D3 MOT Fail after ABS unit replaced

[Prefferle]

Hi folks,
I had recently refitted my ABS module (complete), and pressure bleeded all four corners, and proceeded to the MOT test centre.
The resulting MOT Fail was due to severe braking pressure imbalance on Axel-1 (Service Brake excessively fluctuating offside front). So as explained to me by the Tester, an imbalance of braking force between the front left and right wheels. And he insists the issue is my ‘rusty’ brake discs. The rust is on the inside in the disk vent area.
Looking this up online suggests either brake discs or air in the system. Dial indicator runout looks fine and all I can think of is that I have not activated the ABS pump since replacing the ABS unit as part of my brake bleeding.
I remember a previous comment somewhere where on the d3 it is not recommended to activate the ABS pump using VCDS as damage (?) to a controller can result. Anyone know if I can safely use VCDS to activate the ABS pump to thoroughly bleed the brake hydraulics or should I just try pressure bleeding again?

Cheers,

[steamship]

Originally Posted by Prefferle

Anyone know if I can safely use VCDS to activate the ABS pump to thoroughly bleed the brake hydraulics or should I just try pressure bleeding again?

Cheers,

Can't comment specifically about the D3, but I used both pressure bleeding and VCDS on my D2. I've used the pressure bleeder to bleed them after replacing calipers, and also previously to do a complete brake fluid refresh.

I then used VCDS to do the ABS procedure, but on the first occasion screwed up. I had read the instructions, but not well enough, and missed one important line - that when doing it, the fluid goes to the front left caliper. It helps immensely when you undo the bleed nipple on the caliper, especially when the ABS unit runs for about 10 seconds. I'm sure I ran this procedure twice, as well. Couldn't figure out what was supposed to be happening, so read the instructions properly. Loosened the bleed nipple and ran the procedure, and it pumps out a significant amount of brake fluid. Thankfully, no adverse effect from my previous two attempts at it.

Even with new front discs, pads and fluid flush a year ago, my recent MOT shows a 9% imbalance on the front axle and 5% on the rears. I find it hard to believe that rust in the vented area could be the cause, unless half of them are clogged with rust, and not cooling the same as the other half, and that is somehow giving this imbalance.

[Prefferle]

Thanks Steamship��