Brake Pedal Slowly Goes To The Floor

[Tom Katzenberger]

Good Morning all,

My situation is as follows:

Components: I have installed my 3rd new P30 Master Cylinder on a new 6 Disc brake system. All lines have been replaced with stainless steel solid and
braded where needed. I installed a new combination valve which has the piston removed so it no longer proportions from front to rear. I have Dave
Lindsey's sensitized brake booster with a summit back up vacuum pump.

Problem: After bench bleeding and making sure the plunger is not depressed more than 1" I still have the the brake pedal slowly sinking while under
steady pressure.

I am looking for the wisdom of the members on this site. Again and as always, all opinions and thoughts are welcome and appreciated.

Thank you all.

Sincerely,
Tom K.
--
Tom & Oki Katzenberger,
Kingsville, Maryland,
1977 23' Birchaven, 455 C.I.D., Micro Level, Howell EBL-EFI Spark Control, Macerator, York Air Compressor, 6 Wheel Disc, Quadra Bag, Onan W/Bovee
Ignition

 

[LarryW]

Which 6 disc brake system are you using (rears)?
--
Larry
78 Royale w/500 Caddy
Menomonie, WI.

 

[Jim Hupy]

That sounds to me like an internal failure of your master cylinder.
An old time mechanic taught me this " New parts are just new, they
are not necessarily good or serviceable". I had a similar issue with a new
master cylinder, pedal creep. Turns out the fluid was bypassing the tandem
seal inside the cylinder. When I took it apart and inspected it, the bore
finish was eccentric. Bad machining. Junk. Bought a new OEM cylinder. All
was right in the brake world.
Jim Hupy
Salem, Oregon

> Which 6 disc brake system are you using (rears)?
> --
> Larry
> 78 Royale w/500 Caddy
> Menomonie, WI.
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[Biggamehunter]

Dead head the system. Block off the master. If it holds move to to the combo valve. If it still holds work your way down the system .
--
Michael Orlandi
Fairfield, Ca
1973 GMC Canyonlands
Lenzi knuckles/ hubs

 

[richard h staples]

Tom, If you aren't losing fluid from the system, it is almost certainly a defective/damaged master cylinder. The classic test of a master cylinder
involves first "stabbing" the brake pedal (stepping on it hard and fast). This will often cause a worn piston cup to momentarily seal and hold
pressure. Then release the pedal and rest your foot lightly on it and slowly push down. A bad master will seep fluid back into the reservoir, and
the pedal will sink.

HTH
Rick Staples
--
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

 

[gmcnet1]

I don't believe that stabbing the pedal works on the cursed P30 MC because it uses poppet valves rather than the piston cups you mentioned.
--
Christo Darsch
GMC Nor'easters
1977 Eleganza II - "The Komet"
3.50 Power Drive, Disc Brakes, Alcoas
Weymouth, MA

 

[Tom Katzenberger]

Guys,

That is also what I thought and was my experience in the past.

Rather then installing another P30, I am looking at the 2 stage master and the Hydro boost. The 2 stage master is the easiest and cheapest so I will
try that first.

I am really not interested in installing a 4th P30.

Take care and I will report back.

Thanks again guys,
Tom K.
--
Tom & Oki Katzenberger,
Kingsville, Maryland,
1977 23' Birchaven, 455 C.I.D., Micro Level, Howell EBL-EFI Spark Control, Macerator, York Air Compressor, 6 Wheel Disc, Quadra Bag, Onan W/Bovee
Ignition