Rear Disc problems

I swapped in a Scout 44 rear with a disc brake conversion. It uses 79 cj front rotors, caddilac calipers and pads, and the bracket is from TSM. My problem is no matter what I do I cant get them to work. It will work fine for a bit until I have hit the brakes a couple of times and then I get a brake drag bad enough that I don't even need to push them any more. After it sits for as little as 10 minutes its good to go again. Im ready to bag it and go back to drums, I have tried drilling the hole on the master to be bigger I bypassed the Dist block, and have bleed all the lines. Any ideas if Im missing something please give me your input. TH company from which the kit came said to pull out the residual valves which I figure we took care of by drilling the hole on the master, your supposed to be able to put a screw in there and pull it out but nothing comes out so we drilled it, still to no avail.

 

Sorry I was unclear, the break pedal always has great pressure and works the first time I push it, what I meant was after a few times braking or a few stops the pedal gets more stiff and the brakes begin to drag badly. I have not put in a proportioning valve because I went from four wheel drum to four wheel disc. I even bypassed the stock proportioning valve so its a straight line and I am still getting pressure building up.

 

it sure sounds like a residual valve to me holding back pressure try unhooking line to rear brakes and look in port and see if ball is still in place also on some master cylinders it is not removable even by drilling

 

You can remove the MC residual pressure valve easly.
First remove the hard lines to the MC
Stick a self taping screw into the the hole in the center of the tube seat on the outlet ports.
With a pair of plyers and a screwdriver pry pull the tube seat out, the residual pressure valve is that rubber check valve with the spring behind it.
Then reassemble with out the valve.

 

OK I will give it another shot today and see what comes out. Also should it be fairly difficult to pull that line seat out, I tried once before but I didn't want to ruin it. I'll try again.

 

A 1976 model shouldn't have residual pressure valves. You only use them when the MC is lower than the brakes or at the same level. His is a firewall mounted MC.

 

This is gonna sound stupid but check your fluid level. If it's too full then when it expands (heats up after driving) then it will apply the brakes. I'd still look and see if there is a residual pressure valve. The purpose of the residual pressure valve is to hold pressure on the drum brake wheel cylinder seals to keep them expanded in the bore so fluid doesn't leak past the seals. Some vehicles don't use them because they use an expander in the wheel cylinder to hold pressure on the lips of the seal. Many vehicles do use them because they don't have an expander in the wheel cylinder, regardless of the position of the master cylinder. We're covering this right now in brakes class......
Not saying position of the master cylinder doesn't play a role in this, but it depends on the vehicle...

 

The cadilac calipers I think have self adjusters that may cause problems or it mite be in the parking brakes system. I think I read about it in my research for using the same setup. Just a thought!

 

Could an exhaust manifold/pipe too near a brake line cause this?

 

Yup. If it's heating up the fluid the fluid will expand in the line and do the same thing. Nickmil

 

I used El dorado rear calipers on mine. I got Napa rebuilts. The only time they adjust is when e brakes are set. Do you have a slight amount of clearence on the master cyl. pushrod. ?
Larry

 

This is important. I had mine too tight and experienced the same problem with the brakes dragging. The MC piston must be allowed to return all the way when you release the pedal. Apparently that allows the pressure to bleed off in the MC.

 

I think that is what I am down to, so how do I check the position of the master cylinder piston to see if it is retracting all the way?

 

This is my understanding of it.

Get under the dash and take the rod off the pedal.
Then you can see where it needs to be with just a little slack.
if you have to push it in against the M/C to get it on the pedal.
then you have no slack.
With pedal all the way out against its stop.
Adjust the rod length so it fits on with a bit of slack between pedal and M/C.
Make sure to use a spring on the pedal so you have the slack and it wont rattle around.