Hydroboost on 81

[Old 403]

Been a while since I checked in here but I'm having brake problems and looking for ideas. The original hydroboost brake system has worked great from day one on my 81 Sierra Classic pickup but recently I'm getting only front brakes applying and takes more pedal pressure to stop. Rear wheel up on blocks test shows the rear brakes will apply on a quick pedal push but gradual pedal application gives almost no braking and the pedal goes down more as if pressure is by passing somewhere yet there are no visible leaks. Brake fluid is old and dark. Maybe the master cylinder is weak? The truck is a former diesel but now gas and have run with the original diesel steering pump and brakes since the swap in 86.

[mongocanfly]

The hydroboost has no effect on which wheels stop when mashing the pedal... sounds like maybe your master is bad...have you tried bleeding rear brakes? Also on the proportioning valve, it has the trip feature, meaning the internal spool can trip and send all the fluid to front or back....check and make sure it isn't tripped..mine was one time and all I had was rear brakes.. they make a tool to screw into it to hold the spool centered so you can bleed the system..
Another thing to check is the condition of the rubber hoses, If they're orig,it's probably best to put new ones on,
At the least it sounds like your system needs flushed..

[Dead Parrot]

2nd on the MC. Also replacing the hoses if original. If you replace the hoses, might as well replace the rear wheel cylinders. Inspect the fronts and replace if worn. Get dirty once, have a fresh brake system.

[Old 403]

Originally Posted by mongocanfly

The hydroboost has no effect on which wheels stop when mashing the pedal... sounds like maybe your master is bad...have you tried bleeding rear brakes? Also on the proportioning valve, it has the trip feature, meaning the internal spool can trip and send all the fluid to front or back....check and make sure it isn't tripped..mine was one time and all I had was rear brakes.. they make a tool to screw into it to hold the spool centered so you can bleed the system..
.

How do you check that proportioning valve? Another thing I've been advised is maybe the rear shoes need setting up. This truck, for some reason does not have the slot openings in the backing plate to allow shoe adjustment. Do I have to cut openings there to get in with an adjuster? When the rear drums are off the cylinders and shoes all look good. No signs of leaks or worn out shoes.

[mongocanfly]

some prop valves you can reset by a button on the end of it...my 81 had that button..mashing it recentered the spool..
to adj brakes get them close manually with the adj, then button everything up..start backing up and mashing ebrake..do this several times and itll adj the shoes

[Old 403]

Originally Posted by mongocanfly

some prop valves you can reset by a button on the end of it...my 81 had that button..mashing it recentered the spool..
to adj brakes get them close manually with the adj, then button everything up..start backing up and mashing ebrake..do this several times and itll adj the shoes

I don't remember seeing anything like that button on the proportioning valve but I'll check it out again. When I had the rear wheels on the jack I ran it in reverse and did numerous brake applications thinking the self adjusters might work but I could not see any results from that at all. My parking brake is non operational due to a damaged cable.

[mongocanfly]

"My parking brake is non operational due to a damaged cable"

That's why you can't adj the rear shoes then

[Old 403]

Originally Posted by mongocanfly

"My parking brake is non operational due to a damaged cable"

That's why you can't adj the rear shoes then

No, the parking brake has nothing to do with the self adjusting feature. This from a veteran heavy duty mechanic and a GM partsman. It turned out to be the master cylinder. New cylinder installed and its back to normal four wheel braking.