Brake lock-up

Anybody know why my front brakes would apply itself and not let off until I bleed and relieve pressure?
It happened while I was flat towing it to the trails this past weekend. Also happened all throughout the trail ride. Replaced the master cylinder twice.
88 XJ Power Brakes non-ABS

Wow! I guess no one ever had this problem before

brake caliper froze up?

There are only two reasons a brake will stick on a disc brake:
1. piston sticking in the bore of the caliper
2. caliper sticking on the slider pins that attach it to the steering knuckle.

The easiest thing to do here is to get an assistant, and have him/her press the pedal while you observe caliper motion against the disc with the wheel removed. If the piston moves, but the caliper does not, then you have a slider pin problems. If the piston is NOT moving, then it's caliper rebuild or replace time.

AZ Jeff- Well what's wierd is that the brakes was fine before I hooked it up to flat tow. They weren't sticking or anything. It seems like pressure was somehow built up which inturn applied the brakes while I was flat towing. It happened many times on the trails too. The brakes would be fine then I would feel the XJ struggling so I would let off the gas and come to a complete stop without even applying the brakes. I would have to get out and open the relief vlave on back of the caliper and have my wife pump the brakes twice to let off pressure on both sides. Then the brakes would feel normal. What's happening is it's somehow building up pressure in the system. This thing has got me stumped. When we replaced the master cylinder we adjusted the push rod on the booster and bench bled the master cylinder and bled the lines at each corner. Can it be the proportioning valve?

I am going to ask a dumb question: are you SURE that its' the front brakes that are sticking?

I ask because the rear (Drum) brakes virtually all cars are called "self energizing", which means that the rotation of the wheel actually pulls the shoe tighter to the drum as the wheel cylinder moves under pedal pressure. Thus, it IS possible to get a rear brake "stuck" whereby only rearward motion will release the primary shoe.

Other than what I described earlier, I cannot see how a caliper can stick.

One other thought--are you sure the pedal is not moving downward prior to the brake "sticking"? I am thinking maybe you have some sort of weird power brake booster leakage that is applying pedal autonomously.

Just some thoughts......

AZ Jeff said:

I am going to ask a dumb question: are you SURE that its' the front brakes that are sticking?

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Yes, i'm sure it's the fronts. Both sides. I know this cause when I had to pull over from flat towing it the rim was hot as hek and smelling of burnt brake pads. Also, on the trails when I hit a water crossing they would steam up and sizzle

OK. Sounds pretty convincing. How about the premise that the booster is applying itself somehow? Maybe you can check that theory.

XJ4PINOY said:

... When we replaced the master cylinder we adjusted the push rod on the booster ...

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Without looking very far into this, I'd immediately look at the "pushrod on the booster" and it's adjustment... it may be just enough to hold pressure on...

how'd you do it and how often have you done it before?

I agree with Satan. There is something in the booster/MC that is causing the brake system to stay partially applied.

O'kay, now that I adjusted that push rod out on the booster I need to shorten it and put it back within specs. Anybody happen to know how far out it should extend? My FSM doesn't say anything about the breakdown of the brake booster Help

XJ4, take a straight edge and measure how far the pushrod sticks out from the booster front mounting edge. (Make sure the rod is fully seated to the rear). Then measure the depth where the pushrod fits into the MC bore. (I use a phillips screwdriver to do the measuring). THen allow for the MC flange that fits into the booster. The resulting measurments should equal each other.

If reading this makes no sense, don't worry. It will when you have the master cylinder off and can look at it.

Thanks Runnin!