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MG MGA - Brake Light Switch

I have a 1600 where I have to push "really" hard to get the brake lights to come on. It is probably the switch on the 4-way connector. Has anyone come across a fix for this?

Best regards,

Michael C.

Just replace the switch. It's very easy and usually doesn't require any bleeding afterwards. Once replaced, you can set up a relay to take the load off the switch. There's a ton of info on this in the archives.

Mark J Michalak

I replaced mine with a GM mechanical switch that I mounted using an angle bracket clamped to the steering column. It contacts the brake pedal arm and the brake lights come on as soon as the pedal moves.

Ed Bell

Michael is correct. The new switches seem to burn out quickly. That is why some people add a relay to take the load off the switch. To replace the switch have the new switch ready when you remove the old one. That way you can quickly change the switch and not loose a lot of brake fluid. You will loose a little brake fluid but as Michael states no air should enter so bleeding should not be needed. Any hydraulic brake light switch with a 1/8th pipe thread connection will work. NAPA for example sells one with blade terminals and to use it you would add crimp connectors to your wires.
R J Brown

I don't recall if the MGA has the same thread for th switch as MGB, but if so then Ron Francis sells a quality switch that screws right in. You need to crimp on the included connectors and it looks a bit different than original, but it's held up well on my MGB even with silicon fluid, which kills the current repro Lucas-style switches.
Steve Simmons

Silicone fluid doesn't affect a brake switch. It's just that some recent production switches have cheap and terrible contacts that fail early with normal current. I wouldn't install a relay. That's adding complexity to treat a symptom rather than fixing the problem. If necessary I would go out of my way to find a good switch. - $.02
Barney Gaylord

I did something similar to what Ed did and it has worked flawlessly. There was an article in the MGA magazine some years back. The article is in Mike Ash's tech book published by NAMGAR. It also has part numbers.
Bill Haglan

At least from my experience, this is one of the few parts that rarely fails and if you can find an origional it's probably better than the newer repros (a bit like the fuel sender).

Not that they are in every breakers yard today, but the same unit was fitted to many cars of the period including: Austin Cambridge and Westminster, Magnette, Morris Minor, Riley 1.5, Rover 75 and 90, Sunbeam Rapier, Triumph TR2 and TR3, Singer Gazelle and Austin Healey Sprite and 3000.

Neil McGurk

Barney, I've heard it reported from several sources (including those who well them) that Silicon fluid will attack the current crop of repro Lucas switches and contribute to early failure. I don't know the extent of testing to determine the problem but I assume it involves disassembling the switch.

The most common failure from the fluid is the lights getting stuck on. This actually happened to me a few months back on a car with Silicon, at which point I changed to a switch from Ron Francis. So far so good! :)
Steve Simmons

I don't know that bringing up the issue of silicone fluid causing the brake light switches again will do anything to dispel the myth or not. Silicone fluid does not cause the brake light switch to fail. If any fluid, silicone or other were getting past the diaphragm in the switch, one would see brake fluid oozing from around the terminals as the terminals are not fluid tight. The sad fact is that the replacement switches available today are junk plain and simple (I had one last all of two weeks and it was a mechanical switch used in a MGB that was nowhere near brake fluid of any kind). If the failed switch in Michael's car was an original switch, it probably failed of old age (Michael, the symptoms you are experiencing is a classic failure mode). Over time and untold thousands of cycles, the switches just lain wear out. The bad part is that now you have to find a way to make he replacement switch hold up as long as the original switch did. You can put a relay/arc suppression circuit in with one of the switches available from Moss, Victoria British, NAPA or where ever you find one (see my article at:
or, as Steve suggests, you can spend the extra bucks and get the switch sold by the Ron Francis Wiring Co. at: The part number is SW 32. Either way, the switch will then hold up as well as one of the originals.

The thread on the switches is the same 1/8" parallel pipe thread on all of the switches used in the TDs through the MGBs.

By the way, I purposely contaminated the contacts of a relay with silicone brake fluid then hooked up the relay to a pair of brake lights and a switch to activate it. Over a period of months, I would cycle the switch numerous times a day to see if the fluid would cause any degradation of the contacts - non was observed. Non scientific as it may be, I am convinced that silicone fluid doesn't cause any problems with the switch contacts. However, in the switch that I cut open the extremely flimsy contact that I saw would definitely contribute to a short life span. You can see the discoloration in the picture below.

David DuBois

The tech department at Moss believes otherwise, and supplied the information I quoted above! I would love to settle that debate once and for all. Regardless, I'm not using those garbage switches until better ones are offered whether or not I'm running silicon!
Steve Simmons

David's experiment could possibly be the only 'scientific' analysis that has been done on the brake light switch and that the silicon myth is based on association.

It would be interesting to see how many switch failures there have been on cars with conventional brake fluids. Using my argument of association and going back, say 25 years or more, every brake light switch failure in those days was caused by conventional brake oil contamination! Also, every brake seal failure was caused by conventional oil contamination! Sticking my neck out I would be surprised if the modern day figures did not come out close to 50-50 on the assumption of equal silicon/conventional users. It's just that the silicon instances get reported, conventional do not. It's nearly always that way with any product in every walk of life.

As an aside, but relevant to this thread, just be careful when undoing the switch. If it is overtight, the whole bracket can rotate and twist the attached brake pipes - as I found out when I tried to replace the switch about 5 years ago. I ended up having to disconnect all the pipes, undo the connector from the chassis and remove the switch in the bench vice.

