Rear Brake Bias Valve

Ah the famous MOT failing valve.

So what is is?

It's a 'Force Proportioning Valve'.

What the hell does that mean?

It alters the amount of braking effort available to the rear wheels on disc-equipped cars and it's dependent on the load on the back of the car.

The valve has two inputs from the Master Cylinder and two outputs to each Rear Caliper. There is an arm which attaches to the rear axle to the valve via a spring.

With the car level the full output from the master cylinder will go to the rear calipers. As the front of the car dives down and the rear raises as it does during braking, the spring relieves tension on the valves actuating arm so the valve closes. This means less of the braking effort goes to the rear wheels.

What does it look like?

This is a new one.



Why does it reduce braking effort?

It does this to prevent the rear wheels locking up which isn't a good situation.

So what are the problems associated with it?

Well, they seize up. The location of the valve is under the rear of the car in front of the rear axle.

The valve's arm gets stuck with rust. In the second picture on the right, you can see where the arm pivots.

So how do you free it?

Well WD-40 on the arm's pivot and move the arm back and forth with your hand or use a hammer and a pole. Move it back and forth till it's free. Then to prevent it seizing again spread copper grease about it.

So that should ensure the car passes MOT's and the valve does what its supposed to.

I have lowered the car and the wee man at the test station says valvy no worky no more, what do I do?

Well you adjust it.

How you ask?

Well here's a pic of the springy arm thing taken apart.



These are the components that make up the adjustable part.

The hooked part attaches to the axle. That slides inside the other rod and it clamped there.

This alters the length of the arm thus where the spring sits and the springs pre-load on the valve.

So how do you adjust it then?

Well on the metal rod you can see the nut and the nut end on the rod. You need to loosen these then to get the spring to touch the arm on the valve you need to lengthen the rods length then clamp it back up. And vice versa.

When is it correctly adjusted?

When the spring is touching the bias vales arm and the spring is under light tension.

All very good but I can't get my spanner into the locknut bit where the spring seats onto. What do I do now?

Easy - get a decent sized spanner - 18mm for example, then lever the spring seat off so it breaks the rust. Now you can get full access to it.

What size are the nuts then?

Well I assume they were 13mm at one point but 14mm spanners will fit better due to rust etc.

I don't like the feel now the back brakes actually work. What can i do?

Your a girl, get used to it or adjust the spring so the rear effort is reduced sooner.