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Brake proportion valve


dandan62
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Hi. I have a standard Sierra brake setup on my S7. Disks front, drums back. It has always bothered me that it seems too easy to lock up the fronts when braking hard. The system has the old standard proportion valve between front and back. It has just passed its mot but he always mentions that the rears are borderline pass even though everything has been replaced with new drums, shoes and cylinders. Would fitting an adjustable proportion valve be sensible? Any disadvantages? 

David 

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Not a bad idea, given you're through IVA. I used to have one of these on a competition car with a standard pedal box :

https://www.demon-tweeks.com/uk/tilton-screw-type-proportioning-valve-243412/

Did the trick, allowed me a bit of adjustment.

Your valve, is it mounted identical to the Sierra, ie. at a slight angle? I was always told the angle was very important as it determined the operational behaviour, but I couldn't say if that's true or not (seems plausible). Or maybe it's just knackered? How about removing it temporarily, joining the pipes with a suitable length of "union", and taking it back and seeing how it looks and behaves - before you go investing...?

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The Sierra valve is an inertia type which reduces the pressure going to the rear, so it only works on a moving vehicle, it will have no effect when the car is on rollers. Also it is to stop the rears locking before the fronts which doesn’t seem to be your problem.  Unless you have it mounted incorrectly so it’s pointed downwards?

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That's useful info. From memory the pipe from the servo enters at the lower end. The rear pipe from higher end. It's mounted at about 45degs on the engine bay firewall, so side to side if that makes sense. Being an inertia type it sounds like it should be front to back? 

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I think I'll get rid of it and put a simple valve in. At least if I set the valve completely open and I still get weak rear brakes I'll know its a rear brake issue. I may just be expecting too much from the Sierra drums. To be honest the car stops well but I would prefer not to lock up the front so easily. Or maybe they just all do that in a light car. 

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You have the brakes designed for a 4/5 seat saloon car that was quite heavy loaded. 

The front of our kit cars is very light so the front brakes are going to lock up very easy.

It is more the driver learning how to brake in a very light front ended car that has little weight to hold the front wheels down on the road.

Need I say any more, practice light braking and apply less foot pressure, we don't have ABS and driver aids, just common sense.

 

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1 hour ago, dandan62 said:

I think I'll get rid of it and put a simple valve in.

Hmmm, fluid doesn't really work like that. You'll limit *flow* not pressure. I'm not sure a simple valve will have the effect you desire; it might affect pedal rate - which I know is what you want for the back brakes, but might end up stopping you from applying quick brake pressure at all, including the front, with disastrous consequences! I would check carefully this approach before you take it... there is more to these 'proper' brake proportioning valves than just a tap ;)

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Some interesting info here...

Beware limiting flow....a hard press on the pedal can force fluid into the slave cylinder, but when you let off you are relying on the brake shoe springs

to push it back, which will be slow meaning the rear brakes will come off later than the fronts. BAD news!

https://www.pistonheads.com/gassing/topic.asp?t=1573233

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