70 Cuda front disc brakes dragging

[MR291]

I have replaced my entire brake system from front to back and now have stock power front disc with drums on the back. When turning the both front wheels (when off the ground) I have a lot of resistance on both sides and when driving there is a noticeable rolling resistance. Car will not idle forward on its own and I have to give it a fair bit of gas to get it going. To add to this it is not permanent. It does this most of the time but not all or least not enough for me to notice so I believe that it is not the push rod. Could this be a residual pressure valve issue? I have read that there should not be a valve installed for the disc brakes; just for the rear drums.

Appreciate it.

[1 Wild R/T]

There shouldn't be residual pressure on the front brakes.....  Ok, not exactly true, 2 psi is normal.... Drum brakes typically see 10 psi residual pressure...  If the issue is residual pressure cracking the bleeder should free up the wheel... FWIW slight drag on disc brakes is normal, particularly on single piston designs... If you can grab the wheel & spin it and the wheel spins more than two revolutions chances are it's normal...

[tman]

Did you change out the master cylinder also?  Do you have the correct one?  Power drum mc will have residual valves for fr and rear.  If you do have the correct mc, push rod adjustment may be warranted.  Could be pressing on the cylinder causing partial engagement.  Front wheels should be pretty ez to spin when off the ground.

[MR291]

I will try the push rod. I get no revolutions at all from the wheels so something is locking them up. What stumps me is that it is not all the time. Thanks.

[1 Wild R/T]

Before messing with the pushrod crack the bleeder... If it has no effect the problem isn't hydraulic it's mechanical....

[MR291]

I raised the car this morning to have a look at the brakes and the wheels spin freely, however once I start the car and pump the brakes the wheels are stiff. I then cracked the bleeder valve and the wheels spin freely. I assume then that this is not a mechanical caliper issue. I also have the correct disc/drum MC so that is not an issue. I am thinking one of the following: there is a residual pressure valve in the disc brake portion of the MC (which there shouldn't be), the push rod is not adjusted correctly or perhaps the metering valve is not working (car has the two piece proportional valve). Any suggestions as to where i should look first or other than those listed. Thanks.

[1 Wild R/T]

loosen the nuts mounting the M/C to the booster a couple turns then repeat your test for free spinning wheels after applying the brakes.... 

[solarguy]

I had the same thing happen when I overhauled my brakes some years ago.  The metering valve was plugged with junk and not allowing pressure to come off once the brakes were applied.  I ended up replacing the whole proportioning valve with one from an junkyard.  This was in the days before repops were available and I was not able to find any rebuild kits or services to rebuild the old one.  Today, there is a kit available as well as reproduction valves

Some good info here

https://www.musclecarresearch.com/1970-76-mopar-combo-valve-autopsy

and

http://piratejack.net/proportioning-valves/

[1 Wild R/T]

Valid point.... A quick way to see if it's the master or one of the valves, step on the brake pedal then crack a line loose first at the master, if the wheel is still tight clearly the problem is further down the fluid path.... Tighten that fitting & go to the fitting just past the proportioning valve..

[MR291]

I cracked the line on both sides of the metering valve and the proportioning. Each one relieves the pressure on the wheels. So that rules out the valves. I cracked it at the master and that relieved the pressure. Thinking that maybe it was the master I replaced it. No change. I loosened the master from the booster and that did not make a difference. I checked the booster push rod and it is adjusted all the way in. So in my mind either the booster rod is too long, or there is something mechanical in the pedal system that is not pulling back. Any thoughts. 

[AARuFAST]

You didn't mention the brake pedal.
Is it hard to apply and short stroke.
is it soft and deep applying?

Do u have a brake booster.
Booster to master cyl. rod needs adjusted.

[MR291]

I have a booster and the rod is adjusted all the way in (away from mc). Pedal tends to be soft.

[1 Wild R/T]

I loosened the master from the booster and that did not make a difference. I checked the booster push rod and it is adjusted all the way in. So in my mind either the booster rod is too long, or there is something mechanical in the pedal system that is not pulling back. Any thoughts.

Loosening the master from the booster means the booster & pedal linkage is eliminated....  It still sounds like a master cylinder issue to me... Like your being given a drum brake master....