Hey guys,About 3 weeks ago I took my 78' Trans Am across Florida for a weekend trip. Had a great time as the car ran great, also got some great pictures of the car near the ocean. The trip was around 340 miles, and besides having to fill up consistantly, I had no issues at all. After the trip I parked the car in the garage and let it rest for about 3 weeks. Yesterday I wanted to let it run so I decided to take it to work.About 10 minutes into my drive my brake light came on. On my drive to work the brakes felt normal though. When I arrived, I checked the fluid in the master cylinder reserve, it was filled to the top. I checked the insides of each rim to look for a leaky wheel brake cylinder, but everything seemed dry underneath.I drove the car out for lunch and back, then back home that evening. The entire time the brakes felt fine. When I got home I checked the vacuum pickup from the manifold to the brake booster, seemed fine. I also jiggled the check valve to see if it had gotten stuck. Checked the fluid again and it hadn't moved, still full.Any ideas what this may be?
Make sure your emergency brake is released fully. Try that.
Your brake light is about your parking brake.. Not sure about a 78, but my 71 has a switch that the parking brake itself works. Once the brake comes off the switch when I depress it, the switch closes and gives me the light. If by chance the mounting for the switch has moved, or the wire has grounded out, it'll make the light stay on.
That was my first thought as that is usually the issue with modern cars. Yet the sensor for that light is right on the master cylinder, so I doubt that's what it is.
The switches at the emergency brake pedal (under dash) and proportioning valve (under hood) are both in control of the dash lamp. Since you have eliminated the hydraulic failure possibility, the only other solution is the park brake lamp switch or the centering switch on the prop valve. I haven't seen any old Pontiacs with a sensor on the master cylinder.The pedal switch is where I would look first, it is a fragile part and could easily vibrate loose or break off if corroded. Also, a quick external visual inspection doesn't always reveal a leaking wheel cylinder, you might need to pull the drums off. For what its worth, I have never seen a caliper leak but wheel cylinders are a different story. If you find one bad, better replace the other too!
I haven't notice any fluid loss from the master reserve though either, so I'm doubting a leak. Also, since I've bought the car (about 3 years ago) the brake light on the dash never reflected what position the E-Brake was in, so that fragile switch may have been broken for years now.Do you think that the brake booster itself is slowly dying out? As far as I know it is the factory unit, making it about 34 years old.
Sure, your booster could be feeling it's age. That wouldn't cause the dash lamp to come on though. I'd bet the lamp issue is caused by either that switch or possibly a wiring problem. If you think about it, theres quite a lot of wiring involved for just that circuit. There may be a ground problem or something simple, but that lamp could be on the printed circuit behind the instrument cluster and those are a common problem area too. Best thing to do is lay eyes on everything thats involved in the circuit, then go from there. That will involve getting your head under the dash with a flashlight so yeah it might hurt a little. I'm not much good on wiring troubles personally, but if you take a multimeter to the car and go at it for half an hour, I bet you can get some idea where the trouble is.
Unplug the wire on the proportioning valve that's under the master cylinder if the light goes out the proportioning valve is not centered .The back brakes are probably not working right .Jack up the car and see if back brakes are working . The adjusters might not be working or you could have a leak ?
An easy test of the proportioning valve is to unplug the wire, if the light goes out that's is where the signal is coming from, that means the plunger inside has been pushed one way or the other, that can only be caused by one thing, a lower pressure on one side, that means you have a leak or the shoes are so far out of adjustment that the wheel cylinders had to travel farther to make the shoes make contact with the drums, not likely due to the fact that you just drove it, unless the car has been sitting for a long time and the rust on the drums ate the brake lining off, again not likely. So that leaves only one explanation, you have a leak in the brake system front or the rear. Like idrivejunk said the parking brake and the prop valve is the only place the signal comes from, my experience with this issue lies with the prop valve and they never lie, unless you have been using the parking brake, you have a leak. How the proportioning valve works, the plunger inside is held in the center by two spring one on each end, in the event that the oil pressure becomes lower on one end or the other, the higher side pushes the plunger to the low side making contact with the terminal inside grounding the circuit. Sometimes on old units the plunger gets stuck, then after fixing the problem, it will still send a signal, so you may have to take it apart and clean it or just replace it, sometimes you can use a small screwdriver and push the plunger back and forth to free it up, either way, they don't lie, start searching.Sometimes their just hanging on the lines just below the master cylinder or mounted on the frame just below, on Chevy trucks they where mounted on the front cross member behind the radiator support.An early leak will be detected before you feel it in the peddle.Another possibility rather than a leak, the master cylinder has a check valve at each port going out to the lines, their designed to hold a small amount of pressure on the line to keep the pistons in the calipers from sucking back, if dirt gets in the valve from the bottom of the master cylinder they can't work, so when you push the peddle the piston have to travel farther to make contact, same as if you just completed a brake job. Again, not likely.Back in the 70s I was taught by my Ol-man how to rebuild all of the above +, that was how you did it, parts where not cheep and all the part suppliers had the rebuild kits on the shelf, no *** or China stuff, that includes alternators and starters, you only bought new when it wasn't repairable. Now we have companies that just rebuild that stuff, (remanufactured). GTO guy
Well, I just got back from playing with the car. I put ti up on 4 jack stands tonight, took the tires off, ran it and pressed and held the brakes. I looked for any type of wet spots, but everything was completely dry. Now the fluid level has yet to move either and the brakes still seem pretty strong for the most part. I drove it around my block at around 10 mph and slammed the brakes and it stopped pretty strong (for being an old car). Maybe one of those springs is stuck in proportional valve....
Hey man- sorry if your car doesn't have a parking brake light switch, I know I have been barking up that tree pretty hard. I can't find the part listed anywhere for 70-81 and I don't have one to look at to find out what tree I should be barking up. I did have a pedal assembly in my hand the other day but my memory isn't photographic. In any case, there will be some sort of device that lights the light when the emergency brake pedal is down. Most designs are prone to corrosion and deterioration. I'll stick to Grand Prix questions from here on out.You could always have an assistant hold pressure on the brake pedal while you loosen the lines on the proportioning valve one at a time to see if the light goes off due to the valve having been off-center. If the valve sticks without there being a hydraulic pressure loss ... that in itself is a safety issue (it could just as easily tell you theres not a problem when there is) and would lead me to the question: How old is the brake fluid?
If you have never checked your brake pads and shoes, this would be the next step that I would take....worn out brakes WILL set the light....
I got that Tin Indian GRIN!!
Here's a photo of the emergency brake switch on a 78 Firebird .
right year.. wrong cluster..the TAN wire has only 3 connections..under the hood at the brake safety valve.. under the dash at the parking brake switch as shown above.. the switch shown is turned ON.. as the plunger is not depressed and the collar is in contact with the bracket.. this will ground the tan wire and turn the light on..if both switches are disconnected.. and the light is still ON then there is a short in the wiring.. if you look carefully at the diagram above.. the PINK + circuit supplies power to the brake warning light bulb way up to the right .. the other side of the bulb contact comes to the 5th position up on the right at the connector.. warning. this is a NON GAUGE TYPE CLUSTER Diagram.. there are 2 for each car.. with or without gauges. the wiring connector is the SAME>. but the wires inserted into the connector are IN DIFFERENT LOCATIONS>.if you have installed a GAUGE type cluster instead of the idiot light cluster .. then you will need to tell me or post that info.. i will dig up the second diagram and compare where the wires go..