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MG MGB Technical - Brake Light Stays On

The dash brake warning light stays on. The rear light does not stay on but works as it is supposed to. I have relieved pressure from the front brake lines and have checked the master cylinder fluid level as well as the hand brake. When driving the brakes do not seem to be hanging up. The only thing I have not done is relieve the pressure on the rear lines.

Any ideas will be appreciated!

Jim McDowell

Hi Jim.

What year is the car ?.

Did you do something to the braking system before this happened, if so was it to the front or rear brakes ?.


The later cars also have a switch on the handbrake which lights up the dash light when it is on. It's as small switch on the bottom of the mechanism and I imagine they could go sticky maybe and be stuck on. Easy to check, just unplug it from the loom. That same dash light is driven by the pressure differential failure switch on the master cylinder too. As Don says it makes a big difference what year you car is since the system changed a bit over the years.
Simon Jansen

It's a 77 MGB. Will take a look at the switch on the parking brake when I get a chance. As for what I was doing before the light came on, I rebuilt the engine, transmission and re-wrapped all the engine wires. Problem surfaced when I started the car and took it for a spin.

Safety Fast
Jim McDowell

"... rebuilt the engine, transmission and re-wrapped all the engine wires."

This might be a clue. Perhaps there has been a short created. A wire pinched during engine installation? Or frayed insulation disturbed while wrapping the wires?

Good luck. BTW, I hate electrics.

Derek Nicholson

Jim. As Simon notes, there is a two wire circuit on the lower, left side of the master cylinder. This goes to the brake failure warning light switch. To test if this is the problem, remove the wires from the switch. If the light goes out, there is the problem.

If that is the case, the switch is one which is normally in the on position and is held in the off (open) position by a bar in the master cylinder. The bar has two grooves, one in front of the switch and one behind the switch. When there is a difference in pressure between the front and rear brake sections of the master cylinder, the rod is forced in the direction of the lower pressure, allowing the switch's metal plunger to move upwards and trap the system and leaving the light in the turned on condition. Reset is accomplished by lowering the switch several turns (allowing the spring in the warning light system to return the bar to its central location), then, turning the switch upwards again into is normal position. At that point, reattach the wires and see what happens. You can get rust in that portion of the master cylinder, just as you can in the main bore. In some cases, this causes the rubber O rings to degrade and leak. It is also possible that the rod could, if there is sufficient rust, stick in position.

Let us know what you find.

Les Bengtson


Will finally get a chance to look at the car this weekend. Will try both suggestions and let you both know what I find out.

Jim McDowell

This thread was discussed between 15/05/2006 and 18/05/2006

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