I have a light on, its located on the bottom left corner of the dash. I assume this is a brake light. I first started the truck and the light only came on when cranking, got her pulled into the garage last week and its been there since. I have fluid and the brakes feel great (yes they are power). The only thing i've done since i pulled it in was play with the peddle a bit with the engine off (ensure they are holding pressure) and i pushed and released the emergency brake. I did fuel lines and oil change last night, started her up and now the light is on all the time. Can someone tell me what this light is exactly for (emergency brake or is it proportioning valve related) and what may have caused this to trip?

I have all new rubber brake lines to install, so i will be doing that and a brake system bleed. Will this "reset" that light if its a proportioning valve issue or do I need to do something? Thanks in advance.

Btw I do have a chiiltons coming for schematic reference, it just isn't here yet lol

 

its the proportioning valve, its on the frame by the cross-member under the engine if i remember right or was it on the cross-member, there should be two wires coming off of it.

When i bought my truck the wires were unplugged i plugged them in and the light came on, now i know why it was unplugged....

 

Yup, the spool in the proportioning valve has slid over and caused the light.

Do you have any leaks in the brakes? Wheel cylinder? lines?

 

No leaks, but when i picked it up i had to pump up the brakes. I'm assuming there may be air trapped in one of the lines from sitting so long (although the brakes work well). When I replace the hoses and bleed the system do I have to re-set the proportioning valve or does it do that automatically?

 

I forgot to add, they are not losing pressure, and after sitting I do not have to pump them. The poor girl has sat for a long time though.

 

Been so long, I think it should reset itself? Unless it is full of cruddy fluid and gets stuck.

 

When you replace all three (remember the one on the rear axle?) and bleed the brakes I think it will reset itself. Flush all that old fluid out with brand new stuff. Freshly opened bottle. Previously opened bottles good only for re-fill use.

 

It doesn't automatically reset.

The PV switch has a pin sitting on a detente on the piston. If pressure is unbalanced (as in a brake failure), the internal piston slides to one side and bridges the two switch terminals and voila.. the light comes on.

There is supposed to be a tool to keep the piston centered during brake bleeding procedures. The odd thing is that removing the tool would likely result in air being trapped in the PV. Doesn't make much sense to me.

The more common method to reset the piston is to unthread the switch and with a jeweler's screwdriver reach down the port to manually recenter the piston... one way or the other.

 

Yup got all three. I'll do the bleed this weekend and hope for the best. Appreciate the help!!

 

I don't mean to argue Hio but I've done it before. Not everytime,but possible. When bleeding and flushing alone didn't shut the light off I loosened either a front of rear line at the PV and had a helper push brake pedal down 'till light shut off. Once helper gets pedal all the way down (if) tighten the line back up while helper holds pedal there. No air back in system. If lamp never goes out I tightened that line back up and loosened the other end line. Helper on brake pedal again. Thereby using brake fluid pressure to move pintel to center.

 

If it is stubborn and doesn't want to reset this way, you can tap on it gently with hammer during the bleeding process.

 

Then you've had better luck than me pal. I've tried that to no avail.

 

Plastic low brake fluid warning lamp switch threads into the "Brake Pressure Differential Valve" and has been a notorious POS since day one.

Cracks develop within its threads causing brake fluid to seep out. When this occurres, the low brake fluid warning lamp comes on...and stays on!

1967/79 F100/350, Bronco and Econoline: There are two different types of these switches.

C7AZ-2B264-A = 1967 is one year only, same as all '67 Passenger Cars / C8AZ-2B264-A = 1968/79 is the same, same as all '68/69 Passenger Cars.

 

So i got the drums off finally, discovered the adjusters area all gummed up. The brakes seem in fairly new condition and the drums look like they were just turned. My question, if the front brakes were out of adjustment and not contacting the drum, thereby giving me only rear braking capability would this cause the brake light to trigger?

 

Possible but not likely. You might get lucky though.