Metering Valve vs. Proportioning Valve

Q: Metering Valve vs. Proportioning Valve

I’ve put ’72 LeMans discs and a power booster on my 1968. I’ve got a new propotioning valve and the “round piece”, both for a power disc 1968.

I’ve been told I don’t need the “round piece”, particularly for the later (’72) single piston calipers. I used the larger ’72 master cyl.

Anyone done this? Will I go skidding into a ditch if I do or don’t use the “round piece”? What do I call this insttead of the “round piece”?

A: The “round piece” is a proportioning valve (wrong – Read Next Answer). It’s located between the master calendar and the FRONT brakes. I had this discussion a while back and mentioned to the list that mine started to leak and, at the time, I wasn’t willing to pay $120 for a new one so I bypassed it. After bypassing it, I couldn’t tell any difference on dry pavement. However, I never tested it on a wet road.

The danger is if you get too much pressure to the front calipers, they may lock up on you before the rear brakes fully engage. Remember though, the engineers who designed the system was able to test the brakes on a machine the would tell them which brakes would engage first and by how much.

Years ago, Florida used to have a mandatory vehicle inspection. Part of the test was for brakes. They would make you roll onto a ramp and when the light turned “red”, you slammed on your brakes. These little scaled tubes would fill with liquid and based on the readings, you would pass or fail. I remember taking my 72′ Cuda’ for it’s anual inspection and it failed because the back brakes didn’t engage properly. I went out to the parking lot and back up and slam on the brakes to use the auto adjusters. It took several times to adjust the brakes tight enough to pass.

I went through this long winded explanation to say that the difference between passing and failing was unnoticeable in the vehicle during my daily driving. You, however, will be the judge of your system.

A: No, the “round piece” is a metering valve read on

Before they had the skidpads they had a treadmill kinda like a chassis dyno where you rolled onto a huge motor driven roller that propelled your stationary car to about 60mph and the inspector hit the brakes until the machine groaned and almost stopped. the balance between right and left was measured which was equivelent to a panic stop. They usually did this 3or4 times to really heat up the brakes. I welcomed yearly inspections, it kept the junkers off the road. I did fail in my street/strip driven 1968, it was running so rich it was burning everyones eyes the inspector saw this as a challange and went over my car with a finetooth comb. He even put a rag into the tail pipe blocking it off so he could hear any leaks in the headers/exhaust system. Fresh collector gaskets helped and couldnt find anything except the occasional loss of a high beam indicator. It was a pain to fix, never did find out why it would work sometimes, ran a new ground wire to the steel shell or the indicator bulb and left it that way for 20+ yrs.

Back to the brakes:

I posted something about rebuilding the metering valve(round thing) by using a rebuild kit for an import truck clutch slave cylinder kit. I found the box on the shelf( I guess I dont throw anything away) when I get the application from parts store i ll post it.

there is also a Proportioning valve on 67-9 cars that had a/c , disc brakes and a V-8.This slowed down the high pressure spike in the hydraulics to the rear as thes cars had premature lock up due to being nose heavy. It was a rectangular valve mounted under the drivers seat on the frame rail.

As for someone mixing up brake parts from one system to another, I would advise against it . If you want to change to disc brakes use all of the model specfic parts of that era. You shouldnt use the later combination valve with an earlier metering valve. the combo valve combined all of the metering ,proportioning and stoplight/bias switch into one. Its a much simpler set up but cant be mixed with other systems.

the 67-9 system is basicly the same on the hydraulic part but 1969 changed to the single piston calipers. So swapping 67-8 4 piston calipers with 1969 single piston ones isnt a problem

If you use the 67-9 bird system use all the parts from the same donor.A-bodies are fine donors for the brake parts but I prefer using Firebird(not Camaro or Nova) brake lines ,valving and switch. Also remember to replace the rubber hoses. I also use silicone fluid which when bled properly will outperform any others. A high performance car must have high performance brakes.

A: Thanks for clearing that up. I was wondering why my 1969 350/350 coupe with A/C had this device in the brake line under the driver’s seat and my 1969 400/400 convertible no A/C car didn’t. The thing that confused me even more was that I bought a complete 1969 Firebird disc brake system from a 400/400 coupe with no A/C and it didn’t have this extra metering valve under the seat.

So, since I’m adding A/C to my convertible, and the metering valve to rear brakes is not there, maybe I’ll eliminate the front metering valve to balance the system!!! 😎 Or, maybe someone on the list has an extra rear metering valve they can part with.

A: I don’t mean to beat a dead horse, but I was concerned about any possible confusion on the brake proportioning valve and it’s purpose in relation to the car CG and braking requirements. The valve was installed on V8 cars and Air equipped cars due to the front-heavy nature of these beasts. When brakes are applied firmly (especially front discs), the majority of the car’s weight not only shifts to the front wheels, but also puts the rear wheels in danger of losing traction and “breaking free”. Anyone who has been at the mercy of an uncontrolled spin can appreciate this. The proportioning valve simply limits the hydralic pressure to the rear lines so that the REAR wheels don’t lock up. Here is the exerp from the 1967 Service Manual Disc Brake technical overview:

“The Proportioning valve is used on disc brake cars with V-8 engine and air conditioning. Basically the valve works to limit hydralic pressure to the rear wheels. Up to 380-420 psi the inlet, or master cylinder pressure will equal the outlet or rear wheel cylinder pressure. Above this figure the outlet pressure will rise slower in relation to the inlet pressure. Consequently, above 380-420 psi inlet pressure, braking effect of the rear wheels is reduced in comparison to the front wheels.”

If you metered the front brakes you would only encourage the rear wheels to lock up…I know it was cool when Jim Rockford forced his car into a “power U-turn”, but you wouldn’t necessarily want it to happen unexpectantly with your wife and kids!

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