Re: Rear Brake Weight Compensating Valve (mikerob)
Yes, the later VW ABS Systems do the rear brake proportioning in the ABS distribution block, like a lot of other cars.
For systems that have a seperate proportioning valve on the rear beam...
The system seems to work better if the suspension and beam are not at "full droop". So, if you have the car off the ground when bleeding, try lifting the beam with a jack to its "at rest" position, like it would be on the ground with the vehicle weight. This will allow the valve to open and should solve your difficulties.
Or you can think of it this way... when you slam on the brakes, the weight of the vehicle is shifting forward, unloading the rear. The rear rises and in such the valve starts limiting the amount of fluid to the rear brakes to try to prevent them from locking.
Also, not all Mk III need to have their ABS systems bled with the VW diagnostic tool. Only if the car is equipped with the TEVES 20GI ('97 and on in Mk III) or later systems. And only when these stystems have had air introduced into the ABS pump. Like if during bleeding you let the reservoir run dry or if replacing main ABS components.
Hope that helps.