Hard rear brakes and locking up

[13matsc]

Started my truck today and was about to back out of the garage. After putting it in R and creeping backwards I applied the brakes as normal, but there was a huge clank and the rear brakes locked up. Changed gear to D and drove a bit forward, but you could hear and feel the brakes grabbing in the rear as there were squeeling sounds.

After applying the brakes, the truck does not mange to pull its own weight in either D or R. I always apply the brake when shifting gear. It’s the next time I apply the brakes after shifting the problem occurs. Seems like the «nudge» after changing gears loosen things up.

When jacking the rear up, wheels are spinnning when in D/R. No physical lock up.

Tried removing the load proportioning valve lever and trying different positions of the valve, but still the same.

Not even sure how to verify the correct positions, as it just spins freely around, but seems to have two «positions».

Disconnected the brake booster, that did nothing.
Removed the rear drums and inspected the brakes. Everything looked normal, no leaks.
No locking up in the front, brakes works as they should. This leads me to think the proportioning valve is OK. Tried grounding out the connection to see if it was applied and had a broken cable, but this works and dash light comes on when grounded.

Not sure what to do here? Any tips?
Could maybe bypass the load sensing valve?

All new rear brakes and brake lines in the rear two years ago.
New master cylinder one year ago.

1985 GMC C25
350/400, front disc, rear drums.

 

[Ricko1966]

Started my truck today and was about to back out of the garage. After putting it in R and creeping backwards I applied the brakes as normal, but there was a huge clank and the rear brakes locked up. Changed gear to D and drove a bit forward, but you could hear and feel the brakes grabbing in the rear as there were squeeling sounds.

After applying the brakes, the truck does not mange to pull its own weight in either D or R. I always apply the brake when shifting gear. It’s the next time I apply the brakes after shifting the problem occurs. Seems like the «nudge» after changing gears loosen things up.

When jacking the rear up, wheels are spinnning when in D/R. No physical lock up.

Tried removing the load proportioning valve lever and trying different positions of the valve, but still the same.

Not even sure how to verify the correct positions, as it just spins freely around, but seems to have two «positions».

Disconnected the brake booster, that did nothing.
Removed the rear drums and inspected the brakes. Everything looked normal, no leaks.
No locking up in the front, brakes works as they should. This leads me to think the proportioning valve is OK. Tried grounding out the connection to see if it was applied and had a broken cable, but this works and dash light comes on when grounded.

Not sure what to do here? Any tips?
Could maybe bypass the load sensing valve?

All new rear brakes and brake lines in the rear two years ago.
New master cylinder one year ago.

1985 GMC C25
350/400, front disc, rear drums.

Get it to stick,then crack a rear bleeder. If fluid sqiurts out or the wheels free up it is hydraulic problem,if fluid doesn't squirt out it's a mechanical problem. Let's figure out what we are dealing with first.

 

[13matsc]

Get it to stick,then crack a rear bleeder. If fluid sqiurts out or the wheels free up it is hydraulic problem,if fluid doesn't squirt out it's a mechanical problem. Let's figure out what we are dealing with first.

Was about to try this, but now the rear brakes won’t lock up completely. When releasing the pedal there is about half a second and you can hear it release and the truck rolls. The brakes are still super sensitive and when just rolling forward and backward inside my garage I can lock up the rear wheels.

Could it be a faulty return valve on the master cylinder?

I have the law provided «MOT» check in one week, so this was bad timing.