1967 How to get brake warning light to go off?

[Tom Boehm]

1967 Mustang with power drum brakes. Warning light on dash goes on when there is a brake fluid leak and pressure is lost. How do I get it to go off? I rebuilt the brake system and replaced the fluid with dot 3 and bled the whole system. The factory service manual explained how the sensor works and how to reset it. I tried that but it did not work. The process is to open slightly the sensor outlet connection opposite the last line bled and push the pedal until the light goes off. Then  tighten the connection. 

 

Does anyone out there have experience with this?

[cudaman]

If it works like ones from other makes of the period, there is a piston in the distribution block (look for the part that the brake warning wire is attached to) that is supposed to slide to a midway position it its bore when the brake system is functioning normally.  If one branch of the system has a leak, the piston slides to one end of the bore and makes electrical contact, lighting the light.

 

If the piston is stuck at one end in its bore, it can't slide back to the middle position, even after the brake system is otherwise fixed.

 

Try disassembling the distribution block and cleaning out the piston and bore so that the piston slides freely again. 

 

[Tom Boehm]

Yes that is how the sensor works. And yes that is what I suspected, that the piston is stuck at one end. And yes that is the remedy I thought of. That requires removing the sensor and then re-bleeding the entire system. 

 

I was hoping someone had a way that avoids having to remove the sensor. It's not the end of the world if I have to. 

[Bloo]

It should have worked like you described in your first post. Are you sure it is stuck? They are incredibly annoying and hard to center. Its far easier to go too far than to stop at center.

 

You may need a seal kit. The last one I had apart was for disc/drum. It was combined with a proportioning valve or a metering valve and at least one if the seals was non-standard. I had to get a kit. It probably came from West Coast Classic Cougar, though it might have been Ecklers or CJ Pony Parts or something. Since yours is just a switch, it might have ordinary o-rings inside. They have to be brake fluid compatible of course. EPDM might be OK in brake fluid, but I would verify that first.

 

 

Edited October 28, 2022 by Bloo (see edit history)

[Tom Boehm]

1 hour ago, Bloo said:

 Are you sure it is stuck? They are incredibly annoying and hard to center. Its far easier to go too far than to stop at center.

 

 

Since this is the first time I have dealt with this device, that is what I need to know. Also if I need to take it apart, I should put new seals in it. 

The book made it seem simple. 

[Tom Boehm]

Another thought I had was the piston is not stuck but the vertical post that makes electric contact is stuck in the up or on position. When we were trying to fix this by bleeding various fittings the light never went out or even blinked. This possibly could be fixed without opening and re-bleeding the system. Has anyone out there had this happen? 

[Tom Boehm]

I found out there is a tool available to keep the piston centered while bleeding the system. It replaces the electric switch while bleeding the system. It is shaped just like the switch but the post does not move so as to keep the piston centered. I may still have to take the whole thing apart to re-center the piston and then re-bleed the system anyway. 

[69merc]

Although belated, I saw your posts and thought I'd chime in.  I had my "brake pressure differential block" or "distribution block" or whatever you want to call it, pushed off center when a brake job was done some years ago.  I unplugged the pressure switch to keep the light off until I was able to find a new pressure switch and the tool that holds the piston centered while bleeding the brakes.  (It's very hard if not impossible to reset the switch once it goes off center.) I got a used block to put on the car in the meantime (also uncentered) during which time I had my original block rebuilt by White Post Restorations.  (They also do the proportioning valve)  Then with the centering tool already in, the rebuilt block was installed, then the new pressure switch was put in where the tool was, once the bleeding was done.  Then I could plug in the wiring from the car, and everything is back to normal. 👍