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Camaro Brake Valves

Author: John Hinckley

Version: Saturday, 04-Feb-2023 16:25:44 EST

Camaro Brake Valves

There are several types of valves with various usages in the first-generation Camaro brake systems, and some level of confusion exists about what the correct terminology is, what they do, and under what conditions they're included.
  1. Residual Pressure Valve
  2. Distribution Block
  3. Metering Valve
  4. Proportioning Valve
  5. Combination Valve

Residual Pressure Valve

Drum Master Cylinder and components Drum brake master cylinder
showing the springs(1),
residual pressure valves(2),
brass tube seats(3),
and bleeders(4).
You can't see this one, but it's located inside the master cylinder, behind the brass tube seat in any outlet port that serves drum brakes. In a drum/drum system, there's one in each outlet port, and in a disc/drum system, there's one only in the rear outlet port that serves the rear drums.

The valve is rubber, held in place against the back of the tube seat by a spring, and its function is to maintain 8-16 psi of pressure in the brake system between the master cylinder and drum brake wheel cylinders; this constant low pressure expands the wheel cylinders slightly, pulling the drum brake shoes off their rest stops, and positions the shoes closer to the friction surface of the drum to minimize the pedal travel necessary to engage the brakes. Without these valves, nearly full pedal stroke would be necessary in order to get drum braking action.

Residual pressure valves are not used with disc brakes, so they are not present in the front (disc) outlet port of disc/drum master cylinders or in either port of a 4-wheel disc master cylinder.


Distribution Block

The inlet port side
of the distribution block,
with the terminal
connection for the
differential pressure switch.
Distribution Block
Although this isn't a valve as such, it's frequently confused with one; its sole purpose in all Camaro brake systems is to accommodate the differential pressure warning switch, and it performs no valving functions at all.

Its separate chambers are fed from both the front and rear master cylinder outlets, with a spring-centered spool located in a channel between the two chambers. If there is a fluid leak or air in either the front or rear brake system, on brake application that difference in pressure will shift the spool one way or the other, and the center of the spool will contact the tip of the warning switch; that grounds the switch circuit, which lights the "Brake" warning lamp in the instrument cluster (the same lamp is also operated by the parking brake when it's engaged). With no air or leaks in either system, the spool remains centered in the block due to equal fluid pressure at both ends of the channel.


Metering Valve

This valve, used only on disc/drum systems, is variously called a "pressure regulator valve", "metering valve", or "hold-off valve" in various GM publications or incorrectly referred to as a "proportioning valve" by some people.

The metering valve, which holds off fluid flow to the front calipers until pressure reaches 30-40 psi.
Metering Valve
1969 Camaro disc/drum plumbing, showing the distribution block and the metering valve.
Disc/drum plumbing

The round metering valve is inserted in the system between the front master cylinder outlet (which feeds the front discs) and the front chamber of the distribution block. Its function is to hold off any fluid flow to the front disc calipers until the valve sees 30-40 psi of pressure; this ensures that the rear drum shoes have expanded into contact with the rear drums before the front disc calipers begin clamping on the rotors, balancing initial brake application to avoid the disconcerting front end "dive" associated with "front brakes first" under light brake application.

It also prevents a disconcerting situation which can occur (without the valve) on ice or snow under light brake application (like at a stop sign or red light) where the front wheels are stopped by the disc brakes, but the drum brakes on the rear wheels aren't yet fully engaged and the rear wheels continue to rotate under idle torque, causing the rear of the car to move sideways.

If the brakes are bled with a pressure bleeder that develops less than 30 psi, the plunger at the rear of the metering valve must be depressed in order to hold the valve open so it allows fluid to flow to the front calipers.

The 69 metering valve is similar but slightly larger in diameter than the 67-68 style metering valve.  

Proportioning Valve

Used primarily (but not exclusively) on disc/drum systems, the proportioning valve (also called a "pressure regulator valve" in some GM manuals) is inserted in the rear drum system, mounted to the side of the subframe below the driver's door. The '67 Chassis Service Manual says it was used on Camaros with air conditioning, and the '69 manual says it was used on Camaros with 12-bolt rear axles; usage varied, and no precise pattern of options or equipment has been observed that would dictate its presence.

Its function, according to the November 1969 Chevrolet Service News, is to "limit the amount of hydraulic pressure, at a controlled rate, to the rear wheels in proportion to the amount of pressure to the front wheels. Operation of the valve allows the line pressure to increase normally up to a predetermined point, then the valve limits the amount of increase in hydraulic pressure applied to the rear brakes. This action prevents the rear drum brakes from locking up before the full effective braking effort is produced by the front disc brakes".

OEM industry-standard brake system design parameters always require the fronts to lock before the rears under maximum braking conditions, so the rear end of the car will track straight and follow the front wheels; if the rears lock up first, they lose their tracking capability, and that can result in a spin when the non-tracking rear end comes around with its wheels locked.

The valve design calibration for proportioning is as follows:

      Input             Output
  Pressure (psi)     Pressure (psi) 

       200                200
       270              230-270
       400              290-335
       600              375-420
       800              455-510
      1000              535-600
      1500              735-825
      2000              935-1035
The subframe-mounted Camaro proportioning valve; note the Kelsey-Hayes ID and DDDY julian date code.
Proportioning valve


Combination Valve

Although not used on any first-generation Camaro, all of the separate distribution/valving functions described above (distribution block and warning switch, metering/hold-off, and rear proportioning) were integrated in later second-generation disc/drum cars into what became known as the "combination valve". This valve, usually machined from brass, combined all those functions in one single device, which simplified packaging and plumbing. Multiple part numbers were required to accommodate calibrations for differing vehicle weights and configurations.

A typical GM combination valve, integrating all distribution and valving functions in one device.
Combination valve


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