Help! Self-applying brakes

I'm in the process of troubleshooting a brake issue and I'm hoping some of the bright minds at GRM might have some ideas I haven't considered.

1972 Jeep Commando originally had manual 4-wheel drums. I swapped in axles from a later ('76) CJ7 that have disc brakes in the front and drum brakes in the rear. I changed the master cylinder to a manual disc/drum master cylinder from the same year CJ (reman unit). I replaced the original distribution block with a combination valve (metering/proportioning) from a Grand Wagoneer with disc/drum brakes.

I am experiencing a problem where both front calipers end up stuck clamped down. The brake pedal firms up and pedal travel is reduced. The brakes seem to operate normally at first, then eventually lock down. In fact, I drove the Jeep for about 100 miles or so without the issue (or at least without noticing it), but it eventually became so bad that the front tires were locking up.

This doesn't seem to be a very common problem with brake systems so I haven't been able to find any silver bullets. Because the master cylinder is for manual brakes the pedal rod is part of the master cylinder. One thought is that the pedal travel isn't allowing the master cylinder piston to fully release.

The combination valve has two outputs for front brake lines and I currently have one plugged because there's a T in the line downstream. Not sure if that would cause issues or not.

I'm also not familiar with combination valves, but I've read that there's a relief valve built into some units that can factor into bleeding and I'm wondering if that could be a problem.

My next step might be to run the system without the combo valve to see if I can isolate the problem to the master cylinder or the combo valve.

I'd appreciate any ideas or insight you guys might have. Thanks.

First two thoughts: Are there any residual pressure valves left in the system? These are sometimes used on drum brake systems to reduce the "deadzone" of the pedal.

Second, sometimes damaged lines can act as a check valve, but if this is happening on all four wheels that's probably not it.

Edit: Whoops I see it's just the front...still, that would mean 2 soft lines would have to be bad in an unusual way.

I had a sticking caliper on the front of my 84 f250 that would do that. Drive normal for a while, then start not releasing pressure until you were smoking the pads. backing up would sometimes help unstick it. I just put a pair of remanned calipers on and it was fixed.

Didn't see in your post if you put on new calipers or if they are old.

May be the calipers or brake hoses. One way to check is when they lock up, open the bleeder screw, if the caliper releases, then you may have a bad hose. You can also loosen the line connections upstream of the caliper to see if there is pressure in the system. If there is it may be the combo valve or master cylinder.

I would run both front lines directly to the combo valve and eliminate the tee. It shouldn't cause any issues, but best to eliminate that variable.

Agreed on seeing rubber brake lines fail in that way - also on a Ford Pickup, a 78 F150 BTW - but two going out that way is odd. Not impossible, but odd. Ditto on calipers I'd look for leftover check valves, but then start looking at that combination valve. When an entire subsystem is having issues, the valve that creates that system is a good bet.

I had something like that once on a Dodge charger. Found out the "mechanic" who had replaced my front calipers used vice grips to clamp off the brake lines so the fluid wouldn't leak out. We had words. I learned to wrench on my own cars.

Nothing to add but post some pictures of the Commando.

I would also check the master cylinder to see if pressure is releasing there. Sometimes when a MC is just changed out the position of the push rod that connects to the brake pedal is to long and won't allow pressure to be released from the master. All it takes is to remove the MC and adjust the length of the push rod.

Thanks for the ideas guys. I think I'm going to go ahead and replace all the hard lines and get rid of the T. The hoses are all new and the calipers are new (remans). Not to say those parts aren't the problem, but less likely. The way it locks up both front wheels makes me think it's somewhere between the master cylinder and the front calipers.

Here's a picture of the Jeep. I was test fitting some 35" tires, but it's still driving around on smaller tires for now. 304 V8, 3-speed, Dana 18 (swapped in for the passenger offset rear diff), Dana 30 front w/ a TrueTrac, AMC 20 rear axle with a Detroit locker, 4.56 gears. Suspension is spring over axle with stock springs. Fun project vehicle that the kids enjoy. Scratches the V8, 1970's, convertible, and 4x4 itches all in one vehicle.

I'm with Jimbbski - if the master isn't adjusted properly, the bleed hole will be covered up. This means that expanding brake fluid has nowhere to go, so it starts to apply the brakes. The more heat you make, the more brake line pressure you get. This is in line with a long highway drive not causing a problem. I've had it happen on track, I barely made it into the pits.

My durango was doing that intermittently for a while turned out the shoe that the clipsr slide pins were dry and the calipers were keeping contact with the inside face of the caliper causing heat to build up and eventually build pressure. cleaned and regressed the glide pins and boots. Cleaned and greased the caliper support surfaces (all with hi temp brake grease) No problems since.

I'm also with Jimbbski and Keith (Keith's description is exactly what went through my head after reading the first post). Double check the length of the pushrod on the master cylinder.

theenico wrote: I'm also with Jimbbski and Keith (Keith's description is exactly what went through my head after reading the first post). Double check the length of the pushrod on the master cylinder.

I think I'll start here. A little tricky since the rod comes assembled to the MC. I'm sure there's a snap ring or something to allow its removal. Maybe the pedal assembly can be modified.

The heat part makes sense about the symptoms worsening over time.

Thanks again for the suggestions.

What's on the other end of the rod? Any sort of adjustment at the pedal?

If you can push the pistons back into the caliper, then the bleed hole is open (as it should be). If you can't, the hole is covered. A leftover residual pressure valve would add about 5psi of resistance.

RossD wrote: Nothing to add but post some pictures of you going Commando.

'Improved' that for ya

Small update. I put the Jeep up on jack stands and the front calipers were still clamped down. I then disconnected the master cylinder from the pedal. Presto! Front tires roll freely. Problem identified.

The rod is connected to the master cylinder and has a loop at the pedal end. The pedal's travel is limited by a small bar that contacts the actual pedal shaft. I removed some material on the pedal shaft and the stopper with a Dremel and that looks like it's going to work. That remedy probably sounds worse than it is. There's a lot of material there and I don't think the small amount of grinding did anything to compromise the pedal's strength.

Thanks again for the insight.

Glad I could help!

Good to hear!