American Motors: brake proportioning valve, brake proportioning valve, brake warning light

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Followup To

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Followup To

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Hi Kevin,

I've done much reading about the brake light that goes on in my '68 AMX, but I don't undertstand why a pressure differential exists between the front and rear brakes. There's absolutely no leakage of brake fluid at any of the wheels. Also, do I want to attempt to re-center the rear proportioning valve (near pass. rear wheel) or the brake proportioning assembly below the heater motor? Or, can this be done at all?

Thanks,

Steve

Answer -
Steve,
The valve below the heater motor is the pressure differential switch. If you remove the wire running to it, then the switch assembly it fastens to, you will see a small metal plunger or rod. If your brake warning light is illuminaed on your dash, it's because this plunger has dropped down in the valve. See if you can tell which way the plunger is leaning to give you an idea on which brake circuit created the pressure differential. The top of the plunger will be pointing to the low pressure side. It often will come on after bleeding the brakes. I've had luck in the past moving the piston inside the valve by loosening a brake line on each side of the valve, one at a time, while someone depresses the brake pedal. This in theory can force the piston into the neutral posistion.
When you remove the switch on top of the valve, check for brake fluid inside. There shouldn't be any.
Hope this helps.
Kevin

Thanks for the explanation, Kevin. So, what's the valve for that's above the passenger rear wheel? I thought that was a proportioning valve, too. Is there a cockeyed piston in that device as well?

Thanks,


Steve

Answer -
Steve,
That is the proportioning valve. It is found on vehicles with front disc brakes. The proportioning valve functions to maintain correct proportion between hydraulic line pressures to the front and rear brakes and, therefore, provides a balance braking system. It is not adjustable on your AMX. It is different than the pressure differential valve located under the blower motor. Let me know if you have any other questions.
Kevin

Thanks, Kevin, for your time so far; however, I'm still a bit confused. How would one know if the proportioning valve is bad and needs replacement, or just the pressure diff. switch has a cockeyed piston? My symptom is that the brake light is always on and when I press the brake pedal in a hard fashion, the rear tires will either lock up or skid.

Thanks,

Steve

Answer
Your symptoms could be caused be a defective proportioning valve,or if the brake shoes are contaminated with oil/grease, or if the rear shoes are not properly adjusted, or if the front calipers are sticking.
All of these can cause the rear brakes to lock up. The pressure differential switch does not affect braking performance, it just turns the light on if there is a pressure difference between the front and rear system.
Try bleeding the brakes as described in a previous message. Inspect the rear shoes for contamination. This should turn off the light. Adjust the rear shoes so they LIGHTLY touch the drums when rotating them. Watch the front brake pads apply, then make sure the retract enough to remove pressure from the rotors.
If all is OK, then check for leaks, then a possible defective proportioning valve.
Good Luck.
Kevin