Wire question - brown wire going to proportioning valve
 

What is the brown wire going to the proportioning valve on my 72 with power brakes? I am installing a new Painless Wiring harness into the entire truck, and there is no mention of this wire in the manual.

It's the brake failure warning switch.

That's for the brake light in your dash. Not sure of all the ins and outs, I guess if you lose pressure ( I don't know how much pressure it takes to operate ) the light comes on.
Mike

It should go to the ignition switch, and then to the cluster I think.

I think it is triggered by a large differential between front and rear brakes.

There is a toggle switch inside of the Prop. valve with a diaphram that is pushed to either side by a large difference in pressure on one side or the other it grounds the signal and turns the light on. Also a grounding Emer. brake switch can be tied in to the wire to let you know that your E brake is....not that any of us have ever drove anywhere with our E Brake on!!!

Yeah, mine came on when I had manual brakes right before they started going out on me (before the power brake swap). I was quite impressed that after 30 years it actually still worked.

After 35 years, mine actually came on last November. All of the fluid from the front half of the master cylinder had leaked out, and I fired it up and moved it outside the garage. When I hit the brake, it went to the floor and the light came on. I was like....Woh!
:eek: :eek:

How do you check it to see if it still works?

To check if it still works, simply disconnect the wire from the valve.

I had to replace my prop valve because, I guess, the old one wore out.

TIP: Your old proportioning valve makes an EXCELLENT trotline weight.