Brake Bleeding 101

T

Tom Katzenberger

Well-known member
Jun 26, 2019
795
287
63
Guys,

While bench bleeding a new master cylinder (P30) I only get flow out of the front chamber. No action in the rear chamber. I do get one single bubble
in the rear chamber each stroke. Is the master cylinder toast? Did I blow the rear seal?

Thanks,
Tom K.

--
Tom & Oki Katzenberger,
Kingsville, Maryland,
1977 23' Birchaven, 455 C.I.D.
 
Typical Red Chinese Crap.

> Guys,
>
> While bench bleeding a new master cylinder (P30) I only get flow out of
> the front chamber. No action in the rear chamber. I do get one single bubble
> in the rear chamber each stroke. Is the master cylinder toast? Did I blow
> the rear seal?
>
> Thanks,
> Tom K.
>
>
> --
> Tom & Oki Katzenberger,
> Kingsville, Maryland,
> 1977 23' Birchaven, 455 C.I.D.
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
>

--
Jim Kanomata ASE
Applied/GMC, Newark,CA
jimk
http://www.gmcrvparts.com
1-800-752-7502
 
Try pushing harder. When I recently did my 34mm MC from Dave Lenzi, I ran into resistance after depressing the plunger about 3/4". It turns out that
the full travel is about twice that, so once I overcame the additional resistance of the second spring everything was fine.
--
Christo Darsch
GMC Nor'easters
1977 Eleganza II - "The Komet"
3.50 Power Drive, Disc Brakes, Alcoas
Weymouth, MA
 
Such a timely post. I've been restoring my coach for almost 2 years now and intended to take it on its maiden voyage last week. One of the last things
I had to do (mechanical wise) was to bleed the brakes after I replaced a blown out brake line. Try as I might, I could not get a stiff pedal and
sufficient braking; even with Jim's bleeder. I was faulting a bad MC (even though it's new) or a poor bench bleed. But now I see lots of new tips here
I will employ this weekend. Thanks OP. Great thread.

Shawn
--
Shawn Harris
North Vancouver,
Canada
1977 Palm Beach 403
 
So…couldn’t wait until the weekend. I just checked the distribution valve and the plunger on the end travels about a 1/16 inward from its resting
position when I push on it and pulls out about the same if I pull it outward. So total 1/8 travel. That sounds like limited travel to me but I’ve no
clue how much it should move.

Any thoughts??

Shawn
--
Shawn Harris
North Vancouver,
Canada
1977 Palm Beach 403
 
Shawn,

I'm not positive what you're calling the "distribution valve", but if it's
the device where the lines from the master cylinder connect and result in 2
front brake output lines and one rear brake output line being
"distributed", then it's more commonly called the "combination valve" (or,
erroneously "proportioning valve"). In that case you may want to have a
look at this to see whether your "problem" is one or not (remember that
cast iron housing is subject to lots of rust after 40+ years:
http://www.gmcmhphotos.com/photos/g3510-deciphering-the-combination-valve.html

Ken H.

> So…couldn’t wait until the weekend. I just checked the distribution valve
> and the plunger on the end travels about a 1/16 inward from its resting
> position when I push on it and pulls out about the same if I pull it
> outward. So total 1/8 travel. That sounds like limited travel to me but
> I’ve no
> clue how much it should move.
>
> Any thoughts??
>
> Shawn
> --
> Shawn Harris
> North Vancouver,
> Canada
> 1977 Palm Beach 403
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
>
 
Thanks Ken. Yes, I was referring to the “proportioning valve”. I was parroting Jim’s vernacular.

At any rate, I’ve seen that article you wrote up. Great stuff but doesn’t refer to the travel of the “plunger”.

I will likely just buy and replace but I would like to get this girl rolling sooner than later.

Summer is here!
--
Shawn Harris
North Vancouver,
Canada
1977 Palm Beach 403
 
> Guys,
>
> While bench bleeding a new master cylinder (P30) I only get flow out of the front chamber. No action in the rear chamber. I do get one single
> bubble in the rear chamber each stroke. Is the master cylinder toast? Did I blow the rear seal?
>
> Thanks,
> Tom K.

Tom,
Several years ago I decided to try a P30 MC with my rear disc brake system when my stock MC had a pedal that under constant pressure went slowly to
the floor. The P30, on bench bleeding, I could not get the rear chamber to bleed. So, I took it back to Auto Value for another one. The next one did
the same. Returned it for another, and another. I also asked the counter guy that I had known for years (and also an excellent mechanic) to try it
himself, and he had the same results....rear chamber would not bleed. I called Dave L and he told me to put a lever on the bleeding arm and push HARD.
Eventually it broke loose and bled. While the P30 was a commonly used MC back in the 70's, it is not so common now and is a MC that does not
move...sits on the shelf. Conclusion we came to is that the MC's that I got had been sitting on a number of part house shelves for many years and that
back chamber piston had corroded in the bore, was frozen in place on all of those MC's. With the corrosion in that rear bore, that rear bore was
compromised. After talking with Dave at some length, I chose to obtain a stock MC from a friend, bench bled it successfully, installed it and went on
my way. The P30 has an inherent issue that makes it prone to sudden failure. Personally, I would reconsider the use of that P30 and go back to the
stock MC. Just what I'd do.

--
Larry
78 Royale w/500 Caddy
Menomonie, WI.
 
Larry,

Thank you for the great info. I already have the replacement P30, painted, powder coated the lid, blead and installed. If this fails I will go with
the original, as I plan on checking the break fluid before I pull out and in between stops.

I see that the P30 does not allow for easy refill and checking of fluid. Just getting the rear clip off is a pain.

I have the front fluid chamber hooked up to the front brake system, is this correct for the P30?

Thank again,
Tom K.
--
Tom & Oki Katzenberger,
Kingsville, Maryland,
1977 23' Birchaven, 455 C.I.D.
 
I just double checked. The P30 uses the front chamber for the front brakes.
--
Tom & Oki Katzenberger,
Kingsville, Maryland,
1977 23' Birchaven, 455 C.I.D.
 
You must log in or register to reply here.
GMCMI Fall 2026 Rally
Applied GMC

Share this page

Share this page
Top Bottom
Back