brake light on constantly

[30below]

All new lines,rebuilt m/c,syn.fluid.Brakes work okay,but the light on the dash is on constantly.Light wasnt on before the rebuild.Oh yea,73 Cuda with power brakes

[Bullitt-]

2-things turn on the light...one is the parking brake, the other is the proportioning valve.
 To reset the proportioning valve switch hit the brake pedal HARD, per the service manual.

[73Chally]

Also, the parking brake pull handle might not have enough range of motion to deactivate the sensor that turns the light on.

[ShelbyDogg]

If your prop valve is making the light stay on, have a friend crack open your right rear bleader screw. Use a rag or small hose to catch the fluid. Press the brake pedal all the way down "till the light goes off", then tighten the bleeder before lifting the pedal. If the light doesn't go off after doing the rear, Crack the right front bleeder then press the pedal "till the light goes off", then tighten the bleeder.

This will re-center your proprtioning valve switch which went off-center when you bled your brakes. Your light does go off when you unplug the wire on your prop valve right? It should only take one stroke of the pedal, so don't start pumping like crazy.

Now start the car and press on the brakes hard. the light should stay off unless you lose either your front or rear brakes then the valve goes off-center again.

[dodj]

This will re-center your proprtioning valve switch which went off-center when you bled your brakes.

 the light should stay off unless you lose either your front or rear brakes then the valve goes off-center again.

I thought it was just a regular pressure switch. It is actually a type of differential pressure switch with a contact that moves either side of the electrical connection to supply the ground?

[ShelbyDogg]

Yes.   The center is OFF and the sides are ON. just like SPDT switch with the center position off.(2P3T?)  If front or rear loses pressure, the side with the pressure throws the switch to one side and grounds it causing the light to come on. You just have to figure out which side got grounded.

I had 30below start at the right rear because proper bleeding proceedure ENDS AT THE FRONT BRAKES, so I'm thinking that the rear pressure grounded the front side. He can do either left or right but I figure that he might as well crack the farthest one to blow out some fluid.

[30below]

Thanks guys.I have my job for the day.What if the light doesnt go off when i unplug the prop.valve?

[Bullitt-]

Then the parking brake switch is engaged or your wire is shorted to ground...These switches provide the ground to the lamp. 

[30below]

Well,unpluged the wire at the prop.valve and the light went out.Applied the e-brake and it came on,so that works.Did the front wheel and rear wheel thing with no luck.Guess ill try that again.

[Bullitt-]

Seriously, just stomp the brake quick & hard....it works & what the service manual recommends.

[ShelbyDogg]

Well,unpluged the wire at the prop.valve and the light went out.Applied the e-brake and it came on,so that works.Did the front wheel and rear wheel thing with no luck.Guess ill try that again.

When one of your sides is open, it will feel like you don't have any brakes at all. the pedal will go almost all the down but you will get a little pressure when the closed side applies the brakes, You may have to quick pump it once. You have to get to that point then press a little harder and the light will go off.  You just have to figure out which way you have to go though.

Wade,
 I've had brake prop valves so far to one side that stomping the brakes turned off the light, but it turned back on soon after.  I've had the same thing happen with mine and friend's cars. If pressing hard doesn't "keep it off" then they have to force it the other way to move it a little more. Some of these guys that bleed their brakes are way too aggressive with the pumping and some even bottom out the piston to the area that it should never get to. So they probably have the switch "stuck" on one side.

[dodj]

 I've had brake prop valves so far to one side that stomping the brakes turned off the light, but it turned back on soon after.

You would think there would a set of moderate rate springs either side to re-centre things once pressures are close to equal. Or some kind of manual re-centre ability. I've never had the problem myself, but sounds like it has the ability to be a real PITA.