Owned my 1986 F150 5.0 Supercab 4x4 for a year now and love it. The other day I noticed a brake fluid leak out of the warning light switch in the pressure differential valve. I couldn't locate a replacement switch, but figured that if there was fluid getting into that port in the first place then the o rings on the piston inside the unit must be shot anyway. Instead of paying $250 for a new part, I got one at my local junkyard, installed it and bled without any issues. I've confirmed that the dash light works, but am not sure that the replacement unit (though not leaking) is functioning properly (I've tried to make it trip). SO,

My question: is this unit merely a front/rear brake circuit warning switch (I understand how the pressure differential works), or does it serve a proportioning or metering function? Looking at the design, I can't see how it would work as a proportioning/metering/combination valve.

Thanks for any insight.

 

The switch mostly shows that there is a major difference in the amount of front or rear braking being done. The proportioning valve, as it is called, does not proportion. It merely flows the fluid to the individual brakes. The warning switch shows that one side is doing too much of the work.

 

In the manual it states "This valve provides a higher percentage of braking force to the front wheels to compensate for weight transfer when the brakes are applied"

 

D5AZ-2B264-A .. Brake Pressure Differential Valve Warning Lamp Switch / Available from Ford/Myriad autoparts stores.

Applications: 1980/89 F100/350 & Bronco / 1980/2012 Econoline / 1990/96 F250 / 1990/97 F350 & F(Super Duty) // 1975/78 all FoMoCo Passenger Cars / 1979 Thunderbird/Mark V/Cougar/Ranchero.

This POS plastic switch is notorious for developing cracks in its threads causing brake fluid to seep out. When this occurres, usually the low brake fluid warning lamp on the dash comes on...and stays on.

There are 4 other POS plastic switches like this installed on other vehicles/years (introduced in 1967). The same thing occurs, because...sooner or later, thread cracks develop, so they all begin to seep brake fluid.