I have a 1970 F250 2X4, with drum brakes. I recently upgraded to front disc brakes from a 1975 F250. In the fray of upgrades is a combination valve suited for the appropriate set up.

However, the brake warning switch on the combination valve has one wire, as opposed to the two wires on the original set up. I would like to reinstate the dash light, without going through the trouble of sourcing a switch that would work.

To start with I tested the switch on the valve by means of a voltmeter. With one probe on the terminal of the switch and the other on the body of the valve I found no continuity. With the switch depressed it has continuity.

Would it be correct of me to connect the one purple/violet wire to the switch and the other purple/violet wire to the body of the valve.

 

I am thinking not, but let me go to the site where the diagram is so I can read it and think about it a little bit. I'll be back.

 

Ok, this is what I can make out. The old brake system grounded the circuit to turn the light on, just like your new one is setup to do. I think what they did was run one wire to the switch, and then another one back out(basically the same wire electrically) and ran this to the ignition switch.

When you first turn the key, the ignition switch had a special contact that grounded this other wire, to light lamp during starting for a "lamp is good" test. After that, the wire to the keyswitch did nothing, if you have a problem with the brake system it would ground the 977A wire going to the bulb, turning the bulb on all the time.

I think what you should do is turn the key on, and then ground each wire individually as a test. One of the wires will turn the bulb on in the dash, the other one will not. The one that turns the bulb on is the one you want, hook it to the single terminal on your new proportioning valve switch. The other one that does nothing goes to the keyswitch, I would just tape it up and leave it disconnected.

 

Makes sense! Thanks you, again.