Brake System Help

[CCCC]

The brakes on this 1972 C30 work, and pedal is quite firm, but it feels as though the power assist is not what it should be. Braking feels more like regular (non-power) effect with no real "grab" even with a quick/hard pedal application.

There is good vacuum to the booster and the grommet/check valve seem to be functioning fine - 5 minutes after engine shutdown I get a significant sucking effect when the valve is pulled from the booster - so it is holding vacuum.

Fluid levels good on both sides of the master cylinder and no apparent leaks.

Questions:
Can the booster drum be holding vacuum and yet still not do its proper assist work? How does one check the function of the booster action itself?

Might it be that only one side of the master cylinder is working - giving only 2 wheel brakes - and causing the weak effect even with what may be good booster function? Other than the visible test for turbulence with the lid off the reservoir, or removing a line connection and looking for the "squirt", how to check the MC function on both sides?

What else?

Any insights will be much appreciated.

CCCC

[brak]

If you have a hard pedal, I will go with the booster as the problem.

If the booster was working properly, you should have soft pedal, even if the master isn't functioning correctly.

Just my $0.02

[chris71]

i agree with brak, i dont know too much about the internals of a booster but i have heard that there is a rubber diaphram inside, if the diaphram tore or disintigrated (dry-rotted) , then you wouldnt get the proper assist, but the casing may still hold vaccum, as you have indicated. my money is on the booster, i would just check all 4 wheels and make sure they are in good working order, just for safty sake. when i bought my 55 truck, the rear axle seals were leaking and it looked like you coated the whole inside of the drums with grease, not very cool. good luck whatever you do.

[68gmsee]

Originally Posted by CCCC

The brakes on this 1972 C30 work, and pedal is quite firm, but it feels as though the power assist is not what it should be. Braking feels more like regular (non-power) effect with no real "grab" even with a quick/hard pedal application....

Can you explain what you mean by "no real grab?" Are you saying that the tires do not lock up?

Several things can cause this. Brake shoes contaminated or glazed, front brakes not working properly, could be proportioning valve or m.c. itself.

If you think your booster is okay, then I'd check the operation of the brakes. An easy check is to have someone depress the pedal while you slowly unscrew the bleed valve on the wheel cylinders. Check that each one has a good amount of pressure. Especially the front brakes.

If the master cylinder is not working properly and only the rear side of the brake lines are working, the vehicle will require additional pedal pressure to stop. This is not good.

[68shortfleet]

Have you actually checked to see how much vaccum you have going to the booster? Just cause it sucks air when you pull the hose, does not mean that you have enough "suck". Low vaccum will cause the pedal to feel kinda like a truck with manual brakes, and very low vaccum or bad booster will actually make the pedal harder(firmer) than even a manual brake truck, actually feels worse than manual brake trucks.

HTH
Jason

[CCCC]

Thanks to “brak”, “chris71”, “68gmsee” and “68shortfleet” – this is the type of advice I was hoping for. Have checked the rear linings and drums (a real task on a C30) – no leaks and good lining and good drum surface) and the front pads as well – all good. When I say "no real grab”, I mean that it is difficult to lock up the brakes (and maybe impossible – but I have not yet really jammed them totally hard).

68shortfleet - As soon as I get it home will put a vac meter on the feed line to measure the actual vac (anybody know what the normal back of carb vac should range in a stock 402BB with Edelbrock 4bbl?)

68gmsee - Also will try the bleed method you suggest to check the fluid pressure at front as compared with rear - and have a suspicion that this concern may be caused by pressure imbalance – a poor performing MC on front or rear section – or a prop valve problem.

It will take a while, but will get back on here with the outcomes.

Thanks again. CCCC

[herb101]

Most stock motors will run somewhere around 15" of vacuum at idle. Cam changes make the greatest effect on vacuum as the lift and duration define the window of time that the piston sucks through the intake valve, which is what creates vacuum. Vacuum will drop off as the RPM or load increases, eventually hitting 0" at max load. That is why the brake booster can is so large - it's a vacuum resevoir.

You should also verify your check valve while you're in that far. With it removed, you should be able to blow through it from the small barb to the large, but not from the large barb to the small. Messed up check valves cause a whole host of diagnostic problems and dead-ends and they are only a couple bucks in the "HELP" section to replace.

Good Luck,
Herb