69 GTO front disc brake hold off valve inspection
 

HI,

I have ordered up all new front brake lines from inline and the new pv2 proper combination valve so i can remove the old distribution block off frame and old hold off valve.

the company up here that sold me this stuff decided to give me another brand of brackets that holds the pv2 valve and not the correct brk03a that inline tubes says is the right part. So now i'm on hold for another re-order up to canada which is 6 weeks away. project take a MILLION YEARS I TELL YAH!!

I decided lets come back to the issue and see if i can find something I over looked.

i've been getting air in BOTH front lines - bubbles - with 2 man bleeding for like 2-3 yrs now. only way it would ever sorta half bleed is gravity then i could drive it for 3 - 4 times then pedal to floor. hence the - it's the hold off valve or the distribution block or the line in between. backs have always had no bubbles

so i decided to take off the hold off valve as it's in the front line system of the brakes before the line goes down to distribution block. no one i know has mentioned it to me before and i'm mad at myself for not just taking it apart or at the VERY LEAST BYPASSING IT with a union connect fitting and re-testing.

here are some pics. on the backside where the button is , its completely rusted. when i press the button i can see brake fluid around the edges. i think i clearly found the culprit?

https://youtube.com/shorts/BW-UuSxTxgE

anyway, forum has helped me a lot and yes i haven't solved it yet but just wanted to see if anyone has seen these fail, when they do, do you see fluid at the button behind the rubber?

also wanted to share who has rebuild kits for them:https://www.musclecarresearch.com/gm-3905525-rebuild

I also found another py person wrote this about these valves:

DO NOT hold the button unless you are PRESSURE bleeding.
Using a C-clamp can damage the valve. The proper tool holds little pressure to the button.

The valve allows flow below ~2psi (gravity bleeding) and above ~75psi (manual bleeding). It cuts flow between ~2-75psi. Pressure bleeders work at about 30psi.

Foam can be caused by many things. Possibly the valve is letting air in.
Possibly a bleeder, esp if you are using a vacuum bleeder or speed bleeder.

Possibly the valve is sucking air. They do that sometimes esp if it is the bullet valve type. Bypass it to test.

I have NEVER had to push the holdoff stem to bleed front brakes, even with a pressure-bleeder.

I tend to use 10--15 psi in the pressure bleeder tank.

However, that description--flow at very low pressure, flow at higher pressure, but blocked at moderate pressure--is how the thing is supposed to work. I won't vouch for the actual pressures that close, and then open the holdoff valve. They say 2 psi and 75 psi. I bet the "closing" pressure is more than the ~15 psi that I use in the pressure bleeder.

Yah I think you could be right, my main thing is.. does it leak. for the love of god i hope it is the reason. Taking it apart right now to inspect seals as such.

pics take apart
 

it was pretty gummed up. the bore of the valve cylinder must of had water in it from failure to do brake flushes. So the bore 1st layer had some spots where it was cracked off. so it is not a smooth surface. i cleaned it out brake clean , all parts. put it back together.

attached it back to m/c

then used a vacuum bleeder . put brake sil around the bleeders. vac gauge ran up to 15 psi, and essentially no bubbles on front two wheels.

brake pedal hard.

will fire it up tomorrow and check behind the "push button" on the hold off valve, check for leaks... suspect it still leaks there from either, bad rubber seal or weak spring from 1988.

anyway, just trying to help by posting this for someone else who may look for info on it.

happy holidays and thanks everyone for all your help over last 20 years.

just to close this off. i bypassed this with union fitting and my brakes work fine after bleeding the fronts. go figure all the other money spent for new lines and prop valve was not required. that is hot rodding.

My valve on the 67 GTO died in the 80's and I bypassed it and never noticed any difference in actual operation. Since the valve wasn't installed on later years did the master cylinders change and incorporate a holdoff or did GM just decide it was unnecessary?

Remember that the 67-68 disk brake calipers incorporated springs to maintain pad pressure on the rotors and the springs were simply eliminated in 1969. Did something like this happen with the holdoff valve?

I rebuilt the proportioning valve and the distribution valve in my 69 Lemans with the rebuild kits from Musclecar Research LLC, then used the Motive pressure bleeder to bleed the system at about 12 psi pressure. In my case, I had to push the button by hand to get flow to the front.

Their website isn’t the best but they do have decent tech articles that lead you to the rebuild kits needed. https://www.musclecarresearch.com/5355378-kit

Holdoff valve, safety switch, and proportioning valve moved into the Combination valve, which also became the distribution block.

That was in 71 or 72 wasn't it?

Could be.

These valves had a design issue from the factory.

My experience is that when bleeding brakes using a pressure bleeder you need to use the clip to depress the button. Using the two person method, especially with 4 piston calipers, can be troublesome.

When you depress the pedal, then crack the bleeder, the pedal drops to the floor. When you lift the pedal a vacuum is created between the valve and master cylinder while brake fluid is drawn into the system. This can cause the valve to suck air around the button. I have literally gone crazy a few times not being able to bleed the system properly.

My fix is to keep the valve in good condition and to pack the rubber seal at the back of the valve with silicone vacuum grease.

Also I have had luck lifting the pedal very slowly, so the amount of vacuum in the lines is limited as the system fills.

class j - >that would explain how gravity bleeding only lasted for a couple rides b4 losing my brakes. The theory is if you slam on the brakes that back drums will have the time necessary to come on so you are not slamming your front's on. A car stops with more control starting with the back's applying. hence the name hold off for front's.

i went crazy for months on end chasing my tail because of that valve. did the 70 gto have a combination valve and no hold off ?

i can't see the combination valve somehow having a built in metering for the hold off? Not sure how this all got handled as year progressed.

The hold-off is there in the combination valve. Once cleaned, the GM version works very well.

oh wow there it is ! thanks for showing me this, that is 1970 and up assuming ?

Yes, the combination valve part number for A bodies is listed for 70-72

I have driven the car with it bypassed. Its arguable whether it makes a huge difference. What I can say is that in the past the valve has proven to be dangerous if not in good condition.

I still prefer to two person bleed the car but I do it very slowly and take time after each bleed cycle to lift the pedal slow, allow the system to fill, do a few slow pumps, then do another bleed cycle.

i would agree. i wish i had your advise years ago ,hey go look behind the rubber button. oh gee look brake fluid. ah well some time the hardest lessons teach one a lot. slow return on peddle is a new one for me but i can add it to things to try....when.......stuff like this happens.....

thanks everyone appreciate it