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Tech Question
Terry Duris, Pataskala, OH, 1971 Dodge Challenger 440
A previous owner converted my '71 to front discs using '73 parts. The car was not drivable when I bought it. I have replaced both calipers, rotors, pads and master from AutoZone. The problem is the pedal can be pushed to the floor with little resistance. I am convinced there is no air as I have bled them several times even unbolting the calipers and fitting a block of wood in them and holding them at different positions while bleeding. ( I know the bleeder should point up). Could it be the proportioning valve? Should I try a master with a bigger bore? AutoZone catalog shows a 1-1/32" bore as correct. A 1-1/8" was used on a '71 Hemi. If I pinch off the front hoses the pedal is hard.
Thanks, Terry
Terry-
Before I read totally through your letter, I was sure you had failed during the master cylinder "bench"-bleeding procedure (which is actually easier with the MC bolted up.) If that happens, or the master is allowed to go dry during the wheel-bleed procedure, you're dead until you re-bleed the master.
Typically, I recommend blocking the ports on the master with SAE bolts (snugged, don't go crazy w/torque) and confirming that the pedal is ROCK hard with almost zero travel. If not, the master is either airbound or simply bad (internal leak). Your hose-pinch method seems to have accomplished the same thing, however, with so many reman master cylinders bad (I'd call the defect rate maybe 50%), I'd still like you to try that. You can use the bolts as bleeder screws. I'd be curious to hear what happens with only ONE hose pinched -- this is a good idea for learning if the problem is only one side, or both.
Your test has, however, proved what I already guessed: Everything, and I mean everything, is good. A little explanation: Brake system valves cannot fail except to seize up and/or leak externally. Either way, the pedal will still be firm. And master cylinder bore has no affect on trapped air.
Try this wheel bleeding procedure: First, yes, be sure the bleeder screws are facing up. (I really hope the calipers are front hung and you don't have the taller '73 steering knuckles, which triple the bump-steer, but that's another story). Connect a rubber hose, about 24-30" long, sized to be a snug fit over the bleeder screw's nipple. On the other end of the hose, find a nut, preferably brass, that will just screw over the end of the hose. Drop the hose/nut into a clean coke bottle (clear) that has at least an inch or two of clean brake fluid in it.
Bleed each wheel, starting from the farthest (RR) (you could probably skip the rears, since you seem to have proven that they are OK), this way:
Having the car raised some helps. Using your right hand, pump the brake pedal to the floor, while resting your body on your left hand (on the floor) and looking at the Coke bottle. You'll see bubbles. Hold the pedal at the floor until you see the bubbles stop AND DISAPPEAR. Repeat as necessary until there's no more bubbles. Each time, only permit the pedal to come up SLOWLY to the returned position.
Fixed.
Rick
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