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Old 03-19-2006, 06:09 PM
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Very strange brake issue. Help!

On my Nova (Blower car) i have the large booster and disc/disc master with the standard brass comb valve, 4 wheel discs, Caddya e-brake rears and e-brake is adjusted to where I can just feel the pads moving slightly. Bench bled the master very good and bled the entire system sucessfully, bleeders at the top. Pulling vacum from the carb plate below the carbs (not measured).

With the engine off i have great pedal. Holding pressure, start the car and the pedal goes immediately away, to the floor. If I pump it twice it stays almost on the floor but I have enough brake to stop the car from moving, even giving part throttle it will hold. BUT - it seems to be locked up when trying to back the car up. Put the car in the air, same thing, but given some throttle it breaks loose and is fine, except when applying the brakes gently I can feel/hear a little chatter thing going on until rotation stops, in reverse only.

So, anyone know where my pedal is going? When I shut the car off one pump and I have normal, decent pedal.

I'm tearing my hair out, and I don't have much to begin with, any ideas guys?

Larry

Last edited by 68NovaSS; 03-19-2006 at 06:14 PM.
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Old 03-19-2006, 07:06 PM
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Almost sounds like the rod from the pedal to the booster is too short. With the motor off, your brake pedal should go down about 1" and drop about 2" more when the motor is started.
Hopefully a good brake guy will jump in here.
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Old 03-20-2006, 08:53 AM
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I would also suspect a pushrod problem. Check the booster-to-master rod for correct length. Too short could definitely cause the drop symptom.
Check the booster to pedal rod too. You should have an angled booster on there, so make sure its rod is in the lower hole on the pedal. This will give it the correct ratio as well as the correct angle to keep it from binding.
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Old 03-20-2006, 10:47 PM
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Booster pin depth

Let me as this, are disc/disc master cylinders shallow or deep socket for the booster pin? Mine is about 1 3/4" deep. If I have the correct length pin, can it be a Disc/Disc master?

I've heard rumors it should be a shallow depth piston for discs...
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Old 03-20-2006, 11:19 PM
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It can be either. It depends on make/model it came from, or which aftermarket manufacturer made it. The early Corvette disc/disc masters had the deep hole.

The rod should be sticking out of the booster quite a bit with that master. You can check the rod-to-master clearence with a small piece of modeling clay on the end of the rod, and test fit the master. There should be slight clearence... maybe 1/16"... not more than 1/8" I would think. I dont know the exact spec.
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Old 03-21-2006, 12:06 AM
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A couple of other checks, if the booster has been overhauled and fitted with a less elastic aftermarket diaphragm (or one 'close enough') this will occur. Same if the booster has been chromed - can cause the diapragm to bind.
When the brakes bind pull out the residual (vacuum) valve, if they free up its the booster or pushrod adjustment. If not, crack the front, then rear bleeders to isolate the problem. Proportioning valve? Maybe the 'disc/disc' m/cyl got some one way valves by mistake.
Have fun, snow melting yet?
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