Left front caliper "sticking"

#1

Hey guys, I’ve been using the Book of Kirby for a couple years now, so this is my long-overdue first post.

After a mechanical refresh, the first couple test drives in my car (1989 V12, with ABS) showed that the left front brake was dragging. I rebuilt the calipers and replaced the pads, rotors, and hoses. I found some sort of scale or crystals inside the calipers, which I cleaned out.

Surprisingly, this has not fixed the problem. The left front caliper does not release when you take your foot off the brake. The interesting thing is that relieving the pressure by opening the bleed screw or even cracking the line at the ABS unit causes the caliper to release as it should.

I can take another shot at rebuilding that caliper, but it seems like the caliper is working. I can also try rebuilding the master cylinder in case it’s afflicted with the same deposits as the calipers, but it seems strange that only one wheel would be affected. Last option would be to disassemble the ABS module.

(Robin O'Connor) #2

Try to find a safe place and exercise the ABS unit by braking very hard, it could be a small amount of debris in one of the valves.

(BrianDD) #3

Hi, you need to clean out the ABS module or replace with a good one.
That will cure it.
Cheers, Brian.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #4

The left front caliper
does not release when you take your foot off the brake. The
interesting thing is that relieving the pressure by opening the bleed
screw or even cracking the line at the ABS unit causes the caliper to
release as it should.

That means the problem is upstream of the joint at the ABS that you
cracked. It’s probably one of the valves in the ABS unit. And the
problem was probably caused by someone replacing pads and pushing
the dirty brake fluid back up into the unit rather than opening the bleeder
when pushing the pucks back.

You might try disconnecting that line altogether and then stomping on the
brake pedal hard once or twice. See if you can blow the crud out of the
valve. Nuthin’ to lose! It’ll make a hell of a mess, though.

I can also try rebuilding the master cylinder
in case it’s afflicted with the same deposits as the calipers, but it
seems strange that only one wheel would be affected.

Before you do anything else, you might wish to try adjusting the brake pedal.
The link between master cyl and pedal should be adjusted so that the master
cylinder is fully retracted when the pedal is up, and there is slack in the
linkage. A tiny bit of residual “push” on that linkage can cause weird
problems like this. Normally not a problem unless someone has been
working in there, though.

Last option would
be to disassemble the ABS module.

Others have just dropped into a junkyard, purchased a similar ABS module
off an XJ6, and then combined parts. Otherwise things get expensive.

It’s not your LAST option, though. Your last option, and perhaps the best
and safest option, is to rip that Teves III ABS system out of there and heave
it over the hedge. You can upgrade to a Teves IV from a mid-90’s XJ-S (a
safe ABS system), or you can retrofit a non-ABS braking system from an
earlier model. Either one would be a dramatic improvement in safety from
the Teves III.

– Kirbert

1 Like
#5

This seems to have fixed the problem for now, thanks.

I’ll look at this next. No telling what may have been messed with in the past.

After that I’m going to try a more thorough brake fluid flush.

#6

After 18 months, I am tackling this problem again. I flushed out the front circuits with new fluid this morning.
If memory serves, last summer I would “fix” the problem by making a couple hard stops. This actually cures it for a while, until it sits for a couple weeks. Haven’t tried that yet this year.

I have gone down the ABS electrical checklist and found no problems. The valve function test for the LF wheel does release the brake, as does opening the bleeder or cracking the brake line.

I tried this, and even with the line disconnected the pedal was pretty firm. I didn’t get much fluid out. Ignition off, accumulator discharged beforehand.
Also checked the linkage and found it to be correctly adjusted.

So the problem persists. Thank you all for your help.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #7

That ain’t right! That sounds as though there is an obstruction upstream of there – and if we’re talking about the connection at the ABS valve block that leads to the LF caliper, upstream of there would mean inside the valve block. And if it’s not affecting the RF caliper as well, it can’t be before the valve block, it must be inside.

My standing recommendation for anyone with the Teves III ABS system is to rip that POS out of the car and toss it over the hedge. The pre-ABS vacuum-boosted power brakes are safer that the Teves III. In your case, the recommendation goes double. R&R of that valve block is reportedly stupid expensive. Buying another valve block from a junkyard and swapping parts to make one good unit is cheaper, but why bother? Much better idea to drop off your valve block at the junkyard while picking up what you need to retrofit the earlier brake system.

(scrimbo) #8

If the bleed back is not working could it be master cyl. I was mystified on a motorcycle brake that wouldn’t release because the pedal travel allowed the master cyl piston to park over the bleed back hole…don’t know if that would apply here…but something is obstructing.

(Maynard) #9

Maybe I’m missing something, but I wonder if the brake hose has broken down internally. I don’t remember how the hose is connected to the brake line, but is it possible to loosen that connection and see whether or not the caliper releases?

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #10

If the brake line was disconnected at the valve block on the master cylinder and the pedal remained hard, the problem is in the valve block. If the line had been disconnected at the caliper, my first suggestion would have been to replace that brake hose, not even bothering to check it. Those things collapse internally all the time, blocking flow in or out. But since the report was that the line was disconnected upstream of there, fiddling with the brake hose is not likely to be useful.

#11

Brake hoses are new.