front disc brakes

The 2 lb residual pressure valve in my front disc brakes works too good. It holds the brakes on. the 10 lb valve for the rear drums is fine. Should I try removing the valve to the fronts?

 

I believe so. The rear section of the master feeds the front brakes. The po has installed disc with the dana 30 upgrade and there is a red in line 2 lb valve on the 1/4" line going to the front just before the tee where the line size drops to 3/16 and goes to each side. The master cylinder is clearly marked and I removed the residual pressure valve from it.

 

You may not have enough "free play" in the pedal. Add a little at the pedal to piston adjustment. I had a similar problem. The piston wasn't returning far enough to allow residual pressure to drop.

 

If the piston travel is a problem, which is possible as the connection of the master cylinder to the push rod required me to remove the c clip and install the end of the push rod in and back it up with a washer then reinstall the c clip. this is how the old one was held in place but it may not be right. Having said this if the piston travel was an issue would the back drum brakes be affected as well? They seem to be fine

 

Not necessarily. Remember drum brakes have springs to help the shoes retract and push the pistons back in the cylinders. Disc brakes rely on the seal shape to pull the pistons back slightly for running clearance. If there is too much pressure held in the hydraulic system for disc brakes the seal distortion can't pull the piston back and so will hold the brakes applied. Also, drum brakes typically have a larger running clearance so the issue may not be so pronounced on drum brakes.

 

Today as an experiment I am going to release pressure where the front brake line attaches to the master cylinder without the petal depressed . This should release the brakes if the master cylinder is at fault. I already know that I can release the brakes at the wheel cylinder bleeder valves that are on the b rake side of the 2 lb valve. Hopefully I do not have to remove the master cylinder and fabricate a new push rod , as it is boxed in by the brake lines

 

Well you were right , once I relieved pressure at the cylinder connection the front disc released. now the big question... the push rod has a mushroom head on it and does not extend into the hollow of the piston on the cylinder which is approx an inch deep. do I cut off the mushroom head and weld an extension on so that the rod extends all the way into the hollow of the piston. Currently the c clip with a retainer washer held the push rod in place , but that combination did not let the cylinder travel far enough back to open the return ports. without an extension the push rod will not stay in place.

 

Can you adjust the pushrod? I know on the F-head Jeeps they are adjustable, I can't remember on the V-6 models. Also make sure the pedal isn't binding and coming all the way back.

 

No pedal binding, no adjustments available. I need to be able to secure the push rod into the master cylinder so it doesn't fall out. The mushroom head with the ball on the tip was held in by the retainer clip and a washer on the old system. This doesn't work with the new master cylinder as it can not fully release with this behind the retainer clip.

 

the problem with my existing push rod is that it does not go into the cavity of the piston on the master cylinder there is a mushroom end with a ball on the tip , the only thing that would hold it in place is the rubber boot unless it is held with a retainer washer by the c clip., If I grind off the mushroom tip and weld an extension of the rod then it would be far enough inside the piston not to fall out. Is this feasible?

 

One thing to think about here. The pushrod doesn't push perfectly straight. It move in somewhat of an arc as the pedal moves forward as the pedal and mounting point for the pushrod moves in an arc. If there isn't enough room for this then the pedal won't move freely and could jam up. That's probably why they put a "mushroom" at the end of the pushrod. To allow it to pivot slightly on the m/c piston. Can you grind down the back side of the mushroom a little to allow the piston to come back all the way? Sounds like the piston in the m/c isn't clearanced correctly.

 

You are right. It doesn't travel straight. It needs more thought before any cutting or grinding is done

 

I remember a thread a few days/weeks ago that was eerily familiar to this one. I believe that the final solution was found when someone (Timgr?) suggested putting small spacers/washers between the mc and the bracket which holds it. That would effectively be the same thing as taking a small amount off the pushrod.

 

I wish it were that easy. The push rod in this case is directly attached to the master cylinder and held in place by a c clip. the pedal travel is not an issue. I reversed the cone shaped washer that the c clip retains to gain a little on the piston- rod connection. I will try this and report back