Brake Proportioning valve ??

[oilyman 10]

Hi Its my first post and already looking for help . I have a 1995 VR6 in the workshop that has been re-built with mostly new everything in the brakes and suspension department .We are getting low effort at the rear brakes and reckon it could be down to the proportioning valve. which is new . I have tried it in with the lever both ways and it makes no difference and I am wondering if it has been plumbed incorrectly .

Does anyone have a diagram or a good picture showing the pipe layout . We have a good solid pedal and have bled at the abs unit as well .The front brakes are excellent .

The car was completely stripped for over a year so we could easily have our pipes crossed . Any help would be greatly appreciated .

 

Thanks

Jon

[Simeon 0]

Jon, if you look at the valve there should be some arrows etched into it showing the flow direction through it, so you can check this way. They are a bit of a bugger sometimes, have you been bleeding the brakes with the valve clamped open?

[oilyman 10]

Hi Simon ,thanks for that ,I will have a closer look for the markings ,it is motor factor replacement .I tried bleeding the brakes with the arm fully both ways and got plenty of fluid in both directions which I don't understand . I don't see the reason for it on a car with abs either but there must be a reason .

 

Cheers

Jonathan

[Simeon 0]

The pic on this one has the arrows

 

https://www.ebay.co.uk/itm/GENUINE-VW-CORRADO-REAR-BRAKE-PROPORTION-VALVE-1H0812431C-/142571283105?_trksid=p2385738.m4383.l4275.c10

 

It is my understanding that you clamp it closed (the arm down on the body of the valve) to open it fully. Strange that you can get fluid through with it in either position, I've always struggled to get any flow through them when not clamped in the correct position.

[oilyman 10]

Yes a bit strange ,do you Know if the two pipes from the abs unit are supplying fluid or is one a return , the possibility that they are crossed over is something I need to check .

I have read that the abs unit has three pressure modes full braking half and none ,it would be fair to assume it is stuck on half power .

With the engine running ,brake pedal pressed hard ,you can turn the back wheels by hand but it takes a lot of effort .The front brakes are perfect though .I think the dual circuit brake operates a front and rear brake per circuit which must mean something if it's both back brakes that are weak . Incidentally all the calipers ,pads and discs are new .

An interesting puzzle ,any helpful ideas are greatly appreciated .

Cheers

[Dox 23]

Does the handbrake hold well? What make disks and pads?

 

Did the rears bleed up easily? Yes indicates the valve is open and it’s piped correctly?

 

The more weight in the Rear the more fluid is fed to the rear brakes via the valve, the spring may need adjusting if the valves not reducing pressure with no load in the rear?

[oilyman 10]

Thanks ,yes rears bleed up well and loads of clean fluid has been pumped through ,not sure now what make the components are .Not two worried about the load sensing just need s decent reading on the rears

[fendervg 32]

If the rear wheels are off the ground while you are testing, then the valve is fully closed. You need to clamp it open to give the effect of the car being on the ground and as said above, also hold it open while bleeding.

 

If the ABS is faulty, you will still have full mechanical braking - you can test this by fooling it into fault mode by disconnecting a sensor.

 

Did you bleed the master cylinder and the ABS pump unit after doing all the callipers? Also check your rear solid and flexi brake lines for tight connections and leaks. It might also be worth trying another brake proportioning valve to rule that out - the hardest part with those is getting the old one off, and you've gotten past that hurdle.

[oilyman 10]

Thanks for that ,I did not bleed the master cylinder , I did not know about that .I was thinking of bypass in the valve to take that out of the equation until I find the problem ,a few more possible leads , cheers

[a_riot 0]

I ran into some weirdness like this when I had a shop swap the rear calipers a few years ago. The tech couldn't figure out how to bleed the rears, and then got confused and frustrated when after bleeding, the ebrake didn't hold. You could spin the wheel and pull the ebrake and it had no effect. You could also still turn the rear wheel while standing on the brake with some effort. He had pushed the valve open by hand to bleed them and couldn't get a drop out of either.

 

I can't say what went wrong, but I took it home and bled them like I always do and it worked fine. I always bleed the rears with the rear beam on jack stands. I don't mess with the valve at all as I think it can get locked closed in some instances and confuse the heck out of you. But as long as the valve is plumbed correctly, I would put the beam on stands, and bleed the normal way with a helper at the pedal. Some use the ABS pump, but I prefer to do it manually. Every time I do it this way, it works fine, but any other way, and I get weirdness so I don't argue with success.

[Kevin Bacon 5]

Bin that crap off and fit a MK3 Highline ABS pump. That way you can lose the stupid 1980s proportioning valve, which A) always seizes up unless you drench it in WD40 every other week and B) the bolts that secure it to the rear beam always seize up solid, meaning a new P valve regardless.