Camaro Brake Valves
©2010-13, Camaro Research Group
Written by John Hinckley
Version: Tuesday, 05-Feb-2013 18:51:32 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
Residual Pressure Valve
Residual Pressure Valve
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.
||Drum brake master cylinder|
showing the springs(1),
residual pressure valves(2),
brass tube seats(3),
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.
The inlet port side |
of the distribution block,
connection for the
differential pressure switch.
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 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.
The metering valve, which holds off fluid flow to the front
calipers until pressure reaches 30-40 psi.
1969 Camaro disc/drum plumbing, showing the distribution
block and the metering 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 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:
Pressure (psi) Pressure (psi)
The subframe-mounted Camaro proportioning valve; note the
Kelsey-Hayes ID and DDDY julian date code.
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.