'92 antilock brakes stay on...

Hi All,

I replaced the master brake cylinder last week in my '92 XJ 4.0 I-6 auto tranny 4X4. This vehicle has anti-lock brakes. I did the usual replacement for a MBC: remove the old MBC, swap the plastic tank, bench blead, install, blead brakes at wheel cylinders beginning with the passenger rear and working to the driver front. I didn't do anything special for the antilock system. Worked great, brakes feel fine, no warning lights.

Trouble is three times now the brakes have stayed on. Most recently I was backing out of a parking space, applying the brakes gently. They suddenly seemed to be applied harder. The vehicle became difficult to back or go forward. The brake pedal felt very high and stiff. I jacked each wheel off the ground. With the engine off and the vac hose disconnected from the booster unit, each wheel was siezed. Wheels were cool to the touch.

I released hydraulic pressure in both front and rear systems by loosening the nut holding the steel tube to each of the two ports on the master brake cylinder, then re-tightened. Normal braking operation returned and I drove the vehicle home without further incident.

Did I miss some special thing about the ABS when I swapped the MBC? Anyone had this problem before?

Thanks!
Mike

It is the ABS

Guess I'll reply to my own post since no one did and since I have new info...

Yep, it is the ABS. I started seeing the problem consistantly. Warped the rotors due to the heat after the brakes stayed on while I was on the freeway.

In desperation I disconnected both plugs that connect wires to the ABS pump motor, located just next to the master cylinder. Problems disappeared.

Anyone know how to fix an ABS or where (on the net) I can find a description of how it works? Anything bad happen if I simply leave the system disconnected?

Thanks!

I'm only familiar with the earlier (90) ABS, but it seems as though it might be something to do with the master cylinder, or the installation thereof, assuming everything was functioning properly before the swap. I wouldn't tear into the ABS until you eliminate any problems that may have been introduced with the master cyl swap. Is there an adjustment to set the travel on the master cyl pushrod, that perhaps is not allowing the piston to withdraw far enough to release pressure?
The fact that it is affecting all four wheels seems to point to a malfunctioning master cylinder, rather than the ABS which controls each wheel separately.
Also, its unlikely that you changed to a different type of brake fluid, but I understand its not a good idea to mix regular and silicon fluid, as it quickly deteriorates the seals.

I'm really certain that the ABS is moving fluid into the brake system and is responsible for the brakes remaining on. This is because when I disconnected the ABS electrical the problem disappeared and the pedal operates normally.

The question to be answered is "why does the ABS pump fluid into the system?" The answer to this question may have something to do with the master cylinder swap... but the answer must certainly involve the electrical system in some way. Perhaps the computer that controls the ABS is making some mistake due to a bad sensor, connection, switch...

Thanks for the tip about fluid. I don't know what was already in the system, but will check what I used to see what type it is.

Mike

do you have the old crappy ABS that relies on a pump system and accumulator? you made it seem that way.

woodyjeep said:

do you have the old crappy ABS that relies on a pump system and accumulator? you made it seem that way.

Click to expand...

It's a '92. The ABS unit is adjacent to the master brake cylinder. This unit receives fluid from the MBC resivoirs and has a black motor visable extending out of what appears to be a valve assembly. Both lines of the MBC go to this unit, and lines to the wheel slave cylinders come out of it. There are two electrical plugs. I can't see any names, but maybe there are markings beneath where I can't see...

I don't think the '92 still had the "old crappy ABS" as you so elequently put it, but I could be wrong. If it is the old style, the modulator, pump and several other major components have lifetime warranty.
Does anyone know if the '92 falls under this warranty?

booster??

Well, the brakes still stay on sometimes. I replaced pads/front calipers since they surged a lot and I thought this was somehow pumping the brakes. No luck. I replaced the new master brake cylinder with yet another new one last night. We'll see if the problem continues, but I'm guessing it will.

