69mach1377

So ever since I bought the car in 1989, the light went on and since the brakes worked, I unplugged the connector near the master cylinder.
Now I'm wondering what the real problem is, any clues?

fastbackford351

Yeah, mine just lit up too. It's been going off and on and now it's staying more on than off. My brakes still feel real good and I've bled them nine ways to Sunday and still the light comes on. I'm starting to wonder if my switch is hosed.

Somebody posted a neat link here a while back that showed just how that brake light switch works.

69mach1377

Are you refering to the switch that activates the tail lights? I think that one is located by the pedal lever.

bop11

That light indicates that you have a problem with either the front or rear hydraulic brake lines. The device to which it is connected has a piston in it. On some it also houses the pressure equalizer. One side of the piston is pushed on by the front brake fluid and the other side by the rear brake fluid. Should one of the hydraulic lines to the front leak, the pressure will be lower on that side of the piston and and the piston will move to block it off. When the piston moves it grounds the electrical connector and turns on the dash light. The same happens if the rear hydraulic line breaks, but the piston move to the other direction and seals the rear so you don't lose all the fluid and turns on the light. The light comes on in either case. That your light is on, means that you are not using either the front or rear brakes. Probably the rear as they do not have the stopping power of the fronts and you would notice the skidding. The pressure regulator, which reduces the pressure to the rears to keep from skidding, may be built into the device or may be external.
You should immediately determine if your hydraulic lines and brake cylinders are leaking. Remember the piston seals the line so you can't judge by the amount of fluid in the reservoir.
To reset the piston you need to a0 fix the leak, b) open a bleeder on the side that was originally working and push the brake pedal until the light goes out. When the light goes out, the piston will have been centralized and both systems will, front and back, be working.
The light is a warning that you have a problem and should the good system have a problem you will have no brakes!

69thunder

Not entirely true, the switch can go bad. On the 69 the switch in the event of failure like above will trigger but will often stick so even when the system is fixed the light is still on. If the brakes are all working properly the switch has failed, very common on the 69' from what I remember.

69mach1377

Thanks for the all the info above...
I'm thinking the switch failed decades ago since I've been driving leak free since 1989.
How is the switch fixed or replaced?
I've never been happy with the braking on this car anyways.

bop11

69thunder is absolutely correct. Two failure modes are that the wire is shorted to ground or the switch is kaput. Disconnect the wire and if the light goes out then the wire is not shorted. If I remember correctly, the wire has its own fuse coming from the center terminal of the key switch and through the connector on the dash to the dash light back through the connector and then to the switch.. The block is about $80 from Laurel mountain. It is a safety device and running without it working correctly probably is not a good thing. It should be mounted on the inner fender panel close to the master cylinder. The book says to change it then bleed it by leaving the outlets slightly cracked open and then pressing the brake pedal to fill it until no air leaks out only fluid. I like to do it and bleed the system to get all entrapped air out. You can also center the piston by just cracking the outlet connection on the one side and pressing the pedal to recenter the piston.

69mach1377

Thanks,
The light does go out on disconnection, I drove it that way for 2 decades...
I'll look further into switch replacement if recentering does not hold.

67mustang302

That thing is such a POS. It could wear and leak, allowing pressure to escape from one system and into the other. They're also notorious for not resetting properly. They can trigger from doing brake work, you bleed one end of the system, then when you go to bleed the other side the pressure in the already bled system kicks it over and triggers the light. Or it triggers when you try to bleed the first side of the system.

The idea was to let you know that one end of the brake system failed, while keeping them isolated. It's completely retarded and thoughtless. If one side of the system fails, you'll notice it in the braking performance. If there's a catastrophic failure, you won't even notice the light come on cuz you'll prolly **** your pants as soon as you hit the brakes; and that little light on the dash will be of least concern. Yet at the same time, the unit itself can fail and cause the system isolation to cease, completely defeating the purpose.

I replaced mine with this:
http://www.wilwood.com/MasterCylinde...emno=260-11179

Proportioning valve, system isolator and brake light switch all rolled into one. I like the light switch as well, since it doesn't fail as easily as the mechanical pedal switches do.

69mach1377

That looks like a good solution as part of an upgrade. Like I said, I'm not happy with the braking now. It takes a lot of pedal pressure and several 'whoa boys' to prevent the need for my brown pants as it is.
I had 4 drums stock, but upgraded to front manual disks with no other changes...