Aftermarket Proportioning Valve Install Question

Got a question. I’ve got a gen2 GTS (non ABS) and I’m running gen 3 front calipers with IPSCO brackets and gen2 fronts on the rear. I’m going to install an adjustable proportioning valve to control the rears and make adjustments based on track conditions, pad compounds, etc.

The stock prop valve uses a shuttle valve for the rear circuit (that most people gut) and from my understanding full brake pressure goes to the rear brakes until it overcomes the spring pressure of the shuttle valve and starts restricting the rear brake pressure (in the manual this is referred to as the knee point).

So if I remove the spring/ball for the rear circuit from the OEM prop valve, is it just going to feed full brake pressure to the rear brakes all the time? Or is there some proportioning built into the housing of the OEM proportioning valve and the spring/ball assembly just controls the knee point?

The reason I ask is if removing the ball/spring results in full pressure to the rear brakes all the time then I can put my adjustable prop valve BEFORE the OEM one, gut the OEM spring assembly and still retain the safety shutoff and brake warning light aspect of the stock prop valve. Plumbing between the reservoir and OEM prop valve is also easier to access and farther from the exhaust manifold.

Normally you would always install an adjustable prop valve AFTER any OEM prop valve, but if (after gutting it) the OEM prop valve is just allowing full brake pressure to the rear circuit then it shouldn’t matter…

Anyway if someone that has done this or knows if the OEM valve after gutting it is just a “straight through” to the rear circuit, please chime in.

To disable the proportioning section of the OE Combination Valve: remove the nut at the back, (it's tight!) remove the spool valve and spring, then reinsert the spool and screw the nut back on. It becomes a 1:1 pass-through chamber and still retains the safety valve operation.

The adjustable valve needs to be downstream of the modified proportioning outlet. Otherwise, the safety valve will have a pressure imbalance and cut off the rear circuit completely.

Thanks. And good point on the safety valve. Guess this thing’s going after the OEM block after all hahaha

Great response there GTS Dean. You and I have been down this road, since you have owned and raced your car since the beginning in 1996 I think, you are possibly the world authority on this. I see where Volute took a different approach to the issues the brake systems had, like changing out the sizes of pistons. Stock set up flat spotted many a front tire. One of the reasons Dean knows so much about this. The cheap fix was to gut this valve and the f to r balance got much better.

I remember seeing decades ago in Pegasus Racing Cat I believe, 2 driver adjustable proportioning valves: one was a continuously adjustable knob you turned and the other was a lever with detents at specific settings. The knob you'd have to "tune" while the lever would provide settings which would become known. Tilton made one of them.

...yep, Tilton: 7 distinct settings:

Don't suppose any of ya'll remember what the fitting size is of the rear brake line coming out of the stock proportioning valve is do you? Looks like maybe 3/8 but haven't taken it out yet to confirm...

The 96 manual says it is 1/2-20.

Thanks again Dean. I have the 98 manual and looked in the section where it described the function of the valve. Also looked at the end of the section where it's talking about the torque values and specs of the system and didn't see anything. Maybe the earlier manual is different? Regardless, thanks!

There's a schematic line diagram of the system from MC to calipers. It lists the connection thread size by callout.

I stumbled upon these saved photos that someone else took many years ago when removing the spring. Some folks remove the o-ring and leave the spring. I removed the spring and left the o-ring on the spool. Spool has to stay in because it seals against the hollow cap nut.

GTS Dean said:

I stumbled upon these saved photos that someone else took many years ago when removing the spring. Some folks remove the o-ring and leave the spring. I removed the spring and left the o-ring on the spool. Spool has to stay in because it seals against the hollow cap nut.

Click to expand...

Thanks dean. Great pics. I presume the system must be bled after doing this?

Definitely so. If you don't piddle around while pulling the spring and recapping, you shouldn't have to bleed the MC. The rear circuit needs extra attention.

White New Balance shoes in 3rd pic. I don't know of I would trust that person

I just removed the spring on mine. Just remember it is spring under tension so that nut will go flying across the garage.