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MG MGA - Brake Light Switch

Hi All,

Earlier today while driving I had to brake heavily after doing this the brake lights remained on (when ignition is turned on).

The switch has been in the car for at least 14 years, have always used DOT4 which is changed every 18 to 24 months. Distance travelled approx. 70,000 miles.

I connected an ohmmeter across the terminals and got a reading of 0.4 appears as thought the switch has remained closed. Checked for residual pressure in the brake lines - none. The car rolls easily on a level floor.

Suggestions on next steps

R J Goebel

Disconnect wires at switch. If lights go out, replace switch. Reconnect wires.

Also check the play in the pedal to M/C rod. It may have jammed in the M/C. Lift the pedal, lights may go out. If this is the case, disconnect rod, pull it free and re-adjust. If not replace the switch.
... CR
C.R. Tyrell

I would take the opportunity of removing the wires from the hydraulic switch and run them to a mechanical pedal operated switch - just leave the old switch in the four way union to plug the hole. In the last month I know of 4 hyd switch failures. I fitted a mechanical switch years ago and have had no problems since.

Colin Manley

If that's a replacement switch, 14 years has got to be some kind of record. Barney is replacing them every few weeks with the mileage he's driving.

D Rawlins

I may be sneaking up on better news on the pressure switches. Having failed seven of them in the 4000-8000 mile range, I now have one still running at 12,000 miles in 3 months (with fingers crossed). It is a Duralast RB401 (made in Spain) purchased from Autozone.


If that one fails, I have waiting in the wings a new old stock switch,
Park Remax P118 made in England.
Other numbers on the box are:
Anyone recognize the number format or who the vendor may have been?


Ron Francis offers quality low pressure switches. I've been running one in my A for many years. It looks a little different from stock, but they don't burn out.
Steve Simmons

The Ron Francis switches have been very good (should be for the price), but I have had a couple of reports of premature failures. Without know the particulars about the failed switches, I can't tell what the cause of the failures were, but I found it a bit unsettling that they had failed. Cheers - Dave
DW DuBois

Why not fit a relay to the brake light switch?
J H Cole

This question has been answered many times. If we can find a good switch,no one will ever have to modify the car to install a relay (not to mention extra wire connections and possible relay failures).

Any switch will eventually fail, and every manufacturer will suffer defects. I suspect that the load on the switch determines its life span, and the cheap Lucas reproductions currently being offered cannot handle the load that we need to place on them.

I have a new Lucas switch on my TC which has only two small LED lamps as brake lights. The switch has only been replaced once, presumably because there is virtually no load. But on the MGA, MGB, etc I run the Francis switches because they last a lot longer on those cars.
Steve Simmons

And, the Ron Francis switch is compatible with silicon brake fluid.
George G.

31802/34542 look like Lucas part numbers.

I do have a NOS Lucas screw terminal switch, which I might be prepared to part with, if anyone is interested?
Dave O'Neill 2

Barney, did you ever have the chance to try the "intermotor" switch that some people were recommending?

D Rawlins

Apparently not. It is not on my list of failed switches (yet). I have one additional switch in my trailer, but don't feel like digging it out at the moment just to check the brand name. You should ask if anyone else has recently put at least 10,000 miles on an "intermotor" switch.

I replaced the original brake light switches in both our TD and MGB (both hydraulic switches) with Intermotor switches - they were just about the worst of the lot that I tried over the years. My fix has been a relay.arc suppression ckt on our cars - no problems in the last 10 years. Cheers - Dave
DW DuBois

Reading this I am very glad that I still have the original one on my high mileage A. I presume I also have a spare somewhere from the the Coupe I broke in 1972. Before anyone asks I am not parting with it!

Paul Dean

Removed the old switch today. It is marked "Lucas England" and a number of "A58" or "458". with splade terminals. My 1600 is a late 1960 Australian build, so don't know if it is original fitment or a replacement, I have owned the car for 14 years (so at least that old). According to the vehicle history it has been on the road through most of the last 55 years.

As the failure was the switch remaining on rather than the usual recent failure types, my plan is to cut the switch open, and post a picture.

A difficult removal, in the end I had to remove the 4 way "part" hold it using a vise and use a large spanner, before it finally came free.

I have a replacement switch, not Lucas, that has been in a packet for a long time, looks like packaging from the 60's. So hopefully I will get at least another 14 years of service from the new one.

Now the Question:

Should I use some thread sealant, or tape. One thing for sure I will not make it as tight as the old one.

best wishes

R J Goebel

never, ever, use sealant or tape on brake fittings. They are designed to seal metal against metal to stay leak-free when you stand on the brakes. If it leaks without a sealant it's ether damaged and needs replacing or it isn't tight enough.
dominic clancy

Do not use any sealant on the brake pipe flare nut fittings. Those seal metal to metal between the flared end of the pipe and a conical seat in the mating part.

Do us Teflon tape on the tapered pipe thread on the switch itself. Pipe threads do not self-seal without great torque (after which it cannot be disassembled). With Teflon tape it will seal with very small torque and can easily be disassembled later.

Do not put tape on the first thread, because you do not want fragments of the tape inside the hydraulic system.

I disagree 100% with Barney.

