Intermittent Brake Warning Light

[Swabadger]

Hi, I have a 1985 M1009 that has just recently started to have the brake warning light come on intermittently while driving on the highway. It does not come on while applying the brakes, and will be on for a minute or two, then go out.

The braking action is good, with no sinking pedal or need to pump brakes. No obvious signs of any fluid leaking anywhere and the master cylinder is not showing low fluid, and the fluid looks to be in good shape. It was converted to DOT 3 at some point (not by me) which I confirmed by mixing samples from the master cylinder with water and with DOT 5.

I haven’t bleed the brakes yet, as the light would make sense to me if there were air in the system and the light came on while actually applying the brakes.

The brake warning light does not come on when I apply the emergency brake. The wire is connected, but the light does not come even if I push the E brake to the floor.

Has anyone seen this before?

 

[Skinny]

I think the only things that will illuminate that bulb is the proportioning valve and the parking brake.

I think there is a micro switch on the parking brake assembly which you can test with a meter to see if it's closing when the pedal is pushed down.

The prop valve has a switch in it as well. So in theory which never happens, air will cause one circuit to lose pressure. The other circuit which has pressure moves a valve inside the prop valve. That valve blocks the bad circuit and turns the light on.

I do not believe the master has a low fluid switch.

This is all off of memory. I would disconnect both wires to each of these and see what happens. If the light comes on you have a short somewhere taking place. In other words, neither of the switches are causing the bulb to illuminate.

If the light stays off then suspect one of the switches. Plug one in and try it. If it illuminates the switch is eliminating the bulb for no reason.

Here is a funny story. My Burb had a rear circuit COMPLETELY empty. I'll say one ton brakes stop good if you don't even notice 30% of your capacity missing. Prop valve never did anything. Brake lamp never came on. Master was empty. Like nothing. No fluid leaks anywhere.

So forget the mystery of what the F happened. But the system did not work correctly so do think that prop valve or switch is actually doing anything.

 

[ssdvc]

That happened to me a while back. It turned out to be the parking brake indicator switch, which had come loose and needed some attention. After a tweak and some tightening, all was well.