Hi Guys,
I've been experiencing an issue with my 1985 F-250 4X4 where the brake light in my instrument cluster sometimes comes on at a hard stop; when it does, pushing down or letting up on the pedal will make it go off, and returning it to the same spot will make it come back on. I've replaced the master cylinder and proportioning valve recently, and have bled the brakes by the book multiple times. The brake feel as good as ever, and i'm getting no air bubbles when i bleed them.
Any ideas? I'm stumped.
-James Huston

 

You replaced the master cylinder with what? meaning from where, what brand ?

 

Did you bench bleed the master cylinder? You have to make sure all the old fluid is out and the master is bled. Air may be getting in somewhere triping the warning light. Also lift up the parking brake pedal by wire or string ( I had an issue with it had to adjust the parking break hard breaking would trigger the brake light). Someone else may be able to help you test the prop valve.. What I would do would go something like this (going by a test for a 1 wire)

Set a multimeter to continuity.
if its a two wire, ground one prong to a cleared (sanded spot) on the prop valve
touch one lead to the prong on the male connector of the valve
have someone press the breaks.
if it beeps you have air in the line
if it does not move the the next male connector on the valve
have someone press the brakes.
if it beeps you have air in the line
if not look for loose worn wiring the may be rubbing triggiring the light when braking

 

The master cylinder was a remanufactured one from Autozone. I'll try testing the proportioning valve today.
-James Huston

 

If you have a diesel, you have a vacuum switch that is also hooked to the brake warning lamp.

 

No diesel here, just your everyday 300. I wasn't able to test the proportioning valve because it decided to rain cats and dogs here, but I hope to rule that out tomorrow.
-James Huston

 

Just take the wire off the valve. Also check that wire. If it rubs against metal anywhere and the insulation is rubbed through, the light will come on. All you have to do is ground that wire and the light comes on.

 

I do know now that taking the lead off of the proportioning valve stops the issue; I'll look over the wire in depth tomorrow and report back.
-James Huston

 

The Brake Pressure Differential (some people call it a proportioning) Valve Warning Lamp Switch was a troublesome turd from day one.

D5AZ-2B264-A (replaced D0AZ-2B264-A) .. Warning Lamp Switch / Available from Ford & auto parts stores.

1980/89 F100/350 & Bronco / 1980/2001 Econoline / 1990/96 F250/350.

1970/78 misc. FoMoCo Passenger Cars / 1979 LTD II(not an LTD!)/Ranchero/Thunderbird/Cougar/Mark V.