|
|
|
|
|
|
"This information has not been incorporated in the Chassis Service Manual, and is published here for the first time. This valve limits 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. CAUTION: No attempt should be made to adjust or service this valve in any manner. The valve is not serviceable and must be replaced as a unit. The following chart will aid in understanding the operation of the pressure regulating valve and is intended for information purposes only."
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
"In a discussion with a retired GM brake engineer, he stated that the front calipers (which are not self-energizing like drum brakes are) required higher fluid pressure for effectiveness under max-braking conditions, and those high pressures, if allowed to reach the rear drums, would allow the rears to lock before the fronts, which is a safety "no-no"; OEM industry standard brake system design parameters ALWAYS require the fronts to lock before the rears, so the rear end of the car will track straight and follow the front. If the rears lock first, they lose their tracking capability, and can result in a spin when the non-tracking rear end comes around with its wheels locked. I asked him specifically about the lack of logic in how the subframe valve was applied in production and what other equipment/option combinations would drive its usage, and he couldn't explain it - he said he remembered that there were many ECR's written that changed its usage with option ECL code changes, but he'd have to have all those old original ECR's to look at to understand why it kept changing, and they are long gone.
He also explained the initial reason for the round metering valve (or "hold-off valve", as he called it) between the master cylinder and the front brake circuit; its function was to hold off any pressure to the front calipers until 30-40 psi had been reached in the rest of the system (the rears) to allow the rear shoes to expand out to the drums before the front calipers came into play. This was apparently the result of an anomaly seen during testing in the snow in midwest winters, where (without the metering valve) a car could be sitting at a stop sign or red light in the snow, with very light pedal pressure and the front calipers holding the front wheels stationary, and the rear wheels could still be turned by idle torque and cause the rear of the car to move sideways - very unneriving for the driver. Under normal driving/braking conditions and dry road surfaces it essentially does nothing, as pressure buildup past 30-40 psi is instantaneous, as is opening of the valve."
Many thanks to JohnZ, member of the Team Camaro forum for providing this information on this very puzzling valve.
