Brakes

OK, you guys helped with the electrical problem. Now lets try brakes. I have a new double diaphram 8" booster/a corvette style master with a proportioning valve. Four wheel disc brakes with GM calipers. New brake pads. I have 18 lbs of vaccum at all times. Booster is holding vaccum. No fluid leaks any where. Every once in a while I hit the brakes and the pedal goes to the floor out of the blue. I thought air but. I have power bled the brakes until I'm blue in the face. I can drive for a week and no problem. Then when I least expect it. BAM no pedal. It seems when I do a sharp turn at slow speed it happens. We jacked up the car put it on stands and tested them. One time pedal next no. Blocked off booster and pedal hard as a rock. So, that seems ok? Can't understand it? Thanks :confused:

I'm going to give this a shot. An air bubble is going to give you a 'spongy' pedal, but is not going to cause pedal drop if they work part of the time. Since the rod from the pedal to the MC is essentially a solid link with diaphragms attached to transmit force you can rule out the booster from your search. Since you have no leaks, if the pedal goes to the floor without brake pressure it points to the MC allowing fluid pressure to recirculate back to both sides of the reservoir occasionally. If it were mine I would be changing the MC before it killed me. Am I off base, guys?

Is the proportioning valve meant for 4 whl discs? If it is then it could be the valve is letting fluid drain back which would give you no pedal.
Are there residual valves inline? that too can cause a problem without them due to drain back of fluid.

I would replace that master cylinder - those 'vette MC are just too cheap to risk your life, and since some are now made in a Far East country....... As far as a sharp turn, probably just a coincidence.

These MC range at Rock Auto from $20-$50 depending on the version (I looked at '72 'vette)

I have never seen a proportioning valve, or combination valve, that has an independent return for fluid back to the MC. They simply direct fluid, and thus pressure, proportionally and flow both ways. Likewise residual valves - just a pressure regulator with no alternate flow path. The MC creates the pressure in the lines to the front & rear brake lines, and the only path back to the reservoir is those same lines back through the return ports. If the pedal goes to the floor, it can only be a bad MC, unless I'm missing something. My guess is that you've been running on front brakes only for some time (seals on the rear spool bad), and now the front one is starting to fail, but sealing part of the time.

Replace the master ASAP! Whenever there is a lack of pressure, the master is failing. The MC is the pump in the system, no pressure = bad pump.

Shoot the one on the car is only 5 years old. I guess I had better order one ASAP! Thanks a bunch for the quick response and help!

Sorry I forgot. Yes it is a Disc/Disc valve no residuals but has another adjustable valve under car that you turn for rear brake stopping adj.

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Originally Posted by Steel Warrior

Sorry I forgot. Yes it is a Disc/Disc valve no residuals but has another adjustable valve under car that you turn for rear brake stopping adj.

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That's a proportioning valve.

Surprised you don't have RPV's

Is your MC on the firewall (higher than the wheel cylinders), or beneath the floor frame mounted? Just curious, relative to the need for residual valving.

X2, I kinda hope you'll say the master is under the floor.. If it is under the floor then you need residual valves (2 psi) or verify that the valves are in the master cylinder. Without the residual valves keeping the system full of fluid, that "possibly" could explain why you push the pedal once before you have pressure. Some masters have them built in. Call or e-mail Wilwood and get info on yours before you decide a course of action.

After re-reading your original post, with the info provided, I still feel it's the master cylinder. And I'm hedging a bet that the master has the residual valves built in, but you really must verify all this. Brakes are not something to micky mouse with.