Brake Valve Mod

Brake (Proportioning) Valve Mod

Posted by: kdrolt
Senior Member
Member # 776

Page Established 2/22/98

What is the Brake Proportioning Valve mod?

The Brake Proportioning Valve mod is a modification to the brake combination valve (combi valve) that increases the hydraulic pressure available to the rear brakes and results in less brake dive, more even brake pad wear, and slightly firmer pedal feel.

GM installed the same brake combination valve in the Impala SS that they used in the Caprice with rear drum brakes. Because of functional differences between drum and disk brakes, cars with rear drums use a device which delays the onset of braking to the drums as well as reducing the pressure to the rear calipers. Because the Impala was made in such small numbers, GM didn't bother to design a specific proportioning valve for the SS. Thus, the Impala exhibits excessive brake dive (the nose dips) under heavy braking and the front pads wear out quickly. The rear pads last practically forever, since they are essentially just along for the ride.

This is what the combi valve assembly

looks like when you order it from GM.

The front (left) has a 19 mm vented cap

screwed on to the end. The rear (right)

has a solid metal cap screwed on. The

yellow thing on the top is an electrical

connector for a brake warning light.

The Brake Proportioning Valve mod corrects this GM oversight by removing the delay function from the combi valve and increasing the proportion of brake fluid pressure to the rear brakes. This is accomplished by removing 2 parts, a spring and a red aluminum valve, from the brake combination valve and replacing the vented cap with a solid cap (see "Bolt Mod").

Parts Required

You will need a solid (unvented) bolt to perform this modification. A good source for this bolt is from a new combination valve from GM. The part number is GM #10223533 and it will cost about $49 if you order it from Dal Slabaugh.
Just like the combination valve in your car, the new valve has a vented plug on the front and a solid plug on the rear. Either one can be modified to work well. The solid bolt is simpler to modify (see "Bolt Mod"). The vented bolt is a little harder, but the end result is a "stealth" bolt that looks exactly like stock. Whichever bolt you don't use can be sold to someone else to recoup some of the $49 you spent on the new combi valve.

Tools required

10mm combination wrench, ratchet with 19mm 6-point socket, needle nose pliers, adjustable wrench (approx. 10" long), a Vacula brake fluid catch bottle (about #35 from The Ultimate Garage, call 201-262-0412 to order), hydraulic floor jack, 2 jackstands, wheel chocks, torque wrench.

This shows the brake combination valve (combi valve)

which is located below the brake fluid reservoir and

master cylinder. The red arrow points to the 19 mm

cap which has been replaced as part of the Brake

Proportioning Valve modification

Installation

Place 4 sheets of paper towels underneath the brake combination valve to protect your car from the small amount of brake fluid which will leak out in step 6.

Remove the rubber vented cap and paper tag from the original bolt at the front of the combination valve.

Place the following within reach under the hood of your car: ratchet with 19mm 6-point socket, adjustable wrench, needle nose pliers, modified bolt.

Stabilize the combi valve body with the adjustable wrench while loosening the vented bolt with the ratchet. This will prevent the brake lines which support the combi valve from becoming bent.

Continue to loosen the vented bolt until you feel a gentle "pop". This is the spring inside the combi valve popping the bolt off. Remove the bolt and set it aside.

Using the needle nose pliers, remove the spring and gently pull the red valve straight out of the combi valve. Check to be sure that the red valve has a small black plastic doughnut at one end. If not, you will need to fish it out of the combi valve opening with a pick (or a bent paper clip). The red valve and spring may be discarded.

Install the new solid bolt in the combi valve in place of the one you removed in step 5. Stabilize the combi valve body with the adjustable wrench as you tighten the bolt with the ratchet.

If you installed a "stealth" bolt, then reinstall the paper tag and rubber vent cap.

WARNING: DO NOT DRIVE THE CAR UNTIL THE BRAKES HAVE BEEN BLED!!

Disclaimer: This document describes a modification to your Impala SS braking system. It assumes basic mechanical skills on the part of the reader. If you are not familiar with the techniques involved in bleeding your brakes, then do not attempt this modification without the assistance of a knowledgeable automotive technician. Done improperly, this modification can be dangerous. It will introduce air into your brake system which MUST be bled out prior to driving your car. The author and this web site assume no responsibility for any damages or injury which may result from this modification.

This is what's inside the combi valve.

The vented cap holds in a strong spring

which pushes against the red aluminum

proportioning valve.

The spring and proportioning valve are

removed and discarded and the vented

front cap is replaced with a solid cap

Item:

If you mod the bolt on the combi valve closest to the radiator (on the rear brake hydraulic circuit), then you are doing the proportioning modification.

If you mod the bolt on the combi valve closest to the firewall (on the front brake hyd circuit), then you are doing the metering modification.

The proportioning mod removes the 90:10% front:rear proportioning needed on a Civy disc/drum car (because rear drums are self energizing once applied, so you only need a little bit of force to get them working). You need something more like 70:30% front:rear on a disc/disc car like the WX3/9C1.

The metering mod removes the delay in activating the front brakes. The metering was included by GM because the rear drums don't instantly apply until the brake shoe return springs have been stretched. You don't want the metering (delay) function once you fix the proportioning because in a hard stop, it means the front brakes WON'T come on at the same time as the rear brakes. In an easy stop, you won't ever notice the difference with or without the metering mod because the metering orifice has no effect.

BTW, the combi valve is called a combination valve because is serves the combination of three functions:

1. proportioning front to rear
2. metering the fronts
3. elec (shuttle) switch for the hydraulic failure for either the front OR the back brakes.