MK1 pulling to one side on brake test machine when I'm NOT braking

[Orbis]

Hello everyone!

I have a new problem: a few weeks ago I replaced the brake fluid because it was contaminated with water. After that replacement I did an brake test to check the brake balance.

What happened then (and never before with MOT):
the car started to pull to one side. The clutch was pushed in, gears in neutral, in and I was not braking.
There are also no warning lights or fault codes (checked with VCDS lite). It stopped pulling to one side as soon as I was braking.

Last year I replaced both front ABS sensors because the left one was faulty.

I rechecked the brakes on the test machine with both front sensors disconnected and it was acting normal again.
I also went driving and monitored the wheel speed from the 4 sensors and it was all good and the same.

When I drive on the road, there's nothing wrong and I also don't feel unsafe. It just happens on the test machine.

Any ideas what can cause this?

Thanks in advance!

[2006ransome]

Tyre pressures good ?
After a run jack it up quickly and spin the wheels
Any binding ?

[Orbis]

Tyre pressures good ?
After a run jack it up quickly and spin the wheels
Any binding ?

Hey, thanks for the help! The tyre pressure is good. I check it very frequently. The brakes also don't stick and the callipers are free to move opver the sliding pins. I maintained them not so long ago. I think it's relatet to the ABS or TCS system because if I unplug the front ABS sensors the problem is gone.
The only cause I can think about is that the front wheels are spinning on the brake test machine and the rear wheels not. But it's the same on MOT and there it's behaving fine. If I let someone else drive it in the brake tester and I look at the left wheel, I can see that its braking pulsating, but the driver isn't touching the brakes.

Any more ideas?

[insanitybeard]

Has this only been an issue since you changed the brake fluid? Is so, what method did you use for changing the fluid?

[Orbis]

We did it with a vacuum pump on the bleed nipples. And I kept the brake fluid reservoir topped up. I've done it before but It looks like the ABS or TCS system is flipping because the rear wheels are not spinning by the test. I just don't know why it's doing it now and not al the other times in it's life.

[insanitybeard]

The other thing that might cause it to throw a wobbly whilst on a brake tester is if there's a brake imbalance between the two front brakes, meaning that one front wheel ends up rotating faster than the other- enough for the ABS to detect a difference and attempt to 'correct' the 'fault' by activating the ABS system, this would account for the pulsating you describe.

Having said that, if this happened on the tester without the brakes being applied by the operator, then it could still have something to do with the wheels rotating at slightly different speeds and the difference being enough to cause the ABS system to cut in, by disconnecting the sensors the system would no longer 'see' any speed difference between the front wheels, therefore it would not cut in. Did you say it has traction control fitted as well as ABS?

[johnnyroper]

What condition are the tyres in? And are pressures same both sides?

A difference in rolling circumference from wear/incorrect pressures could cause the TC to have a hissy fit

[Orbis]

thank you for your replies
The car has indeed both traction control and ABS. The tyres are wintertyres and 3 years old. They are still in very good condition and not much worn. The tyre pressures are good as I check it frequently. I think that I'll test it again on another brake tester.