A body disc brake conversion issue

I am having an issue with a disc brake conversion. I bought a complete 1973 Duster front disc brake setup from a guy that had it on his 68 Dart. I put everything on my project Dart along with a new master cylinder. After bleeding the system I can't get a tight pedal. After some investigating I found that if I hit the pedal once it will go to the floor but will get some gripping at the front brakes. Looking at the rear it takes two or three pumps for the rear wheel cylinders to expand. Does this sound like the wrong proportioning valve? I know they make one for the drum front and drum back, and another for disc front and drum back setup. It will be a pain to replace now with the motor in and limited space. Before I go out and buy a new valve I could use a little input from a brake expert or knowledgeable person. Thanks.

Power brakes I'll assume (lack of info)?, new master, did you set the proper air gap between the master and the booster pushrod?...

More info: This is a manual brake setup. I just called Inline Tube techs and they believe it is the master cylinder, which I said is new, The 69 to 74 A body brake setup used the same proportioning valve on both drum/drum or disc/drum and no distribution block. This is the same setup I have in the car. Again why would it take 2 or 3 pumps of the pedal for the rear wheels cylinders to expand.

Having to pump the pedal sounds like air in the lines.
Did you bench-bleed the master before installation? Are the bleeder screws at top of calipers?


Tim

are your rear shoes adjusted tight enough?

X2

could the rubber hose from the body to the rear end be going bad internally ?

X2 on rear shoes being in normal operating range

What brake fluid did you use?

Are the bleeders up?Thats sounds like the probem.Dont ask how I know.Its an easy mistake to mount the calipars with the bleeders pointing down.Rocky

All bleeders are up, master cylinder was bench bled, all lines and hoses new and no air in lines. I now took apart master cylinder. Question should all 4 seals face forward? The second one on the front piston is facing rearward, that is for the rear of the car.I used the bleeder hoses connected to the master cylinder to drain. As I was pumping the brake the larger rear reservoir drained in about 10 pumps and the front still had 60 percent of the fluid in it when the rear was empty. I am thinking the number two seal is backward.As you press on the pedal the front fluid will go rearward. Any input from someone who knows what's correct.

There is a separate block aft of the proportioning valve that you might not have gotten with it when you bought the parts that works the rear brakes.

No, one block only. Inline Tube told me only one used on 69 to 74 A bodies. I found an old manual and it looks like the seals (piston cups) are all in the right direction.I put the master cylinder back in the car, bleed the cylinder, then hook it back together. Bleed the system again, tighten the rear brake to the point of dragging. First time I hit the pedal after all this and it goes to the floor, hit it again and it tightens up about 2/3 down. Pump the brakes a few more times and it stays about the same. Let it set for one minute try again and it goes to the floor again. What the heck.

The pedal issue sounds like air still trapped in the system, are you using an "Inline" brand PV, they are known to be junk, how was fluid flow out the rears when bleeding?

I've had situations where there was trapped air in the front calipers, and no amount of pumping/bleeding was going to resolve the issue, so I would remove each caliper, starting at the passenger side, compress the piston fully with a C clamp, keeping the piston compressed with the clamp I would have an assistant gently pump the pedal/hold pressure then open the bleeder, rinse and repeat a few times, then remove the C clamp and install the caliper/pads and pump that caliper piston into position, then repeat for the drivers side, worth a shot

If you have a faulty PV valve and/or its closed not allowing the rears to bleed properly that's another issue

Sounds like a bad master to me. Plug the ports on the master, pedal should be rock hard.

It could be?, fluid blowing past the piston/seals, and it could be trapped air? often difficult issues, esp trying to troubleshoot via the net...

I would assume that all of the seals should be faceing forward & if one ain't then that is a first for me but with assembly line rebuilds it is certainly possible either from an assembler spacing off or pi$$ed off about something. I would definitely get that resolved however which way you do it then with it bled out & capped, the pedal should be (1) rock hard with (2) virtually no travel with (3) light pedal force & by (4) stomping on it. with the MC resolved then you can uncap/hookup the fronts & as said sometimes calipers can be a bear to bleed. sometimes having a helper vigoriously pumping the pedal then you with perfect timing opening the bleeder on his 4th downstroke will be enough to agitate the fluid & scoot the bubbles out of there & sometimes a caliper needs to be removed & the piston travel blocked & it bled while you hold it in a better position but I think the MC seal is the problem (for sure holler what it ends up being).

We have a winner. Of all the good suggestions and the process of elimination,the only thing that made sense was air somewhere in the system. Took Dayclona's suggestion and took off the front calipers off and clamped them all the way in. Then bled the system again, trapped air came out the right from line. We now have working brakes. Thanks all.

Good to hear!...sooner or later one of us would have a solution for you


Mike

I'm sure glad we didn't talk you into pulling the guts out of the MC!

I did pull the mc apart. Cleaned it out and reassembled it yesterday.That's why I eliminated that as a problem.

In normal operation the rear piston pushes the front piston hydraulically, that's why the rear seal on the front piston faces aft to face the pressure from the rear piston.

Only during a system failure does the rear piston physically contact the front piston.

John you're the man!