hydraulic adjustable proportioning valve experience?

I was wondering if someone could educate me a little bit on adding a adjustable proportioning valve to a standard dual circuit tandem piston master cylinder.

I have had a wilwood adjustable proportioning valve on my race car for many years and have noticed that when i adjust the bias more to the rear, the brake pedal engagement gets lower to the floor. I was recently told that this was not supposed to happen, but I dont see how. If the valve is restricting flow to my rear brakes, unless it has some sort of expansion chamber, its going to increase pressure ahead of the proportioning valve which will cause the piston in the MC that feeds the rear brakes to resist moving. This will in turn cause the front brakes to come on sooner (since it no longer has to move 2 pistons) thus creating "high pedal feel".

Is this understanding correct?

Howard Jones

Most duel circuit single body master valves segregate the two circuits. That is the front system has no fluid connection to the rear. When you put the proportioning valve in the rear circuit it restricts flow to the rear calipers. This restriction reduces rear brake pressure without changing the front circuits pressure.

Wilwood says their valves reduce pressure up to 57% so it will never turn off the rear brakes.

However I have not seen your car or known the condition of the pieces so I will not say what it should or shouldn't do. If all the parts are good and it is put together as a traditional brake system then "it should not do that" is true.

Everything above does not apply to a modern antilock system with electronics involved. I am only taking about a simple duel master, front and rear circuit, and a proportioning valve in the rear circuit.

Its possible that the master is leaking internally from the front to rear circuit. I would recommend a rebuild kit for the master, or at least a tear down and inspection.

I've had a brake "issue" on track.............not a fun thing. Figure it out and fix it. We don't what to read the..... oops! thread.

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I have a new master on order; but if you look at a diagram of a dual circuit master cylinder, you will noticed that even though the front and rear circuits are independent, the pistons that drive the fluid to them are not.

From this diagram, you can see if the piston toward the front of the car were to lose fluid, both pistons would have to more forward before you get any braking from the other circuit as the front piston would have to bottom out on the MC housing before the rear piston could take effect. I dont think that spring in there is strong enough to really do anything other than return the pistons to their rearward position. The fluid behind the front piston is normally linking the 2 pistons together... Make sense?

Because of the relationship between the front and rear pistons in the MC, brake engagement height has to change if there there is a restriction in one of the lines.

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Terry Oxandale

Skinny Man

In my mind, that all sounds reasonable, makes sense, and mimics my experiences with OEM inline master cylinders. I never understood the "proportional" in a proportional valve. It would appear pressure is equal in both circuits as a matter of self-equalizing right up until the valve stops any further pressure, and then it no longer is proportional. If my understanding is correct, this is why I prefer the dual master-cylinder arrangements due to the balance bar's proportioning. But the dual MC arrangements still suffer if one circuit is open just like the in-line ones do.
Dual masters is completely superior in every way, though, converting my car to one of those setups is not something I want to tackle... not on this car at least. But, by researching my issue and really studying how these master cylinders work, i think i can improve the current system quite a bit. I have always heard that these adjustable proportioning valves are a crutch, now i understand why. You should not rely on one of these valves to make up for a poorly designed system....