Steve Gyles

I had that experience too Steve,
By the time I'd got the non-functioning brake switch off, I'd managed to twist the bracket. Fortunately I'd already removed it from the car so I didn't damage the brake pipes, but I had to re-bleed the system.
Dan Smithers

Okay, one more bite. I have been using silicone fluid in my MGA since its first restoration in 1986. I don't remember if I installed a new brake switch at that time, but I do know it failed (high force required to actuate) 130,000 miles later in 1998. The replacement switch now has 84,000 miles in 10 years and is still doing fine.

If you want to blame switch failure on any kind of fluid, I'd guess any kind of fluid has an equal chance of doing it. My common sense says that recent repro switched are poor construction, and subsequent failures have nothing to do with the type of fluid used, as teh fluid does not leak into the contact area. I have seen a couple of these recent production switches cut open after failure, and the contact design is just garbage.

A brake pressure switch that sticks in the ON condition must have the spring diaphragm snapped over center to not return, which would be a manufacturing defect. If the contacts were welded together, that would also be a manufacturing defect or design deficiency. Another possibility would be fluid leaking past the internal o-ring seal to fill and pressurize the contact chamber, but that would require the contact chamber to be sealed on the riveted side, which is very unlikely. In any case, a leaky o-ring would also be a manufacturing defect.

The only way to attribute a leaky o-ring specifically to silicone fluid would be to have some elastomer material that could be attacked by silicone fluid but not affected by other brake fluids. I find that very hard to swallow, as silicone oil is a natural preservative for most types of elastomer material.

The real issue is wanting Moss Motors (or other suppliers) to investigate the switch quality issue, to discover and let us know exactly what problem is (was) causing the premature switch failures, and assure us that they have somehow changed the switch design or manufacturing techniques to eliminate the problem. It does no one any good to continue to sell the same crappy switches indefinitely with no attempt to fix it. When they know the switches are poor quality the only reasonable approach is to trash the whole lot and not sell any more until the problem is fixed and new problem-free switches are produced.

After that statement I am not going to duck. I welcome any critique of the logic and any discussion of what else can be done to eliminate the junk switches from the market place.
Barney Gaylord

A impact wrench will remove a stop light switch without spinning the 4-way. There are lots of times a overly strong impact gun used to disassemble things is much easier on the pieces that trying to twist them apart. I often use a 1/2 inch gun to remove corroded battery terminal pieces. The gun zips them apart where a wrench would just twist things into a mess.
As for they silicon debate I agree 100% with what Barney wrote above. IMHO there really are no good reasons to use the "Paint Thinner" in your brakes. One of the reasons to use silicon is it is less damaging to rubber. Regular brake fluid tends to soften and swell rubber pieces much more than silicon. That makes it better for the diaphram in the brake switch not worse.
Just to see what it looks like I just ordered a switch from WorldPac for $10.76. The brand is listed as APA part # SMB423.
R J Brown

"The tech department at Moss believes otherwise, and supplied the information I quoted above"

I have seen similar statements from other manufactures. I would like to have them explain why one of the mechanical switches they sell, that is nowhere near brake fluid of any kind fails in the exact same mode after only two weeks, or why, if silicone fluid is causing the brake light switches to fail, why the same, wimpy switch that I installed in our TD with a relay/arc suppression circuit installed has not failed in over 8 years, even though is is operating with silicone fluid or why the original brake light switch in the TD took 25 years to fail after installing silicone fluid. Barney's experience with original Lucas (red box) switches, the mileage at which the original switch failed and the mileage he has accumulated on the replacement switch (I am assuming that this is also a red box Lucas switch, pretty well parallels my experience with the switches and silicone fluid. My own feeling, after having fought the battle with cheap brake light switches and my experience with electronics and the failure modes of various components, is that statements like the one that the Moss Technical Department is handing out is an attempt by the manufacture to shift blame for brake light switches onto the owner of cars using silicone brake fluid. I seriously doubt that any tests have been run to confirm the statement. I would be very interested is seeing the test, how it was run and the results if they have been run. Until someone comes up with some documented test that proves the silicone fluid causes the problem, I will stick by what I have found with my own experimentation and my relay/arc suppression circuit. Cheers - Dave
David DuBois

My BGT has been running silicon for many years. The last brake switch lasted from June 2004 through September 2007 over a run of 20,000 miles. It was an original Lucas. At that time it got "sticky" where it would stay on unless you allowed the brakes to remain unused for a spell, then it would eventually switch off.

In the older cars (T-Series and earlier) there has been much talk over the years about older rubber bits (new old stock components) not reacting well to silicon. I suspect the problem with my last switch, reportedly being a NOS Lucas unit, is that the rubber swelled or began to degrade, which cased enough friction to not allow the spring to return the "plunger disc".

I'm not saying the silicon was at fault, but it would certainly fall into the theory. I plan to open the bad switch for inspection and possible repair in the near future. I'll report back what on what I find!
Steve Simmons

I still have an original Lucas switch installed - it's been bathed in Si fluid since 1988, and still works perfectly.

I just installed a used Lucas ignitions switch to replace the third repro "Lucas" switch in four years. The old key still works (worked in all the other switches too) and it has a significantly snappier action and less slop.

It seems to me that the problem is with the quality of the switch contacts generally. Have also had problems with repro starter switches (now back on an old one), repro fuel sender (on a modified Morris Minor one now), direction indicator can (also found an old one that worked).
dominic clancy

Thank you all for you comments! The current switch is from Moss and it lasted for about a month with DOT5 fluid.

I would like to see the suppliers investigate the issue. I'm sure they know.

Best regards,

Michael C.

I've had two of the Moss switches fail within 5,000 miles. I finally used a vintage Lucas mechanical brake switch at the brake pedal after making a bracket for it. This is a serious safety issue and Moss should 'fess' up rather than continue to sell switches that fail and possibly cause an accident. -M.S.
Martin Straka

This thread was discussed between 12/01/2008 and 23/01/2008

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