Hydroboost front brakes locking up after heating up

[Ryanbrown36]

I’ve recently replaced my rear axle and hydro boost in my 1981 sierra grande that came with factory hydroboost and was working before aside from the seals being gone and leaking everywhere. I’ve purchased a replacement unit for I believe a 1978 c3500 due to not being able to get a bolt on replacement for my truck. I’ve shorted the pushrod to what I think is the right size (20 thousandths smaller than the distance of the master to the hydroboost piston) there’s a healthy amount of play in the pedal before it engages the piston but as soon as thermal expansion happens they still slowly lock up until I crack a bleeder and release the pressure. Anyone got any ideas of anything else to check or try?

 

[Ricko1966]

Crack a line at the master and see if that still releases them if you have to crack the bleeder to release them you probably need new brake hoses.

 

[xm20k]

Crack a line at the master and see if that still releases them if you have to crack the bleeder to release them you probably need new brake hoses.

This either swollen shut or the liner is damaged causing it to act like a one-way check valve.

 

[Dmack]

I had the same issue. Cracking the caliper line helped me get home, but would happen again at random times. Finally traced it to the master to hyrdoboost interface. I loosened the nuts securing them together and and it released the brakes. I played with spacing and found a spacer made out of .100" aluminum cured my problem. Experimented with washers to get the spacing.

 

[HotWheelsBurban]

I had the same issue. Cracking the caliper line helped me get home, but would happen again at random times. Finally traced it to the master to hyrdoboost interface. I loosened the nuts securing them together and and it released the brakes. I played with spacing and found a spacer made out of .100" aluminum cured my problem. Experimented with washers to get the spacing.

Yes the hydro boost units are prone to this. If the pushrod effective length is incorrect, it'll make the brakes lock up after a few stops. BTDT, this is the thing I had the most problems with on my OBS one ton Crew Cab longbed.

 

[Ryanbrown36]

Crack a line at the master and see if that still releases them if you have to crack the bleeder to release them you probably need new brake hoses.

Okay I will try the master and see what happens

 

[winfred]

if cracking a brake line above the brake hoses releases the brakes it's more likely a hybroboost issue, if only at the caliper/below the hoses, it's a hose

i modified a 2010 2500 duramax hydro booster to fit my 78 c30, swap the firewall mounting plates, and shorten the rod, then got these little brass drop in's for the fittings, later ports are o-ring seal, early ports are flared seal, the adapters are LS swap parts and cheap at amazon to old style lines work in a modern pump, but they worked in the booster too