Lack of consistent brake pressure

[richinwaco]

Hey guys I'm hoping someone out there has experienced what I'm experiencing and can give me some advice.  I've got a 70 Chally with manual brakes and 4 wheel disc.  The mc, proportioning valve, and rear calipers came from Dr Diff.  Front brakes are stock disc that I got from Rock Auto.  I redid the lines as part of the rebuild.  It's a brand new system on a car that doesn't run or drive yet. 
My problem is that I cannot get consistent pressure on the pedal.  First pump goes to the floor even after power bleeding.  If I pump it 4 or 5 times I get good pressure, but when I stop for 15 seconds and try again it goes to the floor.  If I pinch the left front hose I get good pressure on the first push.  If I pinch the right front hose I get mediocre pressure. 
I've used a power bleeder to bleed the system, I've plugged the mc and tested it, I've replaced the left front caliper and hose.  Both calipers work great when there's pressure in the system.
I can't think of anything else to replace or test.  Any ideas?

[mopar jack]

Make sure the bleeders are in the top position on the calipers.

[anlauto]

Did you bleed the master cylinder first before hooking up any lines ?

[richinwaco]

Bleeders are on top and I bench bled the mc after the first round of bleeding. 

[anlauto]

If you have some time, try gravity bleeding the whole system by opening up all four bleeders at the same time and let it sit. Keep an eye on it and when fluid start coming out close off the bleeder.

It sounds as if you might have a faulty master cylinder

[richinwaco]

I thought it might be the master cylinder too but it seemed odd that I got good pressure when pinching the left front line but not the right front line.  And I got good pressure when I plugged it completely.  I thought there might be air in it, that the caliper was bad, and the hose was bad.  But none of that seems to be the case.

[anlauto]

How about the proportioning valve ?

[Skunkworks Challenger]

I'm guessing a faulty master cylinder but only if you didn't let the mc go dry during the bleeding procedure

[Oldschool]

I was helping a friend of mine with his '71 Chevelle with a 572 BB. He used aftermarket brake parts along with stock pedal assembly. We had the exact same symptoms on his car. After doing all the normal things, rebleed lines, calipers, MC, and make sure bleeders were on top. We ordered another MC and installed it with the same results.  After much head scratching, we discovered that the MC pushrod was just a tick short. It would pump up after several strokes because even the short pushrod would eventually move the diaphragm enough to build pressure, but it would fade as soon as it sat for a minute. When we replaced the short rod with a longer one, the brakes worked perfectly. Just a thought when dealing with a blend of aftermarket and stock parts.....   

[richinwaco]

Interesting.  The pushrod is still stock but the mc is aftermarket.  It'd sure be a lot easier to try a longer one than another mc.

[Oldschool]

Interesting.  The pushrod is still stock but the mc is aftermarket.  It'd sure be a lot easier to try a longer one than another mc.

Let me know it this helps....   

[richinwaco]

Unfortunately the longer rod didn't work.  However the longer rod enabled me to discover that the pedal wasn't actually going all the way to the floor, it was going to the end of the mc.  Also, when I add 15 lbs of pressure to the brake system using my power bleeder, the pedal pressure is good.
So after replacing everything I can think of it seems as though the mc is the problem.  It holds good pedal pressure when it is plugged, but doesn't generate enough pressure when it is hooked up to the brake system.
Am I correct?  What do you guys think?  Everything I've done so far is leaving me with the same result.

[HP_Cuda]

This might sound crazy but replace the power booster block off if you have one that has 2 inlets for manifold pressure. My guess is one is for manifold pressure and the other is so you can test the amount of vacuum getting to the booster.

The guy I bought one of my cars decided to electrical tape one when it developed a crack. You really couldn't tell either unless you looked at it close, replaced it and whammo had instant pressure again.

[richinwaco]

Its a manual system, no power assist.  That's what is baffling me, it shouldn't be this complicated!

[richinwaco]

Hey guys I'm still fighting this brake issue.  I got another m/c, bench bled it, installed it, bled 3 quarts of brake fluid with a power bleeder, and am getting the same result.  If I pinch the rubber hose on the front left caliper I get good pressure.  After replacing the m/c AND caliper, I'm a little lost as to what to do next.  I don't want to replace more parts that don't need to be replaced.  Any advice would be appreciated.