vtraudt

Recently, the front brakes on my 2003 AR locked up (clamping, barely able to move the car).
Bled the brakes. Start car, drive. Lock up again.
First thought stuck caliper, but both at the same time?
Now thinking a hydraulic issue. Ran VAGcom output test (fine, nothing abnormal).

What might be causing the fronts to lock up?

Are the 2 'channels' of the master cylinder the old fashioned diagonal way (one channel front left/rear right, other front right/rear left), or are front and rear on separate 'channels'?

Kneale Brownson

Do they release after the car cools down?

vtraudt

No, only after opening the bleeder screw.
Also, NOT sure if rear also lock up (still need to jack up on all 4 corners to check).

Kneale Brownson

Maybe an issue with the ABS or electronic stability control?

vtraudt

Nope. Only when opening the bleeder screws.

vtraudt

Possible. Just didn't think that the ABS pump would generate pressure.
Is there a fuse to just kill the power to the ABS pump?

vtraudt

Correction: All 4 wheels locking up/dragging. Takes only short drive and warm up to occur.

Update: replaced the brake lines (heard the inner liner can collapse), no change.
Also note: ABS/Brake light is on.

Could the ABS module cause the issue? I had the codes early on after the auto to manual swap, took a while to find a soft coding combination that eliminated the lights, but now back again.

Or could it be the brake booster or check valve to the booster (in the vac line)?

Any troubleshooting suggestions to narrow down the cause?

indoor

Looks like ABS unit is acting up. Service manual says some hydraulic abs units have to be bled via computer(VCDS). Check your abs unit type if you have the service manual(they have a picture of each type).
Maybe you got some air in the unit, or it's leaking...
P.S.: I would lube the caliper guide pins even though chanses are not all four seized up out of nowhere...oh' and clean the abs sensors too just to rule them out...

thejulex

Can you describe in more detail when it locks up?

E.g. you drive normally and all of the sudden it brakes on its own?
You brake lightly and it locks? or heavily that would prompt ABS to activate?
Right after coming to a stop first time after starting the car (ESP pump self-test, ESP pump generates pressure in brake system without driver's involvement)?

Either way it sounds like a valve in ABS is acting on you, or more specifically perhaps ESP channel pressurizes the system as if traction control is needed and never let's go.

It must be ESP pump, not abs. ESP pump normally activates when traction control kicks in as at that time you naturally have zero pressure in hydraulic system (you're not pressing brake pedal when accelerating).

Send me your email, I will send you factory repair manual for brakes for fun reading (130 pages PDF... covers pretty much every nitty gritty detail)

josepmasnou

Hello, my name Josep and live in Spain.

Sorry for my bad English.

I have the same problem in 2002 allroad.

When I drive and traction control (ESP) actived, and sometimes is triggered not work, the brakes lock.

The fluid brake can not return to abs module to the pump. If I take the return pipe brakes are unlocked.

I changed the ECU module abs, ESP control unit and master cylinder, the G201 sensor that goes in the pump pressure.

If the diagnosis step indicates only error brake pedal switch, but is new.

Someone could help me?

Thank you very much.