Here is an issue I've never even heard of anyone having before. The brakes on my '90RR aren't releasing completely when the pedal is released, with the result that there is tremendous drag on the vehicle.

When it has been sitting a few days and the system is depressurized, I can jack it up and turn all the wheels by hand. After I turn it on and let the ABS pressurize, I can still turn them by hand. After one push of the brake pedal however, things are very tight and will not budge. Obviously the engine has enough power to turn the drive train but my acceleration and top end are severely limited, and I can stop at an intersection without using the brake pedal (I have a 300Tdi and R380). If, after letting it sit for a few days, I drive it without applying the brakes, there is a very noticeable improvement (until the first time the brake is applied). And, no, I'm not driving all over smoking the brakes.

My thoughts are that it must be the ABS booster, with one of its internal valves malfunctioning. I don't think it is the pressure conscious reducing valve because the front brakes are affected too, not just the rear which the PCRV deals with. There is no reason that bleeding should have done anything (it was just done). I've pored over the schematics and can't come up with any other place thefault might be other than the booster. Anyone else had this problem or have any real insight?

Thanks;
Greg Haugen
Fargo, ND USA
'90RR 300Tdi ex-V8
'99RR 4.0
'00DII 4.0

 

This is a tough one. I drive a Classic, and a few guys here also do on a daily basis. I ran this by them, and we are all scratching our heads. So --- to try and help I have to ask you questions aboutwhat may be a lot of obvious things that should be checked out. Is the vehicle used in a part of the country that lays down salt on the winterroads? Have you confirmed that the emergency brake drum is not hanging up? (they can do this with no rhyme or reason attached to when --and cause drag). Did you confirm that you have proper caliper piston retraction at each corner?. (It is time consuming, but the test is to have the vehicle with all four wheels off-- One corner at a time --- depress the brake pedal, release, and check that corner to see if the brake pads have retracted enough to turn the brake rotor. If not -- you have calipers that need to be rebuilt or replaced --- as the pistons are hanging in the bores. Aggravated by local condition where salt is used in the winter.)

We have not seen any malfunction of the ABS system that would result in the condition you are experiencing.

Hope this helps.

regards,
Geoman
www.eurotekapg.com

 

The more I think about this and experiment with various factors one-by-one I think it will turn out to be the brake calipers needing to be rebuilt. Even before the first post, I had put the vehicle on stands, and after the first application of brake pedal, the fronts would not budge by hand, one of the rears wouldturn with great effort, and the other rearwouldn't budge by hand either. I'm going to rebuild them all and see what that does. Since all of them seemed to be severely affected I had initially thought about something central (like booster) which would cause a global fault like this, but it may just be that after a period of not having been driven, various and sundry environmental insults (read "I like to go wading") etc it will be the calipers.I'll let you all know.