Hi guys,

OK. I read all the forum advice and I actually have worked a lot with brakes (drum systems). However I have a problem I haven’t encountered before because I did something new to me and it has complications. Just checking to see if y’all think I got a bad, brand new bad master cylinder or if I have a proportioning valve problem.

I put in an all new brake system that now has been vacuum bled by me, pressure bled, helper bled with me watching, bled with new speed bleeder screws finally installed, then bled with those screws in and me watching with a helper pushing. Each time there was air in lines. Each time I bench bled master cylinder (5 times and had air each time in rear port). Each time the master cylinder would have a rock hard stroke stop on the bench.

Finally today I think maybe I got the air is out of lines and wheel cylinders and calipers, but with screws closed the brake pedal went halfway to floor to start nd will leak down a bit on steady application. So, if this classic sign of master failure is so obvious, what is the question?

Well, I was about to conclude I had a bad master cylinder but didn't want to do so...because it is brand spanking new and I worked hard on all the re-works! You should know I did the Uglytruckling.com / Ray at Helitools front disc conversion .I added in the Mopar proportioning valve at same time to proportion fluid pressure to a brand new Mopar 10 inch drum setup in rear.. I had to change pushrod and drill a new hole in the pedal arm to get the right stroke for a 2 compartment master wich the new one is. I am getting full pushrod stroke up and down with self bleeders open, so pedal stroke length is ok. The small port on the cylinder front feed the front brakes; the large port on cylinder rear feeds rear brakes. Pumping pedal one time pumps up brakes good and vacuum at 18.5at idle inches works booster just fine. Unfortunately the old classic foot on pedal leaks down a bit after a few seconds, but not to floor.

Upon mashing the pedal I have usable brakes at rear, but the disc calipers mash pads too tight for the front rotors to turn and won't release. This confuses me. I don’t know if the front piston in the master is not coming back enough to release pressure (even though the pushrod is all the way out) or if a caliper piston is sticking. I pulled calipers and pads and measured piston movement with a digital caliper and found maybe .130 movement out with no pads in place. I can squeeze these pistons in with a C-clamp until calipers will slip right on so that small movement may be enough, if the piston is retracting. Mopar FSM says clearance between pad and rotor is to be .005 or less, so I see we are dealing in less movement here than with drum systems and thus don't know how much a piston should move out when the pedal is depressed to floor with no pads in place.

I was debating next to remove the proportioning valve and see if it was the problem, but it seems to make no difference whether the valve is adjusted to provide pressure all way to rear or all way to front discs. Still, the cylinder leakdown spells BAD NEW CYLINDER in my previous experience, which again is only with drum systems. I just can't see fluid leaking from the rear of the master cylinder at any time. I don’t know if there is a relationship between caliper piston retraction and proportioning valve, or if the failure to release pad pressure on the front rotors is just more evidence of a bad dual port master cylinder wit only an internal leak of some sort. Would y’all change the master or pull the valve next? Or both? And of course it is 92 degrees, humid and skeeters are out. What a #@@&!!*.

rock
'64d100

Don't know if this is causing your problem, and don't know what kind of master cylinder you used, but in almost all cases, the rear portion of the master feeds the front disc brakes, while the front portion of the master feeds the rear drum brakes. The rear "half" of the master for disc brake systems has a larger fluid reservoir and the passages are differently calibrated as far as location and size, since disc brakes require more fluid displacement over their lifespan.
In that case, it just might be a front/rear reversal at the master.
This also suggests front/rear reversal at the master.

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