Need Help with Hydro-boost

My '78 Chevy C-30 is acting up, and I've never had to work on a hydro boost system before. The power steering is fine, but the brakes are all wonky. Sometimes it acts like there's air in the brake system. Sometimes I have almost no pedal at all until I pump it. Sometimes the pedal acts like the master cylinder is bad, with the pedal continually creeping towards the floor. Sometimes it's rock hard and even kicks back a little. Sometimes the rear brakes lock up during braking. It all seems kind of intermittent.
Any advice from the experts?

Even if the hyroboost fails, you should still have a pedal, just lacking boost. I'd check out the rest of the brake system. Pulling the drums off that thing will suck, look for fluid leaks down the outside of the backing plate first. Are you losing axle oil? If any of its making it's way to the brakes, all kinds of weirdness ensues. Pinhole in a line sucking air?

does the PS fluid level get low?

if so, the booster might need to be rebuilt/replaced. look for a trail of PS fluid down the firewall by the booster.

Yeah first of all, I'd disable the hydroboost (have no idea how you do this) and drive it unpowered to check for more fundamental brake problems.

Kinda hard to disable the hydroboost, high pressure line runs to the hydroboost and then to the steering box.

It does sound more like something in the brake system. Check all the usual suspects for leaks, and check hoses, they may be getting too old.

I don't know about Chevy trucks, but on old BMWs that have hydroboost brakes there's a nitrogen filled accumulator underneath the master cylinder (nicknamed as the brake bomb, because it looks like an old timey round bomb from the movies.) If that thing fails the brakes get all weird similar to what you describe. I'm guessing Chevy included a similar accumulator in the system somewhere.

They suck. I hate the pedal feel, and when they go stupid, they get a really soft low pedal.

Sounds like maybe there is air in the power srteering system, or the master cylinder is junk.

Sounds like a M/C problem to me. I work on industrial vehicles that use hydroboost of all different types and I've never had a bad one.

The hydroboost is pretty simple and bulletproof. Its simply taking the place of the vacuum booster, so if it were failing you would typically have a consistent rock-hard pedal.

The rest of the system is plain-old boring GM parts-shelf stuff. Sounds like a brake thing and not a hydroboost thing.

One thing hydroboost will do is overpower the limit spring in the M/C. On a typical vacuum boosted car, the harder you press, the harder it gets to press. you reach a point where the pressure in the system overcomes your leg strength and it doesn't go anymore (unless you have air or a bad M/C). With hydro, there reaches a point where the fluid pressure assist will overcome. Depending on which M/C you have, even with a properly operating system, you might be able to bottom the pedal at which point the P/S belt might squeal and the pedal seems to kick back a bit. That's not an indicator that something is wrong, its just that hydroboost creates a LOT more peak assist than vacuum.

Thanks for all the suggestions, guys.
I've owned the truck for many years, so I'm pretty familiar with how it feels when it's working, and there's definitely something wrong. I also have a lot of experience with vacuum boosted brakes, and while I'm sure I haven't experienced everything possible, this thing is acting very weird. It does have a power steering leak, and I have to refill the res. often. Could that create a problem or damage the booster? I don't lose brake fluid, so I don't have a leak in the system.

I have an '85 C30 that used to have hydroboost. I took all that crap off and replaced the booster with a normal vacuum booster. Never saw the reason behind having the power steering tied into the brakes. It's a fairly simple swap. I got the parts from the local Pull A Part, but any decent Chevy pickup with the right options will be a good donor vehicle.

ncjay wrote: Never saw the reason behind having the power steering tied into the brakes.

because vac boosters suck especially on gm trucks and the hydroboost gives so much more positive feel.

In reply to patgizz:

Pat, do you have any insight into my problem?

bravenrace wrote: It does have a power steering leak, and I have to refill the res. often. Could that create a problem or damage the booster? I don't lose brake fluid, so I don't have a leak in the system.

doubtful. The power steering requires more pressure than the hydroboost, so if you're getting power assist steering, you should be getting enough to run the hydro.

I'll tell you the truth, though... I have never ever ever ever saw, heard of, or fathomed a failed hydroboost. I didn't think it was possible.

i don't think they "fail" so much as have the seals go bad and cause havoc.. i rebuilt the hydraboost unit in my Regal a couple of years ago in about a half hour. mine worked fine, but had a bad enough leak to lower the fluid level enough to cause the pump to whine about every 1000 miles. you can get a couple of different rebuild kits from www.piratejack.net and rebuild it yourself, or take a gamble on a reman booster for $120. i chose the $40 option..

someone mentioned the pressure reservoir- the older Chevy trucks didn't have one of those, and since my Regal uses the same booster, it doesn't have it, either.. but it does somehow store enough energy for one pump of the pedal after engine shutdown.

I guess the thing that's bugging me about this is that I've never seen a brake system act this way, and I've got a LOT of experience with brake systems. What I don't have is experience with mal-functioning hydro-boost systems. But I hear you guys, so next question. Is there anything about the hydro-boost that would make a mal-functioning brake system act differently than it would with a vacuum booster? I'm mainly suspecting the master cylinder hear, but it's not acting like a typical bad MC.

sorry i've been absent from this thread. here's my input: ______.

that's right, i have nothing to add. i have zero experience with hydroboost.

Re-reading the initial post, I'll first apologize for speculating even though I'm definitely not the requested "expert"...

Beyond that, while the symptoms you describe are spectacularly strange, they all seem like stuff that would almost certainly be master cylinder if for no other reason than it defies Occam's Razor to try to make them into, say, brake line issues.

I'm going to suspect some sort of bizarre combination of degraded seals in the M/C. When they're bad, they cause the sinking pedal you've got. It's still a reach, but I'm wondering if you've got a torn or otherwise deformed M/C seal so that you sometimes get the sinking pedal, other times you get nothing 'til you pump it (perhaps flopping the torn seal back into rough position?). Even the rear brakes locking; as bizarre as it sounds to have the intermediate piston stick (due to a folded seal?), could that cause the rears to lock?

It's obviously a guess, but trying to ascribe that combination of symptoms to anything other than the M/C just seems wrong. Were I in your shoes, I'd start there.