One thing... When I put the new new MBC onto the booster unit I happened to look at the actuation rod. This rod comes out of the booster and goes into the MBC. When you press on the brake pedal it is this rod that presses on the bore of the MBC piston and moves it forward to create hydraulic pressure.

The end of this rod is domed. On my rod it appears that the dome is loose. I can't tell if it is broken or if it is somehow screwed on. It is possible that this dome could be changing its position on the rod from time to time and might sometimes make the rod slightly longer than at other times. If the rod were longer this would account for the brakes staying on.

Does anyone know if the dome is welded, turned, or screwed onto the end of the booster actuation rod?

Thanks!

Do you ever hear the ABS pump come on before you have this problem? Also, the FSM has a good write up about how the ABS works.

lc
92 xj

I've got a '90 Limited with the "crappy ABS." Did anyone else have a problem where the brakes release for a split second before grabbing hard? I've been to 3 or 4 places trying to find the cause of my crappy braking distance and their sudden desire to lock but nothing has worked...new master cylinder, rotors, drums, pads, even had the slider arms ground down.
Maybe I'm attacking the wrong side of the problem...any help would be great!

Yes, in the earlier posts I did say that unplugging the ABS corrected the problem. That's the frustrating part. The problem comes and goes randomly. It doesn't seem to happen to me as much as it does to my wife. After the ABS was unplugged the brakes didn't come on for many days. But they are comming on now and I'm still searching for the cause. With the ABS unplugged I can't blame it for adding pressure to the system. It might have a 1-way valve in it that holds pressure after I apply it with the brake pedal, but I doubt this since if I release pressure (by loosening the flare nuts on tubes at the master cylinder) the problem goes away. A 1-way valve in the ABS would still hold pressure after I release. Right now I've found the rod from the vac assist into the master cylinder has a broken dome at the end. I've ordered a rebuilt vac assist and will install it ASAP. Maybe that will fix things...

Thanks for the offer of documents on the ABS. I may have to take you up on it if the new vac assist doesn't cure the problem.

The 92 is NOT the crappy 90 Bendix system....it is the Tevis system far better. Mine never had a problem but i pulled it out to run a 44 in the rear .....

You can always gut it out.....but if you have the new master may as well try to fix it....you sure your relat for the ABS is not sticking closed?

also if you got air in the pump you need to hook it up to a scan tool and do a power bleed..

chewy

mhead, did you ever figure it out?

i don't know anything about ABS.... i'm really suprised EAGLE didn't post something to this....
mike

Hi All,

Here's what I hope is the last word...

A recap of the brake problem is that after I replaced the master cylinder (MC) the brakes would sometimes (but only sometimes!!) stay partially engaged. When this occurred it could be corrected by loosening the hydraulic lines at the MC and allowing the system pressure to fall to zero by letting a few drops of hydraulic fluid drip out. Brakes would work fine thereafter for a minute, hour, day, week,... until the next lock-up event.

First guess was that the ABS was somehow comming on and pumping the system up. Solution was to disconnect the ABS electrical thereby denying the pump in the ABS electrical power. At first this seemed to work, but the uncertain nature of the problem made it hard to tell. After a while the problem came back....

Second guess was the MC itself. Replaced this with another rebuilt unit. Problem continued...

I've solved the problem, or at least have implemented a solution that has now worked without the problem appearing for several weeks. It turns out that I noticed when replacing the MC that the tip of the rod within the Vacuum Brake Booster (VBB) assembly seemed to be broken. This rod goes from the VBB into the MC. Normally the tip is domed, with the dome being formed by a special part pressed into the rod. In my VBB this domed part was loose. It must have flopped sideways every once in a while and jammed the rod in the MC piston at the wrong place, thereby making the rod slightly longer and effectively holding the brakeks on. In this condition continuous force would be applied and there would be no way to relieve the jam except by releasing hydraulic pressure in the way described above.

I replaced the VBB with a re-built unit. Problem hasn't been seen since.

Perhaps this is the end of this story/thread.....

Thanks for all the help in the comments above from all!