Parking brake dash light

[Tucson38]

Question about the parking brake warning light on the dash ('85 GMC). Does this light only indicate when the parking brake is on or can it indicate a problem with the main braking system?

Mine starting lighting up and turning off randomly yesterday although the parking brake was off. I'm concerned that it could indicate a fault in the main braking system or does it only indicate that the parking brake is on? I checked the fluid level, it was normal, no leaks anywhere.

[TexasJeff]

Mine is any 1980 GMC and I have a wire that goes to the proportioning valve that I had to unplug because it caused the brake light to come on and stay on, to this day I have never been able to figure out why.

[Dead Parrot]

The light does both. Your park brake may be out of adjustment or the switch may be going bad. Or you could have master cylinder going bad. The master cylinder can go out and the fluid level be normal.

I think there is some kind of reset switch on the prop valve. Don't know if it is for the brake warning light or it resets the valve itself somehow.

[Cape Codder]

Stepping hard on the pedal will reset the light sometimes. Best done while parked. It is good to test the brake lines on old trucks once in a while anyway.

[Tucson38]

Thanks to everyone for the responses. Appreciate the input.

[Ice&burban]

Mine came on the other day, I was wondering the same thing. I was teaching my son how to parallel park my suburban, he hit the brakes hard a couple times, then I notices it was on. He did really good for his first time trying to park a land yacht. I'll look into some of the suggestions, but if anyone else some ideas, I'd be glad to hear them as well.

[Tucson38]

Dead Parrot made the call, it was a master cylinder. I was launching my boat and the brakes wouldn't hold, had to hit the emergency brake. That got my attention.

Anyway, got it replaced.
Life is good.

[chengny]

There are two switches used in the brake warning circuit. If the contacts in either switch are closed, it will create a path to ground and illuminate the BRAKE light on the dash.

One of them is operated by the parking brake lever and closes when the pedal is pressed down (or is hanging down). That is more of a reminder that you need to release the parking brake. If not, you'll wear out your brake pads/shoes and when you press on the gas pedal the truck will tend to go really slow.

Now the more serious of the two causes of the BRAKE light coming on:

There are 2 brake lines that exit the master cylinder. Before they lead off to any of the actual brakes they go to the "brake proportioning/combination valve". It is there that the 2 main brake lines from the master cylinder are split into 4 lines. Those four lines then go to the individual wheel cylinders. The internals look like this:



There is an electrical switch built into the proportioning valve. The switch is operated by a spool that is normally centered within the valve. If there is a loss of hydraulic pressure on one side of the valve when the brakes are applied the spool shuttles (either the left or right depending on which braking circuit loses pressure), closes the switch and the warning light on the dash illuminates.

From what you describe (having to pump your brakes to get pedal resistance, your brake warning light is lit and it requires more effort to stop the truck) there is most likely a serious problem in your hydraulic system.

First thing is to pull the cover off the master cylinder and check the brake fluid levels in the two reservoirs. You will might find that one of them is either very low or empty. You can try to refill it and maybe the brakes will self bleed. It may take a lot of pumping, but these brake system are very forgiving about entrained air. Very often, if the air is near the front only, the air bubbles will just rise to the master and vent out.

But - no matter what - if you find low levels in the master, you will have to find out where the brake fluid went. You should immediately inspect your brake lines for leakage.

[chengny]

This is to Ice&burban:

While I was still working on the post above, the administrator - for what ever reason - decided to revoke my right to edit my own work. Ignore it. It was written in response to a guy whose brakes were soft and had a brake warning light. Your situation is different. This is where I was when they cut me off:

Before doing anything else, find out for sure what is causing the brake warning light to be on.

Here is how to determine which switch (proportioning/combination or e-brake) is causing the brake warning light on the dash to illuminate:

Both of the switches are normally open. The lamp in the dash is always hot on the positive leg. When the contacts in either switch close, a path to ground is created on the negative side and the lamp lights up.

To determine which one is creating the path to ground - and lighting the dash warning - you are going to pull the harness connector from each switch (one at a time).

Do the prop/comb valve first - it's actually easier to get to than the e-brake switch.



If the dash warning goes out, that switch is closed and you have identified the problem. If it stays on, plug the harness connector back on to the and go under the dash to the e-brake switch.

Pull it off



If the warning light then goes out - that switch is the problem.

If it stays on, leave the harness connector off and go back to the prop/combo valve. Pull that harness connector off.

If the warning light goes out only with both disconnected - there is a problem with both switches (that is so rare you shouldn't even consider it).

If the light remains lit even with both switches unplugged - you have a wiring issue. There is a short to ground in the negative leg of the lamp circuit.


So , at this point you should know which switch is causing the warning light to be illuminated. I am guessing it’s going to be the one in the P/C valve. If it's the e-brake, make sure the pedal isn’t just hanging down under it’s own weight and pushing on the switch.

But even if it is the P/C valve switch, no worries. From what you describe - the warning light came on when your son nailed the brakes – so there is most likely not a serious problem in your hydraulic system (i.e. a big leak). That switch can also be triggered by sudden, violent application of the brakes even if the system is tight.

First thing to do is to pull the cover off the master cylinder and check the brake fluid levels in the two reservoirs. There is no reason to believe they will be low, it’s just procedure. If you find that one of them is either very low or empty, that is part of the problem. It’s also an indication that you probably do have a leak - just not a big one. Refill the reservoir if it was down and continue. If the levels were normal, there is no leak. The switch just tripped to the alarm position due to the hard brake application.

But - no matter what - if you do find low levels in the master, you will have to find out where the brake fluid went. You should immediately inspect your brake lines for leakage.


Next you are going to want to reset the switch to clear the dash warning light:

I have always been able to get them to reset themselves with one real hard stop.

Make sure your M/C chambers are full.

Then go out and drive the truck - at the first stop sign, red light or even if you just find a quiet stretch of road - slow down to about 5 mph and then suddenly apply the brakes. I mean hit them hard. If the first time doesn’t work, try it again and SLAM the brakes on.

That should do it. One hard brake application from a slow roll will usually center the piston in the valve, open the switch and extinguish the warning light. You’ll have saved yourself the hassle of diving under the truck to open the lines .

[Tucson38]

Thanks Chengny - you are well versed on this. Appreciate your taking the time to respond.

[Ice&burban]

Thanks for all the information chengny, very well detailed. I'll get my son out to give me a hand going over everything, this way he can learn.

[gramps77]

ive tried everything listed above and find that when I disconnect the proportioning valve the light goes out. Ive tried the HARD braking and the lines are absolutely free of any air, all I get is good clean fluid out of all 4 ports. Brakes were bled with the button depressed at proportioning valve. What else should I try? Or should I assume at this point the valve is bad?

Tucson 38 thanks for the post