Hi all,

My brake warning light is illuminated in the cabin. My fluid level is good to go, I don't have any line leaks, and I've personally verified that all my drums look more than okay.

I spoke with somebody that seems to think the proportioning valve that's original to this 1972 F100 is most likely faulty; probably sticking at times and giving a false pressure reading that's tripping the warning light.

Oh and one more thing - if my foot is pressed on the brake pedal, especially at max pressure -- the light goes off. But as I release the brake and put less or no pressure on it, the light warning light turns back on.

Any ideas? It's not the end of the world, since I know the brakes are operating okay. Just a tad annoying to see that light on all the time.

 

Try cracking loose the brakeline connections at the master cylinder then retighten them. This will sometimes balance the pressure in the proportioning valve.

 

first did it just come on? has it been on a while? its been on a while you might need to take it off to reset it.

use a mirror and light and remove the switch at the valve. look which way its slid. then crack the to line to the breaks on the opposite valve line and have someone pump the breaks slowly till the valve centers and re-tighten. bleed the breaks with a vacuum pump farthest to the closest.

 

A '72 F100 won't have a proportioning valve. It would have a pressure differential brake valve.

Proportioning is a function for the rear brake circuit, that was incorporated into the brake valve for discs front/drums rear trucks, beginning in 1973. Proportioning was one of three functions in the '73-up brake valves. The other two functions in the '73-up brake valves were pressure differential sensing between the front and rear circuits and metering to the front disc brake circuit.

Under normal circumstances, there are only two things that will cause the warning light to come on; Either there is (or has been) a brake component failure, causing a drop in hydraulic pressure between the primary (front) or secondary (rear) brake circuits.

Or, the second thing that can cause the warning light to come on is opening up the hydraulic system --to replace a MC, wheel cylinder, flexible or hard brake line and then bleeding the brakes out. This can easily cause an imbalance in system pressure, front vs. rear, which can shift the valve and turn the warning light on.

To correct the problem, the pressure differential valve spool will have to be recentered. You'll have to determine which side it's shifted towards and open a bleeder screw on the opposite brake circuit, slowly apply pressure on the brake pedal until the light goes out, close bleeder.

Internal view of the brake pressure differential valve. (Upper half of photo).



Ford procedure to recenter the differential valve.

 

I few weeks ago I did bleed all the brakes with the hope that excess air might possibly be the cause, which didn't solve the problem. But you're saying to identify the side opposite the valve shift and only bleed that side, which I will definitely try next.

Excellent, thanks for the advice.

 

when you bleed the breaks did you do the pump pump bleed. open valve in jar of fluid and pump, or vacuum bleed.