Front Disc Brakes Lock Up

I recently upgraded my 1974 CJ5 with front disc brakes. They work well, except for one problem that I haven't been able to figure out. The pressure in the front brake lines seems to build up after repeated braking while driving until the front brakes are "dragging" almost to the point of being locked up. Oddly, when I am stationary and repeatedly pump the pedal the pressure does not build up. I suspect the problem has something to do with the Combination Valve or the Master Cylinder.

I do not think that the calipers are sticking, or that the new rubber brake hoses are damaged inside because if I release some pressure by backing off the plug for the Metering Valve on the Combination Valve until a small amount of brake fluid drips out then the brakes release and are back to "normal" until it happens again.

Does anyone have any suggestions for this problem?

 

Are your lines reversed ?

 

No. When I first got the Jeep back from the mechanic who did the install the lines WERE reversed. A friend told me that the line coming from the larger reservoir on the MC is supposed to go to the disc brakes. So I replumbed them that way and thought the problem was solved. It wasn’t. But that makes me think the MC is wrong. In the FSM picture of the MC the larger reservoir is the one farthest from the firewall and the smaller reservoir is the one closest to the firewall. My MC is the reverse of that. Could that be the problem? Did I get the wrong MC?

 

Check the pedal free play. If you do not have sufficient free play, the master cylinder may not be fully releasing.

 

^ Agree.

I can’t remember how tight it is under there, but next time it happens, chock the wheels and see if you can pull the pin on the plunger at the pedal and slide it off. If releasing that solved the issue, you found the problem. The plunger needs to go all the way back to its home position to release any built up pressure.

 

Did you remove the check valve in the front brake outlet on the MC? If that's a drum MC it will keep slight pressure on the front brakes causing them to drag.

 

If the a MC has a Larger reservoir, it is always for the fronts. Some of them are equal size , but the front is the front . You have the wrong MC, I believe. Pirate Jack is a great place for these.
Everyone that responded here, has good tips to look for , but I would make a phone call & get a dual GM MC: tell them you have front discs / rear drums .
Make sure you bench bleed the new MC, before installing.

 

I had this exact problem on my rear disk brakes. There was a misalignment that caused a pad to drag. This overheated the fluid in the caliper and locked them up. If I cracked the bleeder, fluid would shoot out and release. When I fixed this misalignment all my issues went away.. So, It could be pedal free play, misalignment of the caliper, too much residual pressure due to proportioning valve or something else.

 

Thanks for that info. I have checked the pedal and linkage and that all seems good. I will look to see if there is a check valve in the MC outlet port. That makes the most sense. Except that I can't "pump" the brakes when the jeep is parked and get a hard pedal that doesn't release. I think that I would be able to do that if a check valve was the problem. The dragging and "lock up" only seems to happen while driving. I have read several times the description of how the combination valve works and it seems pretty confusing. Could there be a malfunction in the combination valve that would cause this problem?

 

Thanks. That's a good lead. I will check it out.

 

I had a similar problem when I upgraded to discs. Pads were dragging causing the fluid to heat up and lock up the brakes. I upgraded the MC but used the old pushrod, they weren't compatible and caused the MC to not release all the way, difference of about a 1/16".