgive them all, save those who hurt the dogs. They must answer to me in hell" - ol Andy, paraphrased

 

[johnny]

And when I posted that I was referring to a hand - held squeeze vacuum pump. Then I remembered, I have a perfectly good HVAC vacuum pump, what would
be the argument against using it with a capture bottle? Should it not be the same as the pressure bleeder with 29.92 PSI on the master cylinder?

Physics gurus?

--johnny
--
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
"I forgive them all, save those who hurt the dogs. They must answer to me in hell" - ol Andy, paraphrased

 

[Matt Colie]

> And when I posted that I was referring to a hand - held squeeze vacuum pump. Then I remembered, I have a perfectly good HVAC vacuum pump, what
> would be the argument against using it with a capture bottle? Should it not be the same as the pressure bleeder with 29.92 PSI on the master
> cylinder?
>
> Physics gurus?
>
> --johnny

Johnny,

29.92 is Inches Hg.
14.7 PSI.
One place I worked used to pressure bleed at 30 psi right up until a guy shot brake fluid into his eyes.

I have been successfully vacuum bleeding for year. I like it because:
A - I have a little electric vacuum pump that makes it real easy.
B - It works for the newer and strange cars that have a plastic fluid reservoir.

It works even better if you wrap the bleed screws with teflon tape. That tape also keeps the bleed screws from corroding into the bodies....

Matt
--
Matt & Mary Colie - '73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan
OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Twixt A2 and Detroit

 

[justin brady1]

Thanks for the offer Billy,
That seems like a great tool to have around so I'll probably make one up!

Does the brake need to be depressed for it to work or do you just pressurize and the crack the bleed screws as usual?
--
Justin Brady
http://www.thegmcrv.com/
1976 Palm Beach 455

 

[justin brady1]

> > And when I posted that I was referring to a hand - held squeeze vacuum pump. Then I remembered, I have a perfectly good HVAC vacuum pump,
> > what would be the argument against using it with a capture bottle? Should it not be the same as the pressure bleeder with 29.92 PSI on the master
> > cylinder?
> >
> > Physics gurus?
> >
> > --johnny
>
> Johnny,
>
> 29.92 is Inches Hg.
> 14.7 PSI.
> One place I worked used to pressure bleed at 30 psi right up until a guy shot brake fluid into his eyes.
>
> I have been successfully vacuum bleeding for year. I like it because:
> A - I have a little electric vacuum pump that makes it real easy.
> B - It works for the newer and strange cars that have a plastic fluid reservoir.
>
> It works even better if you wrap the bleed screws with teflon tape. That tape also keeps the bleed screws from corroding into the bodies....
>
> Matt


That's my biggest complaint with the vacuum bleeder, it sucks air around the screw so you can never quite tell if you're getting bubbles from the
brake lines or from the air leak. I'll pull them when I do the job and wrap them
--
Justin Brady
http://www.thegmcrv.com/
1976 Palm Beach 455

 

[LQQKatJon]

getting plate secure on top of master cylinder takes the most time, so it does not leak out the sides. then pump up the tank to about 10 psi(that
is what I do). then I take my jar and hose and go around a couple times opening up the bleeders and letting the fluid drain out. I start from
passenger side rear, then go to driver side rear, and front passenger side and finish up with driver side front. I might go around twice for good
measure.

when you feel confident you have no air coming out and clean fluid, shut the bleeders off, and remove the plate and it is done. no need to pump the
brakes at all, or even go inside the coach while bleeding brakes.
--
Jon Roche
75 palm beach
St. Cloud, MN
http://lqqkatjon.blogspot.com/

 

[Jim Hupy]

Justin, what the pressure bleeder does very well is to pressurize the whole
brake system to the same pressure. It also forces all the air in the system
to the end of the lines, including the siamese brake lines at the rear
wheels. When you crack the bleeder valves, the pressure forces the air out
first, and follows it with the old brake fluid in the system, and finally
the fresh fluid from the pressure tank. The entire master cylinder
reservoir as well as the supply in the pressure tank is full of brake fluid
without ANY AIR mixed with it. I tried to vacuum bleed a GMC one time.
That is why I created the pressure bleeder kit. I know what works really
well, and what does not. Dozens of GMC owners use my kits. Do what you
think will work the best for you. ( My wife calls the "pump and hold"
method of brake bleeding a DIVORCE PROJECT.)
Jim Hupy
Salem, Oregon
78 GMC ROYALE 403

On Tue, Apr 16, 2019, 3:22 PM Jon Roche via Gmclist
wrote:

> getting plate secure on top of master cylinder takes the most time, so it
> does not leak out the sides. then pump up the tank to about 10 psi(that
> is what I do). then I take my jar and hose and go around a couple times
> opening up the bleeders and letting the fluid drain out. I start from
> passenger side rear, then go to driver side rear, and front passenger side
> and finish up with driver side front. I might go around twice for good
> measure.
>
>
> when you feel confident you have no air coming out and clean fluid, shut
> the bleeders off, and remove the plate and it is done. no need to pump the
> brakes at all, or even go inside the coach while bleeding brakes.
> --
> Jon Roche
> 75 palm beach
> St. Cloud, MN
> http://lqqkatjon.blogspot.com/
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

 

[richard h staples]

Justin wrote "... I've got an intermittent spongy pedal which I assume means an internal leak..."

Your assumption is probably correct. Approaching this logically; the vacuum power booster helps you push the pedal DOWN. If it fails or loses
effectiveness, the pedal will be (much) harder to depress, and/or you won't be able to push it down as far. Not your case it seems.

OTOH, the master cylinder and the hydraulic pressure it develops RESISTS you pushing down on the pedal. A failure or loss of effectiveness here
will result in the pedal going low and/or sinking beneath your foot. (Other things may cause this too, but will usually result in a loss of fluid to
the outside world.)

The classic proof of a bad master is a pedal that slowly sinks away when you push GENTLY on the brakes, but "revives" when you "stab" the pedal
hard and fast. The fluid seeps past a seal or seals due to wear or or damage, but the sudden pressure of a "stab" on the pedal spreads the cup seal
enough to momentarily seal and hold pressure.

If it's bad, buy a new one. I used to rebuild brake cylinders years (decades) ago, but old cylinders tend to develop pits from corrosion/rust, and
dual master cylinders are a pain to overhaul. Only calipers are worth rebuilding IMHO, and only if mostly free of rust.

HTH.
--
Rick Staples, '75 Eleganza, Johnstown, CO

"Advice is a dangerous gift, even from the Wise to the Wise, and all paths may run ill." -Tolkien