Sloppy breaks -1972 K5 4x4

[Sagesurfing]

I’ve been dealing with a sloppy break issue for years now. Seems like the breaks lose pressure, particularly after driving high speeds/rpms (freeway driving.)
I find that the peddle starts ok (not the best, but works,) and after driving a bit it just ends up going soft, to the floor. In the past I’ve had to throw the truck in neutral and press the pedal with both feet to get it to slow to a stop.

Just to make it easier, here’s what I’ve tried so far:
- 2018 new break booster
- 2020 (new engine, master cylinder
- 2021 bled the master cylinder again and all the break lines (farthest to closest.)
- 2023:
• replaced front breaks (calipers/pads/rotor)
• replaced rear pads, re-spun the drum
• adjusted the break arm length that goes from the booster to master C
• bled the master and all lines twice more
• seated the breaks (adjusting drums, following seating procedures)
• also ensured the proportioning valve shuttle remained centered during bleeding process and checked to see that it isn’t tripped.

I’ve also checked for vacuum leaks, searched for like leaks etc’…

Again, I drive around town locally and it’s pretty much fine. But as soon as I drive on freeway kinda higher RPM’s, at some point it’ll suddenly lose pressure (pumping does nothing at this point.)
If I stop at that point and just kill the engine for just a couple seconds, the pedal will pressurize again with one pump and I can restart and go again.

Any thoughts?

[Steeveedee]

That is the symptom of the seals in the master bypassing fluid. I don't see it on your list of changes.

[A1971Blazer]

have you replaced all the rubber hoses? I just recently replaced the entire braking system on my ‘70…it will easily skid all 4 tires when you cram down on the brake pedal

[Sagesurfing]

Originally Posted by Steeveedee

That is the symptom of the seals in the master bypassing fluid. I don't see it on your list of changes.

Well.. I did replace the entire master cylinder. I would think that would address that issue, right?

[Sagesurfing]

Originally Posted by A1971Blazer

have you replaced all the rubber hoses? I just recently replaced the entire braking system on my ‘70…it will easily skid all 4 tires when you cram down on the brake pedal

Not sure if the rubber hoses would explain why the breaks come back entirely if I kill the engine for like 10 seconds.

[dmjlambert]

So you stop making vacuum for 10 seconds and it's back to normal. This seems really strange. I don't know much about power brakes, so I'm staying tuned to find out what is the deal.

[Steeveedee]

Originally Posted by Sagesurfing

Well.. I did replace the entire master cylinder. I would think that would address that issue, right?

Doh, I misread that!

[dagnabbitt]

Originally Posted by Steeveedee

Doh, I misread that!

Not uncommon, mind you, for even a brand new MC to be junk. I know that isn't very helpful but it happens quite a bit.

[Sagesurfing]

Originally Posted by Sagesurfing

Not sure if the rubber hoses would explain why the breaks come back entirely if I kill the engine for like 10 seconds.

Today I tested the theory that maybe the vacuum is the culprit. I thought that maybe the bladder in the booster collapses and that killing the engine stops the vacuum and allows the bladder to take its form again. I pulled over when the breaks went soggy and unplugged the vacuum from the booster for a bit without killing the engine, then plugged it back in but the issue persisted. It’s just weird because I don’t understand how the engine would have influence on the break system other than through the vacuum line. 🤷*♂️

[Ironangel]

What motor are you running? Have you taken a vacuum gauge inside the cab with you and monitored the vacuum through the various engine speeds and loads? A lot of overlap in a cam will kill vacuum availability. Also, the rear wheel cylinders can leak past the piston and trap fluid between the push pin seals and pistons. If your not loosing fluid I'd suspect the master cylinder piston seals are not sealing completely. When bleeding the wheel cylinders take some thick dielectric silicon grease and seal the threads on the bleeders so air cant be drawn back in on the release. Only open the bleeders a quarter of a turn to minimize the air being sucked in by a vacuum bleeder...

[A1971Blazer]

Originally Posted by Sagesurfing

Not sure if the rubber hoses would explain why the breaks come back entirely if I kill the engine for like 10 seconds.

sorry..I missed that..sounds like you've replaced everything that could cause the issue, but more than a few times I've done that only to find out that a new or reman part is bad