67 Ford dashboard brake system warning light

[Tom Boehm]

How do I get this light to go off? 1967 Mustang with power drum brakes. The light won't go off after I rebuilt the brake system, changed the fluid, and bled the entire system. The light goes on when there is a brake fluid leak. A piston in the sensor moves toward the leaky side and makes electric contact. Bleeding the system caused it to go on. The factory service manual says to open slightly the outlet opposite the last line bled and push the pedal slowly to re-center the piston. This did not work. I understand how the sensor works. Does anyone out there have experience with this ? Responses to my post in the Ford section say this is very finnicky to get right. Does the piston get stuck? Does the electric contact get stuck and not the piston? 

[8E45E]

Check your parking brake switch.  It also lights up when the parking brake is applied.

 

Craig

[Lahti35]

Yes, the piston can get stuck in there. I have one of these headache inducing units on my '68 F250. They can get junk in there and get stuck pretty good. The piston can move both ways but only comes out/goes in to the body one way. If you didn't clean it out when servicing the brakes that may be your issue. 

 

Here is some info on centering (which you have already attempted) and a nifty tool to keep it centered when bleeding.

https://www.ford-trucks.com/forums/113549-centralizing-brake-pressure-differential-valve.html

 

If it were my valve body I would take it out, disassemble, clean and reinstall with the locking "tool" to establish baseline functionality since trying to reset with the factory procedure hasn't worked.

 

One could also just unscrew the contact switch and press the end to see if the switch itself is bad first. 

Edited November 2, 2022 by Lahti35 (see edit history)

[JACK M]

If there is a wire at the ((brain just quit)) anyway there is an equalizer thing that makes ground if one end of the brakes fail, or they are not bled properly. (Near the master cylinder)

There should be a procedure to center that switch in a moTors manual or the likes.

I guess a guy could just unplug that wire and the light should quit.

[rocketraider]

It ain't only Fords that do it...

 

Worked on one in a 72 F100 for a couple days once trying to get the accursed thing to reset. That was before I ever knew about that marvelous tool.

 

 

[F&J]

I can't recall doing a Ford, but on same age Chevy the switch is so simple to diagnose. 

1 hour ago, Tom Boehm said:

Ford section say this is very finnicky to get right. Does the piston get stuck? Does the electric contact get stuck and not the piston? 

Not usually sticking,from rust or gummed up.  That piston in there does get shoved to one side during bleeding without using a factory dealer tool that holds the piston in place.  

 

You likely don't have 4 wheel brakes working at this point. That is because the piston is then blocking flow to one end of the car. 

 

Does one end of the car have very little flow when bleeding?  That would prove that you don't have 4 brakes working.  That switch rides on the piston shaft, to a high spot or low spot, (i think it is a low spot in center)   The off position is at the center of that piston shaft.

 

You can pull the switch out to see.    Some pistons are a bitch to get centered, and you need to know which end of the car has no brakes, then open the bleeder on the opposite end of car then press harder.  Sometimes I have to manually center the piston when it just won't bleed out.

 

You could try loosening one line right at the input side of the switch valve, then see if you have good flow and pressure there.  Then do the same to the other input line of the valve.  If there is one line coming from master cyl to that valve that is air-bound, then bleeding the opposite end of the car's brake system won't/or can't make the piston move back to center.  I would try this first. 

[JACK M]

1 hour ago, rocketraider said:

That was before I ever knew about that marvelous tool.

Yes, (as the light goes a little brighter) I forgot about that piece. I have one just in case.

[Tom Boehm]

Thank you Lahti 35 and 8E45E!    Now I know what I have to do. The link to the Ford Truck forum was very helpful.

1. Check the parking brake switch.

2. Check the electric switch on the pressure differential switch. I now know that can be removed without fluid leaking. If fluid leaks out when it is removed then I need to replace the O rings inside. 

3. If the switch is not stuck then the piston is probably stuck. Time to remove the whole thing to clean it out and replace the O rings inside. And re-bleed the system. 

4. There is a tool available to keep the piston centered while bleeding the system. It temporarily replaces the electric switch while bleeding. 

 

When the dashboard light is tripped on, it does not prevent an otherwise healthy brake system from working properly. Front and Back. 

 

[Lahti35]

4 hours ago, Tom Boehm said:

Thank you Lahti 35 and 8E45E!    Now I know what I have to do. The link to the Ford Truck forum was very helpful.

1. Check the parking brake switch.

2. Check the electric switch on the pressure differential switch. I now know that can be removed without fluid leaking. If fluid leaks out when it is removed then I need to replace the O rings inside. 

3. If the switch is not stuck then the piston is probably stuck. Time to remove the whole thing to clean it out and replace the O rings inside. And re-bleed the system. 

4. There is a tool available to keep the piston centered while bleeding the system. It temporarily replaces the electric switch while bleeding. 

 

When the dashboard light is tripped on, it does not prevent an otherwise healthy brake system from working properly. Front and Back. 

 

No prob!

I'm actually going to be doing mine this weekend. I've got to find some X-rings (quad-rings) once I get the piston out. I couldn't really find a rebuild kit in stock save for one but they didn't list what the seals were made of. I'd assume EDPM or something compatible with brake fluid.