Hi, I'm a new member and I'm hoping someone can help. My xk8 1997 has just had a 'new' second-hand but serviceable abs module fitted after the usual problem of the ABS traction control and stability lights came on. The new module has sorted the light problem. Now the rear brakes are binding as soon as they are applied. I can release the pressure from the bleed and they are free again until the next application. Am I missing something obvious?.
Any help gratefully received.

 

Just drove the car in a field, when the brakes are applied the rear ones stick on, accelerate hard so the traction control comes in and it releases the brakes. Brake heavily and the ABS works fine but the rear brakes stick on. accelerate and allow the traction control to come on and the brakes release again. Does anyone have any ideas please?

 

Take the binding rear caliper off and see if it properly "floats" in the mount and you can compress the piston easily with a clamp. I've had rear disc pistons go sideways in the bore and bind on other cars and typical pin corrosion could keep the caliper from floating, resulting in bind.


If the calipers are good, I'd look again at the module.

 

Seems like there is something wrong with the way the ABS pump is operating. I have not done any work on this, but is the module/controller and the pump the same thing (one assembly)? Have you changed both? I believe replacing the module involves opening the hydraulic brake lines. Were these put back correctly for sure? I suppose you could get an engine compartment picture from someone and figure this out.

The other thing I can think of is to make sure that after opening these lines, the entire setup is properly bled, but it is more of a shot in the dark. I assume you have done this already, otherwise the pedal would be soft and spongy.

The last thing would be something wrong with the master cylinder. I believe there is a spring inside it that is supposed to return the pistons to the right spot. Not sure how to test that...

Best of luck, keep us posted.

 

Thanks for the advice. The brakes have all been serviced. Nothing is corroded or sticking and the pistons are all free. Good idea to recheck the bleeding again. I've done it several times but it is worth a chance to get it checked again. It points to the mechanical side of the abs I think.

 

Well I have bled the brakes again and the problem persists. Apply the brakes and the rears stick on. There is residual pressure holding in the rears only. Nothing from the master cylinder or fronts. All very strange. My next step is to strip down the rear calipers again.

 

Is there a fuse for the ABS pump? Maybe, IN A SAFE SPOT, you can take it out and test the brakes. If there is no binding anymore, then the calipers are fine...

 

Do both rear brakes stick? I've heard of calipers binding when there's a delamination inside the flexible brake hoses that causes them to act like a one-way valve.

But to have that happen on both sides when a braking component has just been renewed would be a massive, massive coincidence.

I have to say that if you've just fitted a replacement ABS unit and the problem has immediately arisen, I'd only be looking in one place...

 

I've had the exact thing bladerunner919 described happen on the flexible hydraulic line on a Miata's clutch. The line looked good externally and both master and slave were new, but the line only allowed 1 way fluid flow. Even just blowing air through it was 1 way.


It would be weird for both brake lines have the same problem at the same time, however.

 

Thanks for the advice everyone. Brake lines are all good. The problem remains though.

 

I've seen this when there was no freeplay between the pedal and master cylinder. Check valve doesn't reopen to allow the backflow to the MC.

 

Thank you everyone for the help so far. Unfortunately I am still struggling. the brake lines only allowing fluid one way has been checked, all is ok. the master cylinder returns ok. I just had the rear calipers off again and tried to push them back in. they won't move and more surprisingly when I push the l/h in the r/h tightens up and visa versa. The front brakes remain free all the time.
I have the ABS electronic module disconnected. I think that its the hydraulic module that is faulty in some way. this is the second one I have put on though that shows exactly the same problem. I am really confused and fed up.
terry

 

trying to find if out if you fixed this issue
i am experiencing the same issue

 

Only fixed by fitting a second-hand ABS pump. No problems now. It was obviously a fault with the pump unit. Fortunately it does not need coding in. Good luck.