Proportioning valve

There is a proportioning valve on our VR4 mounted between the ABS unit and the rear brakes (Workshop Manual 35A-2, 35A-7 or pages 87 and 92 of the translation Vol.2). It’s wondering me, because the ABS unit should do the work. Or am I wrong?

I’m asking this because my car didn’t pass the Dutch MOT. There was a difference of more than 60% in brake operation between rear right and left. So I was thinking about a problem in the ABS unit. But there is no dash bulb warning. Looking closer under the bonnet I saw the proportioning valve.

So I have two questions:
- Why is there a proportioning valve
- What can be wrong with my rear brakes. I did a complete revision some years ago and pistons etc are 100% ok. Can it be the ABS unit or maybe the proportioning valve?

Thanks for your attention.

Is the rear brake balance difference on the footbrake, or the handbrake?

The proportioning valve applies different levels of braking effort to the front and rear. Side-to-side is not affected by the proportioning valve.

The ABS unit only acts as an "on/off switch" for the individual brakes to release the braking effort to a single wheel during moments where it detects no movement of the wheel. But the main brake bias between front and back is governed by the physical sizes of the brakes, as well as the proportioning valve to further fine-tune it based upon the weight distribution of the car.

As elnevio says - are you talking about the handbrake, or foot brake?

Quote:

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Originally Posted by elnevio

Is the rear brake balance difference on the footbrake, or the handbrake?

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It's on the footbrake.

the proportioning valve lives on the engine bay bulkhead behind the rear turbo.
you will see it as it has the brake lines running to it.

i am not exactly sure how it work but i do not believe it is an active vale it is just a static splitter of braking force.
So i would imagine it is just ratio of holes sizes inside the valve that governs the split of braking force.

i would have to agree with @Confused garry, the proportioning would only be front to back not side to side as that would be dangerous and tend to put the car into spin if you applied the brakes hard.
an imbalance in the rear foot brakes would probable be down to a sticking callipers pistons or guide pins if you have standard VR4 brakes.
it is very easy to use the wrong grease and cause the rubber seal on the top bolt to swell up and stick into the bore of the slide bolt or to much grease on the slide bolts when rebuilding the brakes and causing a air bubble/lock in the slide bolt, so instead of operating like a slide bolt it act like an air spring.
you apply pressure to the brake peddle and instead of that being applied to both pads evenly the trapped air forced the sliding calliper away from the disk reducing the braking force being applied to that side of the car.

the reason why i know this can happen is i have caused both scenarios to occur on my car in the past and i have learned from my mistakes.
the tell tail result is one of the pads wares more than the other. this tells you the calliper is not sliding correctly and the fact you should be able to move the calliper on the slide bolts quite easily with your hand it should not be very stiff.

if the pads are warn almost all the way down this can cause the pistons to stick in the callipers as they start to twist in the calliper bore and the pistons do not slide back into the calliper correctly. there is nothing to hold the piston square with the calliper if they get pushed out of the calliper to far.

just to be straight with you some years ago is a long time for brakes.
brakes should be stripped and clean and re greased twice a year in my book to keep them working correctly.
this does depend on mileage and type of usage e.g. track or fast road heavy braking.
higher mileage check more often, harder braking with track use check more often.

@Davezj, thank you for your comment. The reason I brought the PV up is that I studied its function as soon as I became aware of it. The other reason is that another VR4 owner in The Netherlands had exactly the same problem and the experience that replacing the PV solved the problem. So I was surprised it helped him. In case it is a malfunction of the PV I ordered already a new one.

About the brakes themselves: good point about the grease and the possible consequences. I never thought about it. I will check this soon (and also the possible change of locking and guiding pin).

Maintenance of the brakes twice a year, as you suggest, is too often for me. The VR4 is not my daily, drives max 2000 miles/year and is standing high and dry if not in use. I check once a year when there is a MOT.

Keep you informed!

i believe the only reason the proportioning valve should make a left to right difference in the rear will be due to a blockage inside the valve.
maybe some debris gets into the system from the fill point and then get forced through the master cylinder and into the proportioning valve which effects the distribution of the braking pressure.
probably a removal and clean would be the first thing to do to see if it makes a difference.

i would imagine you could clean the valve by doing the following, but i have never needed to do this.
and i would only ever recommend doing this unless you are sure there are no moving parts in the valve like rubber seals that might get damaged by the solvent used to clean out the brake fluid from the valve.

remove the valve.
give it a clean out with carb cleaner and then a blow out with compressed air.
but make sure when you clean it out and blow it out hold the valve in white kitchen paper then if anything gets washed out or blown out you will see the evidence on the paper.