'72 K20 Braking variations

[72-c20]

stock frontdisks, drums rear.
was braking fine, but needed a bit more bleeding...

after I bleeding, the back wheels would lock up really easily. if you pushed slowly slow, no lockups.

then it switched tonight. harder pedal than before. like no boost at all. thought I wouldn't stop in time a couple of times! and I could head a sound like the pressure bleeding off coming from under the hood....after I let off the brakes

then on my way home, on about the 3rd time stomping on the brakes, I got 'em to lockup again, and now it's back to the quick lockup again.

I've searched through all the brake problems, and didn't see one where it went back and forth....

depending on whichpost I read, I lean towards the booster, then lean towards the check valve.

thoughts?

[68gmsee]

Good question... The master cylinder could have defective seals or have internal contamination. Lots of internal stuff that can go bad in the booster also. Since master cylinders are cheaper, I'd start with that.

As for the p-valve. It seems that you may have had air on the lines going to the rear brakes. Front brakes do most of the stopping and you may not have noticed if they were not working before. It's possible that after bleeding, they're now working they way they should and the proportioning valve is bad.


The proportioning valve reduces the pressure to the rear brakes. Regardless of what type of brakes a car has, the rear brakes require less force than the front brakes....

If equal braking force were applied at all four wheels during a stop, the rear wheels would lock up before the front wheels. The proportioning valve only lets a certain portion of the pressure through to the rear wheels so that the front wheels apply more braking force. If the proportioning valve were set to 70 percent and the brake pressure were 1,000 pounds per square inch (psi) for the front brakes, the rear brakes would get 700 psi.

[72-c20]

should also add that prev. owner had put in a new master cylinder, due to brake problems, and couldn't get it bled properly, so the truck sat for years before I got it....

[myoldchevytruck]

Just priced a new 72' Master Cylinder at Auto Zone. About $40.00. Reconditioned was about $20.00 plus core.

[70rs/ss]

If you want to do it once and be done, buy the whole unit, master and booster for like $125. The wooshing sound, sounds like the booster is leaking, so I'd do the unit and be done. If the rears are locking after the new parts, they may be adjusted too tight?

[72-c20]

Ok. just took it for another drive, and the noise from under the hood was a lot worse. parked it, but kept it running, and after messing with the hoses, found the culprit (at least for why my syptoms varied).

the hoses that were being used for my PCV, and last part of the brake booster hose were collapsing. need to get the proper hoses yet and a new check valve just to be sure... but to get me home (I was parked at Menards) I got a hard plastic elbow, and coupler, and eliminated the areas that could collapse.

brakes working as "normal" again... (as in when I first got it back after the bleeding. if I "stomp" it, I can lock/semi-lock the brakes, but normal braking pressure, it stops nicely....

[70rs/ss]

If the rears grab before the fronts, try backing the adjustment off a turn at a time on the back brakes. When adjusted correctly the rear shoes should slightly drag on the drum, when doing one side it is better to have the other side apart so that drum doesn't drag causing you to go too loose. I wondered if it was just a hose issue, good hose (not heater hose!) will prevent that from happening. the valve is more for an incase the motor dies, than normal operation. The valve holds pressure for an in case situation. Be sure the vacuum is coming from the bottom rear of the carb and is the only draw from that port, too little vacuum can cause a braking issue as well, the PCV valve needs to draw from another vacuum source, usually into the air cleaner. Glad you seem to have gotten you hands around it!