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Own a '71 F250. When I bought it, brake warning light remained lit with ignition on. I replaced shoes on front drums which showed some uneven wear. Replaced dual reservoir master cylinder which was leaking on front brake portion of the system. Bench bled master cylinder prior to installation and presssure bled all four wheel to get clean fluid throughout. Have not checked condition of rear brakes (heavy duty rear axle) but truck brakes evenly and emergency brake holds nicely. Brake warning light remains lit with ignition on. My understanding is that pressure differential valve is not centered for some reason.
I just bled mine like you would the wheel cylinders (except there is no bleeder valve, you just loosen the fittings) and the light went out. I think there is a specific procudure to center the valve but I didn't need it.
Jas88--Did you disconnect the brake lines from the valve in any specific order? Did you bleed all the lines? Did you vacuum bleed, pressure bleed, gravity bleed? Thanks again, Oldbluer.
You can center it (assuming it ain't stuck) by bleeding at the wheel cylinders. You said the part of the master cyl that controls the front brakes was leaking, so I will guess the rod has moved toward the lines that connect the front brakes to the dual brake warning switch. So try this.
Have your assistant hold the brake pedal down with the key on so they can see the brake warning lite. Open the bleeder on one of the rear brakes slowly. Have your assistant yell when the lite goes out and stop bleeding. If you go too fast the lite will go out then back on like a blink. If this happens you need to move to one of the front wheels and bleed there using the same process.
What you are doing is centering a small rod in the brake warning lite switch by allowing the fluid flow to move it. If you bleed a couple of pedals full on the rear with no change then you should try the front.
Hope that helps.
If the light is on, the equalizer valve is not centered, true. what keeps this light centered is equal brake pressures in the front and rear brake systems. (they are separate) Once it has been pushed out of center, even by the normal bleeding process, it must be reset. Some of these switches have a reset button on them. If not, have someone sit in the cab with the key on so the light is on. Then bleed one or the other end of the sytem just enough till the light goes out. It will probably take several tries, as the person in the cab usually can't react fast enough to the off light until the switch overcenters and the light turns on again. You may have to try bleeding not only on one side of the truck. But the basic point is that you can return the switch to center by bleeding the fluid from the opposite side of the switch. good luck
It depends on what circuit (front brake circuit or the rear brake circuit) tripped the pressure differential valve spool and which circuit the spool moved towards to turn on the warning light.
Front and rear brake pressures have to be equal to keep the PDV spool centered, for the warning light to remain off. If the rear brake circuit, for example, experienced a low pressure situation (either through a brake system component failure where pressure/fluid was lost or if it was caused while bleeding the brakes out), a bleeder/brake line fitting would have to be opened, on the opposite brake circuit, and pressure applied to the brake pedal to recenter the valve spool until pressures force the spool back to center and turns the warning light off.
My 69f-250 had a stuck piston in the switch housing. I had to remove the housing and was almost unable to remove the frozen piston. A little heat from a micro torch persuaded it to come out. At 50 years old it is likely frozen also. If this turns out to be the case I have a source for the rebuild kit. You won't find it anywhere else.
I just went through all of this the last couple days and am not having much luck. I bought the oring kit from Muscle car research like Ultra mentioned. I took the valve out and got the piston unstuck and cleaned and soaked parts in amonia and then cleaned with alcohol after a light steel wool. Reassembled everything with the piston centered and checked the light on the dash before I started bleeding and when I started the truck the light came on then went off like I understand that it should. Then I bled the brakes like normal and now the light is on all the time and I cant get it off. I tried centering it per the instructions that Ultra listed above and no luck. Although the light is NOT on when the key is in the accessory position, only in the ON (run) position. I thought it was supposed to be on in either position if it wasn't centered. Anyway, not sure what to do now. I thought about taking the valve back out and centering it, then bleeding the brakes with a hand pump bleeder. Thinking maybe that would keep the piston centered better during the bleed process. Any thoughts? Thanks
The light is supposed to come on when the ignition switch is in the "crank" position as a way to prove that the bulb and circuitry is functioning correctly. It shouldn't come on in any other key position.
On '68-'72 models, the warning light will do this. '67 has only one wire on the pressure differential switch and doesn't have the Prove Out circuit of the 2nd wire to make the warning light momentarily flash in the start/crank ignition switch position.
Ok, well that's good to know because it is working properly, thanks guys. Whats not cool, is that I still have to figure out a way to recenter the piston. So if I undo it and manually re-center the piston, do you guys think using a brake bleeding pump would eliminate some of the pressure difference in the valve during bleeding, allowing the piston to stay centered? Thanks
You need to get one of these. Once you get the spool re-centered, insert this valve lock tool, before you start bleeding the brakes out, and the spool will be held in place so it can't shift off-center again.
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