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MG MGB Technical - Brake Check Switch

I have a 1971 mgb roadster. Is anyone familiar with the "Brake Check Switch" or "Brake Failure Check Switch" (Moss 141-280) on the upper left side of the dashboard? What is its' function? How does it work? How do you determine if it is working properly? How do you maintain it in proper working condition?

I am about to get my car inspected for the first time in 15 years, and I am not sure if this swtich is working correctly. Thanks for any information anyone has.

The Brake Failure indicator should only light if the shuttle valve located between the front and rear systems is moved off center. This would happen if either side of the system lost pressure due to a leak. There is a switch function in the indicator that is a press to test. That should light the lamp when pressed to check the condition of the lamp. Other than checking the lamp there is no maintenance needed for the indicator/switch. If it is on then you may have a brake system problem.
Bill Young


You are saying the switch will light in the test mode when pressed. That is not happening now. Is the lamp replaceable, or do I have to replace the switch? Or should I start tracing the wires to find faults? Thanks.


The bulb is replacble but shouildn't have burned out. More likely are dirty contacts on the plug.
Dan Robinson


You may have already found the problem with your brake pressure failure warning light, but if not, perhaps these hints will offer some assistance:

The PURPLE (always HOT, FUSED) circuit powers this light. The purple circuit also operates the bright light flashers, the horn, the interior light, the cigarette lighter...

One side of the light bulb in the warning light switch is connected to PURPLE. The other side of the light is connected to a WHITE/BLACK wire which runs to the warning light block (left inner fender). If the pressure between the front and rear brake circuits becomes too great, a valve moves and the switch is grounded -- and the light illuminates letting you know that since your foot is nearly on the floor and the brakes feel horrid that the brakes are, indeed, faulty. But, what if this warning light doesn't work? How will you know that your brakes are dangerous?

So, another WHITE/BLACK wire runs back to the warning light assy to a switch. The other side of that switch is ground. Now, when you push the switch the power runs through the bulb, to the switch in the brake block, back to the dash unit, through the switch to earth and the light illuminates.

By 1975 the Fed deemed us irresponsible or too stupid to check the warning light switch on a regular basis, so that wired the warning light to the starter circuit. So, after 1975, when you turn the key switch to START, the warning light illumninates. This is done through a diode. When the diode fails you can engage the starter motor by pulling up on the handbrake handle. Some improvement!

Hope this helps.

John Twist
University Motors Ltd

John H Twist

This thread was discussed between 11/08/2005 and 13/08/2005

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