soft pedal

[GoMangoBoys]

I have bled the brakes, but still have a soft pedal.  Is there something with the proportioning valve or the metering block that I need to do to get them properly bled?  This is a 70 Challenger that we have converted from power drum to power disc front and drum rear.

[Bullitt-]

  Could be the rear brakes need adjusting tighter
On both my '73 after rebuilding the front disks & rear drums and another member's '70 who had recently paid for a complete brake job the pedal would go to the floor with the engine running, but seemed normal with engine off. (He had just driven 300+ miles when his problem appeared) I had discovered that my rear drum adjusters were not engaging and sure enough when I checked his weren't either. Correcting this & getting the rears adjusted properly solved both cars issue...

[GoMangoBoys]

I previously adjusted the rears until they drug the drum.

[Dakota]

One of several issues I had with a similar brake set up was having too big a gap between the end of the pushrod coming out of the booster and the cup in the back of the master cylinder. 

https://forum.e-bodies.org/wheels-tires-brakes-suspension-and-steering/12/71-340-cuda-brake-booster-rod-travel/21992/msg267864#msg267864

[Ramman]

It's been my experience that shoe and pushrod out of adjustment causes low pedal, not soft pedal. The only thing I've found to cause soft pedal is air in the system somewhere. Fluid is not compressible. Air, obviously is.

[GoMangoBoys]

So my pedal IS low I believe.  We converted this car to 4 speed from auto.  We had 2 different pedal assemblies.  I used the one that had the brake pedal that was the same length as the clutch pedal that I got from Brewers which was for a 70 Challenger.  I did a check where I cracked open the line on the MC while my wife pressed the pedal all the way down.  The pedal was not touching the floor or firewall anywhere.  This made me assume that the MC was getting pressed to the end of the stroke.

[GoMangoBoys]

He is the pic of my pedals.  The one with the tape measure is looking across the clutch pedal with the dummy end of the tape measure resting on the brake pedal.  This shows that there is about a 2" difference in the pedals.

[Bullitt-]

   I don't know about 2-inches but every Mopar I've seen from the era had the clutch pedal higher than the brake..

  From '73 service manual ..

[Ramman]

I did a check where I cracked open the line on the MC while my wife pressed the pedal all the way down.  The pedal was not touching the floor or firewall anywhere.  This made me assume that the MC was getting pressed to the end of the stroke.

That is what dual master cylinders do. They are actually two separate systems in one. If you had crack both lines the pedal would have no doubt went to the floor. Although that's not a good idea. It can damage the cylinder piston. If you have a failure in either front or rear brakes, you still can stop on the unaffected brakes.
Try this. Pump the brakes 3 times as fast as you can. Does the pedal come up? That's a classic sign your drum brakes need adjusted out more. What you're doing is " catching" the wheel cylinders before they have time to retract and hitting them again with the shoes partially out.

[GoMangoBoys]

At this point, I am now wondering if I have a bad master cylinder.  I unbolted it from the booster to adjust the rod.  I just moved it aside and left it hanging on the coiled brake lines.  There was a puddle of fluid in the front cavity of the booster behind the MC.  Maybe the MC is leaking out the back??? I bought this MC from a buddy.  Not sure if it was new or rebuilt?

[pschlosser]

At this point, I am now wondering if I have a bad master cylinder.

That was going to be my input.  I had a soft-pedal on an E-body that further declined over a few years to the pedal sinking to the floor at a stop light.  A master cylinder rebuild took care of it.

[Dakota]

I bought a new master cylinder from Dr Diff... ran about $105 including shipping.   This was almost exactly a year ago.

[GoMangoBoys]

So I replaced and bench bled the master cylinder.  The brakes are way better, but still not right. The brake light is on dimly.  Is there something that I need to do with the proportioning valve and distribution valve when bleeding the brakes?  I have read that there can be issues with that.  It may just need bled better.  I have been using the motive power bleeder that I borrowed from a buddy.  He says it works great.  My son is home today so I will try the old-fashioned push the pedal method.

[Bullitt-]

 I rarely hear anyone mention this but there are times you may need to hold open the proportioning valve while bleeding... They even had a tool to hold it open.



Some discussion here https://forum.e-bodies.org/wheels-tires-brakes-suspension-and-steering/12/tool-c-4121/13836/

[Dakota]

Will holding this valve open primarily help the rear brakes or will this also help get air out of the front lines too?