Anyone have experience with Hydroboost???

[Motor City GMC]

When i test drove this truck the first time before i bought it the brake pedal was very soft and went to the floor, ot would stop the truck barley but when it did it would do so by locking the back brakes, so me and the owner bled the brakes out and added some fluid and the pedal came back and all was good, i drove it home a hour and a half.
The truck sat for the next few weeks while i worked on it and the next time i drove it i still had a nice stiff brake pedal but the rear brakes was doing the same thing again very tou hy and locking up when braking.
Fluid is full, the only thing i am questioning is that i know i have a small power steering leak, could this be the problem?
What do ya think????

[cadillac_al]

Either a leaking wheel cylinder or axle seal. You should check the gear oil in the rear axle to make sure it isn't dry, since you said you aren't losing brake fluid. I have a car that leaks a little brake fluid. The rear wheel only locks up for the first few stops then it smooths out so I don't sweat it.

[mongocanfly]

I wouldn't think it would be the hydraboost.if the hydraboost didnt work the pedal would be hard to push.just like pushing brake pedal with engine not running. If its a soft peddle with no visible leaks it would more likely be the master cylinder. If you only got back brakes your proportioning valve may be tripped and needs reset. Mine was that way when I got it. Po had replaced front brake lines. What happens is it was built in for safety in some trucks in case you blow the brakes on say, the front, the valve trips and sends fluid only to the back brakes.and vice versa.to reset it you would need to loosen back brake bleed screw "to create a leak" and slowly push the brake pedal and the valve will "reset"..that fixed mine..or it could be the front side of master cylinder leaking internally.. I think what he's talking about with rear diff level is he means oil leaking thru axle seal onto brakes but if you only have back brakes I don't think that would be it.

[Motor City GMC]

Lack of gear oil in the pumpkin could cause this??

[jdf915]

I'm not sure on the gear oil but you should take the drums off and see what your parts look like and you should make sure they are adjusted properly.
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[Motor City GMC]

I did notice a couple runs of fluid on the outside front of the master and also a couple behind the hydroboost against the firewall (where it bolts to the firewall) last time i drove it...also my pedal is kinda stiff but only got like that after bleeding the brakes.
Could that valve bieng tripped cause this?
When resetting it do you open the bleed screw and pump the brakes or only push down once, hold and retighten before releasing em?

[mongocanfly]

I just had to change my hydraboost the other day because it was leaking power steering fluid onto my headers but it had no affect on my braking.if you have a leaking hydro and master cyl I would replace both and start over..this is the first hydro I have owned and I tried and tried bleeding brakes but couldn't get them to work. So I went to a buddy of mine that does alignment and brake work and he told me about the prop valve. He loosened one bleed screw on rear and I slowly pushed on pedal until it reset...seems like it took a couple tries..he said any time you open a brake line and push the pedal it will do it again.. when it sees unequal pressure from front to back it will close the low pressure side and it will have to be reset again..or he said I could replace prop valve with just a standard one and eliminate that problem. I'm not sure you have a prop valve with a reset ,,mine did,, he said not all of them had that and I don't know how to tell them apart.I would change hydro and master first if both are leaking and go from there.

[Motor City GMC]

How do you know if that valve is reset or not when you do that?

[mongocanfly]

We had it on a lift and it went from no front brakes to good front brakes...I was in the truck but he said you could hear it click when it went back but I didn't hear it.

[hatzie]

If the Combination valve is not centered the Brake light will be on...