Proportioning Valve Keeps Tripping?

[KyleSeal]

UPDATE 4/18

Since my last update, I have bench bled the master (again), replaced the rear brake hard lines and the single soft line, and cleared the tee of debris.
The rear lines had some junk, nothing significant, and the tee had some as well. The rear hose flowed fluid about as much as I would say the new one does. Was this a problem? Maybe, but either case, the hose was 5 years or more old, and the lines were beginning to strip on the line nuts. So not regretting I did those in any capacity.

These made no impact on the outcome.

I still have zero pedal.

I checked the clearance for the push rod from the booster to the MC, and it moves immediately when pedal input is applied, and moves smoothly.

For S&G I adjusted the pushrod WAY out to preload the brakes, see if I could even stop the trucks rear end while in the air, it took 5+ pedal pumps to stop it, but once stopped it did not allow the wheels to free spin in gear again, as if they were locked on. I did not apply throttle to see if it would do anything.

This is honestly the most response Ive gotten from these brakes, and I still have no idea of the operation of the front brakes as I havent had enough confidence to even allow the truck to move under power.

So, next I will install block off fittings on the master to see if I can have any pedal when flow is 100% sealed.

If this master is bad, I may have to move to some sort of Wilwood or something. I would prefer not to, as it will not fit the trucks looks whatsoever, but it may be my only option at this point.

[geezer#99]

Change your pedal ratio.
Check where your connection pushrod is on the brake pedal and Drill a hole 1 1/2 inches lower. Attack the pushrod there and test. Your pedal ratio should be close to 4 to 1 then.

[Sheepdip]

Can you post pic's of your brake calipers as you have them installed? I'm just shooting in the dark here....on a thread a while back there was a situation as yours where the owner kept stating the bleeders were on top, and they where.. But they were not on top of the piston bore to remove all the air. He finally rotated or swapped the brackets which put the bleeder actually on top and problem solved.

[MikeB]

Originally Posted by KyleSeal

UPDATE 4/18

So, next I will install block off fittings on the master to see if I can have any pedal when flow is 100% sealed.

Yes, that should eliminate the master cylinder.

Also, I think the problem is something other than pedal ratio or master cyl bore size. As an example, when I installed 85 C10 spindles and disc brakes on my 69, I actually drove the truck for at least several weeks with its stock manual drum/drum master cylinder, and the brakes worked pretty well. Sometimes I wonder why I went ahead and installed the 71-72 master cylinder, booster, brackets, and pedal linkage! However, the system has worked great since then, minus some issues with the 50+ year old combo valve, and brain fade on my part when bleeding brakes recently.

[RichardJ]

>>I checked the clearance for the push rod from the booster to the MC, and it moves immediately when pedal input is applied, and moves smoothly.<<

That doesn't tell you anything about the correct booster pin length.


>>For S&G I adjusted the pushrod WAY out to preload the brakes, see if I could even stop the trucks rear end while in the air, it took 5+ pedal pumps to stop it, but once stopped it did not allow the wheels to free spin in gear again, as if they were locked on. <<

Adjusting the pedal rod longer just makes a problem with incorrect booster rod worse.

MC pistons must be able to return to seated position so that ports are opened to allow fluid to return to reservoir when pedal is released.

Adjust pedal rod for freeplay.
Adjust booster rod with actual measurements. If you don't have the measuring tool you must be able to move the MC far enough to take measurements.
A - B = C, which is what the tool does.
After "C" is determined, reduce pin height ( C ) by 1/32 to 1/16".

I set the clearance at 1/32 on a new booster for my Vette. After a couple months, I noticed at stop lights with slight grade, the vehicle would no longer roll with brake off. (manual trans) The seats inside the booster had settled with age and spring pressure. The pin had extended past my original setting and was holding the MC slightly engaged. I won't set one that close again.

[KyleSeal]

Originally Posted by geezer#99

Change your pedal ratio.
Check where your connection pushrod is on the brake pedal and Drill a hole 1 1/2 inches lower. Attack the pushrod there and test. Your pedal ratio should be close to 4 to 1 then.

This is going to be my next step.

Originally Posted by Sheepdip

Can you post pic's of your brake calipers as you have them installed? I'm just shooting in the dark here....on a thread a while back there was a situation as yours where the owner kept stating the bleeders were on top, and they where.. But they were not on top of the piston bore to remove all the air. He finally rotated or swapped the brackets which put the bleeder actually on top and problem solved.

Yes, once I get back into the shop I will snap some pictures. To my recollection, the bleeders point vertical physically so I imagine that is not an issue, but always good to check.

Originally Posted by MikeB

Yes, that should eliminate the master cylinder.

Also, I think the problem is something other than pedal ratio or master cyl bore size. As an example, when I installed 85 C10 spindles and disc brakes on my 69, I actually drove the truck for at least several weeks with its stock manual drum/drum master cylinder, and the brakes worked pretty well. Sometimes I wonder why I went ahead and installed the 71-72 master cylinder, booster, brackets, and pedal linkage! However, the system has worked great since then, minus some issues with the 50+ year old combo valve, and brain fade on my part when bleeding brakes recently.

Personally I do believe myself that the issue is not the bore size or pedal ratio either, as I see some people run some crazy hodgepodge of a booster/MC/pedal setup and those guys stop their vehicles just fine. This is just such a simple system so its super frustrating that I am having this issue. And after 3-4 bad MCs... you start to think its possibly yourself...

Originally Posted by RichardJ

>>I checked the clearance for the push rod from the booster to the MC, and it moves immediately when pedal input is applied, and moves smoothly.<<

That doesn't tell you anything about the correct booster pin length.


>>For S&G I adjusted the pushrod WAY out to preload the brakes, see if I could even stop the trucks rear end while in the air, it took 5+ pedal pumps to stop it, but once stopped it did not allow the wheels to free spin in gear again, as if they were locked on. <<

Adjusting the pedal rod longer just makes a problem with incorrect booster rod worse.

MC pistons must be able to return to seated position so that ports are opened to allow fluid to return to reservoir when pedal is released.

Adjust pedal rod for freeplay.
Adjust booster rod with actual measurements. If you don't have the measuring tool you must be able to move the MC far enough to take measurements.
A - B = C, which is what the tool does.
After "C" is determined, reduce pin height ( C ) by 1/32 to 1/16".

I set the clearance at 1/32 on a new booster for my Vette. After a couple months, I noticed at stop lights with slight grade, the vehicle would no longer roll with brake off. (manual trans) The seats inside the booster had settled with age and spring pressure. The pin had extended past my original setting and was holding the MC slightly engaged. I won't set one that close again.

I bought the H shaped tool and have adjust the booster pin to 1/16, which I will double check again, so hopefully that is all proper. When I did set up my phone to check for booster pin travel, it seemed to travel what I would imagine a normal piston stroke might be, judging by when I was bench bleeding the MC.

[KyleSeal]

So I am going to put a close to this thread:
I installed my new master cylinder from CPP (aluminum style) and bled the air out - still had no pedal, so I moved under the dash and moved the pushrod down about 3/4 of an inch - immediately the pedal was better. I have now driven the vehicle about 100 miles and the brakes are great - issue resolved.