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MG MGA - Brake Light Switch

While on a road trip with my car club, the following MG told me my brake lights were out. All other lights work as required. Back home shows power to the brake light switch.

I have SILICONE brake fluid in my system and we have all heard about the "Urban Legend' of how this causes premature brake switch failure.

My question is a dumb one but I am having a senior moment.

When I repalce the switch, will I have to bleed the system due to air entering the 4 way brass connector where the switch mounts?
Gordon Harrison

You may not need to bleed it at all. Otherwise install the new switch finger tight, then have someone press down the brake pedal, When fluid starts to ooze from the threads on the switch, tighten it up.

After using a NAPA switch that failed every two years with the silicone I ordered one from Moss, eight years, no problems yet
J Heisenfeldt

I'll go for a record attempt- 20 years with silicon fluid and the same brake light switch.
dominic clancy

Dominic - you win, so far. Mine is just 13 years with silicone fluid (Automec) and the same switch.
Bruce Mayo

Thanks guys, Gordon
Gordon Harrison

A technique that I've used to minimize leakage from the brake system when working on individual fittings is to place a couple of layers of plastic wrap film over the open M/Cylinder top and then screw the cap back on. (Remove the plastic when the job is finished!. Seems to help, although I haven't tried it with brake switch replacement. Mine is at least 37 years old and running Silicone fluid for about the past 5.
Tom Heath

Gordon - The problem is not silicone fluid as two people have attested to, the problem is crappy replacement switches. There are two ways to insure that the replacement switch will last. First make or purchase a relay/arc suppression circuit and install it at the same time you install the replacement switch. For instructions on making and installing the relay/arc suppression or ordering one can be found on my web site at: Click on Other Tech Articles, then on Brake Light Relay. The second method is to order switch, P/N SW 32 from Ron Francis Wiring at: This is a heavy duty switch that others have had good success using. Cheers - Dave
David DuBois

Gorden - You say that there is power to the switch and are you assuming that there is no power after the switch because the rear brake lights failed? You may have done this already but have you had someone activate the brakes with the ignition on whilst you test the output side of the switch? There are a number of bullet connectors at the rear of the car for the brake wiring and these can give problems since they are exposed to the weather etc.
J H Cole

Just had a look at the SW32 switch shown on Ron Francis Wiring website which David posted. What kind of connectors are on that switch? The switch in my car has spade connectors. I've also seen them with screw connectors.
Andy Bounsall

Andy - The switch from Ron Francis comes with it's own connector, which is different from the ones we are used to. Cheers - Dave
David DuBois

I had two brake light switches fail within 12 months of replacement. On fitting the third, I had a relay fitted and this seems to have solved the problem. I understand it inhibits excessive arcing and carbon build-up across the switch contacts.
Joe Gates

This thread was discussed between 15/07/2008 and 18/07/2008

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This thread is from the archive. The Live MG MGA BBS is active now.