I have an issue with an inconsistant pedal on my 70 Cuda with a manual disc brake master cylinder.
This is what I have:
Factory 71 disc setup on the front from a 71 Charger, calipers overhauled and new hoses.
Original 10in brakes on rear with new wheel cylinders.
New replacment distribution block with disc brake hold of valve.
Manual disc brake master cylinder from Master Power Brakes with what I think is a 15/16 bore (they said this would reduce pedal pressure needed as I dont have a booster).
The problem I have is when the car has been sitting the pedal travels near to the floor on the first aplication then comes up for the second aplication.
I have bled the master cylinder and seem to get a nice bubble free stream of fluid at both front wheels when bleeding.
I have clamped of all three flexable hoses and the pedal stays high and consistant. I released the clamp on the rear brakes and the pedal stays consistant. I relesed the clamp on the flexable hoses to the front discs and the pedal drops!
When I purchased the master cylinder I noticed it had a residual line presure valve in the front disc side which I removed on advice from MPB as they said it would make the front brakes drag.
Any advice on how to rectify the problem would be gretly apreciated.
Thanks Tim

on the mopar master the front line goes to the rear brakes---the rear line goes to the front brakes if you have rear drums they have to have the residual valve----if the residual is gone on drum brakes you have to pump them up every time---bob

The front of the master is feeding the rear brakes and still has a residual valve in it, the rear of the master feeds the fronts and has the residual valve removed. The rears are not an issue the problem is in the fronts.

if the brake pedal/rod to master isnt adjusted to release all the way it can be a problem----other than that no other ideas---bob

There is a possibility of air still being trapped in one of the front calipers, I've had this happen on a number of occasions, some internal castings can hold air pockets,...what I do is remove the caliper from the rotor, remove the pads, use a large C clamp and compress the calipers piston, bottom it out, now have an assistant carefully pedal bleed the system as you normally would, as you bleed the caliper (no need for hard pumping) except leave the C clamp compressing the piston all the time during the bleeding process, do each caliper one at a time, then refill the master to the proper level....you won't damage the caliper doing this, just eliminate any air trapped, you do have the calipers mounted correctly lEft/right, front or rear mount? with the bleeders as high as possible

I was suspicious there may still be air trapped in the front and have bleed them three times now, however I will give you method a go and see what happens! Thanks

Update, i finally had time to rebleed the brakes with the calipers clamped and the bleeder at 12 oclock, however this has not fixed the problem.
I spoke with someone at the local brake shop who said I may have an issue with master cylinder volume as it only has a 15/16 bore size and it may not be providing enough fluid volume to the calipers on the first aplication and this is why the pedal goes down on the first aplication and the comes up on the second.
On further investigation I have found the factory manual disc cars had a m/c bore size of 1 1/32.
Could this be my problem?

I have had problems with the master cylinder in my Duster. I tried a master Power non power M/C that measured almost 1 3/16". THAT dang thing was terrible! Extremely hard pedal with stopping ability equal to putting my bare foot out of the door. I have read from many members here that suggest either a 7/8" or 15/16" bore unit for non power applications. Anything more than that results in high effort and poor stopping. To really drive this matter home, PM Doctor Diff or The Ram Man. They are respected members here.
Jeff

anyone?

I did this same thing to my 71. But I used a MC from Car Quest that was made for a 71 with power disc brakes. Both of my cars have power brakes even with heathly cams.

I used a Ebay used propotioning valve for a 71. My system works great, it has 11" drums in the back.

I know you don't want power brakes, this is the part number I used on my cars. I sold a number of these on E bay years ago and would buy 6 at time and resell them.

I've read here that there has been problems with new propotioning valves, like I said I used used valves and they work fine with no leaks.

OK Mis-adjusteed rear brakes can cause what you are experiencing but from what I have read it sounds like you have narrowed it to the front brakes.

To me it sounds like the caliper/piston may be retracting and the first push resets it. The second push is solid because it is in place.

If it is retracting it can be caused by the seals placing tension on it, The rubber bushings on the caliper pins or the anti-rattle springs.

Here is what I would do.

Place the car on jacks with the tires off and about level. Pump the brakes so the pedal is tight. Release the pedal and check for space between the piston and the pad, the caliper and the pad and the pads & the rotor. You should only need a feeler guage to check.
Check both sides.

Wait until the pedal would be soft and re-check the gaps. If the gaps are opening up something is causing the piston to retract.

A residual check valve would fix it but start by greasing the bushings, then removing the anti rattle clips (if it has them) and check for tension on the piston seal.

Hope it helps.

Why don't you put the residual valve back in the MC and see if it helps or hurts your situation?

It sounds like you need it?

did you bench bleed the master b4 you installed it in the car?

I do have the anti rattle clips installed and the rubber bushes are greased.
Just after the front brakes are apllied the front wheels seem to need a bit of force to turn by hand. After a minute or two the wheels are much easier to turn and you can just here the pads draging on the rotors slightly.

Update, I have gone up to a 1 1/32 master cylinder and I now have a consistant pedal!