The switch is in the same system as the other connections in the brake piping. Therefore the same rules apply over the WHOLE system. If you want to trust your braking to a thin film of teflon tape, which is not pressure tested or designed for brake system pressures, go ahead, but this will always be the weak point in the system.

The braking system wasn't ever designed for sealing with teflon tape. If you need tape to get a leak-free seal, something is damaged and needs to be addressed. This is not household plumbing, your life depends on brakes working 100% - if you have an accident and have teflon dependent brakes, I'm sure your insurance will prove to be teflon coated too.

The original system works great, without teflon tape. If you have brake problems, go to the root of the problem and replace the part with an in-specification component rather than bodging a fix with household plumbing solutions.
dominic clancy

The problem with putting teflon tape on pipe threads is not that it will cause a leak (far from it), so much as getting fragments of the teflon into the system as Barney mentions. You can do as he suggests, and not put it on the first thread, which should keep it from getting into the system on initial installation. However, you also need to consider that if (when) the replacement switch fails, bits of teflon will be left behind in the female threads in the fitting, which may very likely end up pushed into the brake system when the replacement switch is installed.

D Rawlins

No teflon tape anywhere in the system. If it doesn't seal without using tape then the threads are no good. I've never had a switch leak and have never used tape.

Considering the threads are steel to copper, I suspect the copper threads re-form themselves slightly as you tighten anyway.
Steve Simmons


Opened up the old Lucas switch, different design than the one on Barney's site. A rubber washer deforms under pressure and presses on a thick copper disc which in turn presses down onto a set (two piece) blade contacts. The top blade makes a circuit and when the pressure is released the smaller blade springs the top blade thus breaking contact. The terminal holding the blade(s) looks like solid copper the other terminal is sliver coloured, silver perhaps. The underside of the contact blade is also silver in appearance.

There is some discolouration of the contacts, however this had not stopped the brake light from operating....quite the opposite it would not turn off.

Looks like a well made part, and quite possibly had been on the car for more than 14 years.

Reassembled the 4 way brake splitter and new switch earlier today. Never had any leakage from the brake system in 14 years so altogether without any type of thread seal. Tomorrow will bleed the brakes and hopefully many more years of service from the new (old) switch.

Picture of the components

Thanks to everyone for comments


R J Goebel

On 02 October 2015, R J Goebel Queensland Australia wrote:
>>"A difficult removal, in the end I had to remove the 4 way "part" hold it using a vise and use a large spanner, before it finally came free".<<
This is why you should use Teflon tape on the threads. While the brass threads may be somewhat compliant, the male threads are most often rolled or cut on a pot metal die casting (or occasionally a forged steel part). The male threads can be a little rough and difficult to seal. This requires strong tightening of the threads to affect a seal. That makes it difficult to disassemble later. Dissimilar metals can also lead to galvanic corrosion that will effectively weld the threads together.
In my case, I have changed the switch at least a dozen times in the past few years. I do not want to damage threads by over tightening, and I do not want disassemble the brake pipes to get it apart (had to do that a couple of times). If you are careful with position of the tape there is near zero probability of getting any of it inside the fluid circuit. I have never seen that problem, NEVER.

On the lighter side,
On 03 October 2015, R J Goebel Queensland Australia wrote:
>>".... presses down onto a set (two piece) blade contacts".<<
Great picture. Now we are getting somewhere. This is the first switch I have seen (disassembled) that has the proper electrical contacts. Gotta love it. They should all be made this way. Not only is it good contact materials, but it could be made to be a snap action contact (although I don't think this one is). This design will be a very long lasting switch. Long Live Lucas! Can we have a couple more pictures showing different orientation of the switch body and external terminals.

There should not be any galling or significant galvanic corrosion between the switch and union. Any corrosion taking place will occur solely on the switch threads, not on the brass union itself. This is one reason why most MGAs still sport their original 60 year old 4-way unions with their original threads in perfect condition.

The switch should not require a pipe wrench to tighten or loosen. Even if you do use teflon tape or some form of lubricant other than the inevitable brake fluid all over the threads, the union should still be held in place firmly while removing or installing the switch, to avoid twisting against the brake lines.
Steve Simmons


Was there a date code on your switch?

Dave O'Neill 2

I think it was a mistake using tapered threads by the designer. Better would be straight threads and seal with a flat face + washer.
But since it is tapered, should use tape.
Art Pearse

Why do you feel it was a mistake? Thousands of vehicles used the same thread, and no manufacturer I'm aware of ever put tape on them. Will it hurt? Probably not, if done carefully. But it shouldn't be necessary, and if it is then I would repair the threads or replace the faulty part.
Steve Simmons

Dave in answer to your question;

Found the date code and part number. Date is 8/65 making this a replacement switch. My earlier post indicated the switch had splayed terminals. So sometime between 1965 and my purchase in 2000 this switch was changed. Part number is 34542B. In any event it is 50 years old, and manufactured to last a long time.


Have e-mailed some additional pictures.

Thanks to all for comments and thoughts

R J Goebel

I finally found the other brake switch in my spares kit. It is an Intermotor 51620 switch (Nottingham England) with a Moss part number on the box (no markings on the switch itself). If the switch made in Spain ever craps out, the Intermotor switch will be the next one to test.


This thread was discussed between 30/09/2015 and 05/10/2015

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