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MG MGB Technical - Brake/Stop lights not working

This follows on from "lost all lighting" a few days ago...

When I tried to fit a centre rear brake light to the roll bar of my '64 car, connecting it up to the red & purple wire that is in the back of the engine bay that goes to the rear brake lights, I lost all brake lights. I think I got the centre rear brake light to work very briefly but there has been nothing since.

I disconnected the new brake light. I have gone over the wiring but when I put my foot on the brake pedal I get no brake lights.

When I connect the red & green (indicator) wire to the green & purple (brake light) wire ledft turn signal flashes my brake lights, so the wiring from the engine bay backwards is working.

Have I blown the switch in the brake lines that picks up brake pedal pressure?

Is this switch replaceable without draining the brake fluid?

All advice appreciated!

John Prewer

I would have thought that if anything, you have blown a fuse. But if you say the green/red (btw, you should refer to cables as the main colour first and the trace colour second - there is a red/green colour that feeds the courtesy light, but as you state the brake light flashes, I assume it is in fact green/red..) is live, then it is not a fuse UNLESS you or PO have upgraded the fusebox so that the brake lights are on a different fuse to the indicators. Have you?

Also, you say red/purple wire - there is not a red/purple on your car. You mean green/purple, yes?

Can you burn out the switch? Yes. You can even burn out the wire. But the fuse would have prevented this, so if your brake lights are fused, then I doubt very much you have burnt anything.

MGB brake switches are notoriously poor. I would hazard a guess yours is gammy.

Where did you connect the green/red to the green/purple? Instead, try bridging the green and green/purple wires on the brake light switch - this will give you a better indication of what is happening, or not. If the lights work (remember to switch on ign) with this bridge, then, yes, it is switch. They can be changed with the fluid in the lines, but there are several horror stories out there of the switch not unscrewing and twisting the union... how would you fancy leaving it be and putting in a mechanical switch? Much better.
Hal Adams

The factory installed Lucas switches, installed in the brake circuit, were and still are very high quality. My '67 still has the original switch. As Hal has suggested, jump the two leads, at the brake switch, and observe if the brake lights come on. This will narrow down the problem. Yes, you can remove the brake switch without loosing any brake fluid or needing to bleed the system sfterwards. I've had great luck in removing the stock hydraulic switch and have never had a problem with the union rotating. Maybe I've just been lucky, though I doubt it. RAY
rjm RAY

Brilliant idea guys and far too simple for me. Jumping over the switch I get brake lights, with the switch connected properly and my foot on the pedal I don't.

Can the switch be replaced without draining the braking system?

The switch to the best of my knowledge is 48 years old and original. If it was ever replaced by the previous owner it is still over 35 years old.

John Prewer


Sorry I misread and you answered my question.

I will now start looking for my spare, there's on in the garage somewhere!

John Prewer

I've not had luck with the Moss or other aftermarket brake lamp switches available in the US. They often start to fail after even a few weeks, requiring ever increasing pedal pressure to operate.

Check the archives about installing a relay and arc supression diode. Dave DuBois has several postings. Below is a website showing Dave's solution which I used. It has worked perfectly for years.

Also here is a link regarding installation of the relay and diode.
Robert McCoy

When you change the switch, remove the cap, the put a sheet of cling film over the top of the brake fluid reservoir and replace the cap. This will stop the brake fluid running out through the switch union. Then swap the switches. Remove the cling film. Check the brakes. You should be lucky and not need to bleed the brakes.
Hope this helps, Mike
Mike Standring

Ron Francis sells a heavy duty switch for about $30. They have gotten rave reviews. RAY
rjm RAY

G'day John,

I recently changed mine for the same reason. I sourced a Hella 4565, a German made switch which is used on VW, Volvo, and most BMC cars. It fitted perfectly and I didn't have to bleed the system afterwards. At worse I only lost a drop when removing the old switch.

I also fitted a relay as per Dave DuBois' instructions on the link given above. Hopefully I will now have a problem free brake light system for many years to come. I would highly recommend the relay.

PG Crowell

For me, the pressure switch is not as socially responsible as a mechanical one as it require pressure to build up before completing the electrical circuit. This could be well over a second before the person behind is aware that you are braking. A mechanical one on the other hand, well adjusted, will complete the electrical circuit the instant you hit the pedal.

I never did understand why they went to all the expense to make pressure reliant switches when it would be so much cheaper and more reliable to have a mechanical contact switch at the top of the brake pedal.
Hal Adams

I agree Hal, and I have fitted mechanical switches (with relays) to both my MGs.
Mike Howlett

An air free hydraulic system acts almost like a solid steel rod, and there is no discernible delay when operating the brake pedal - certainly not over a second, more like milliseconds.

I would say that there is probably more chance of a longer delay in the take up of a mechanical linkage than a serviceable hydraulic system (puts on steel helmet).

The system has worked well for 30 to 40 years - it's just the poor quality of the modern replacement switches that give us the problem.

Replacing the switch with a quality one and fitting a relay is probably an easier option than retro-fitting a mechanical system, I would say.

PG Crowell

I disagree Phil. The hydraulic switches you buy these days seem to need quite a pressure before they come "on". So when you are braking gently up to a stop light for example, no brake lights show. I have fitted mechanical switches to both my B and my midget and they come "on" immediately the pedal moves, even before there is any discernible braking effect. Much safer IMO.
Mike Howlett

I too have been a victim of the poor quality moss replacement switch which cant take the current or needs a hard press on the pedal to activate. In my car I fitted some replacement LED stop & tail light bulbs. These can be had off ebay & are a direct replacement for the standard bulbs. Because they draw hardly any current a relay is not needed, the switch handles them fine & they come on a lot quicker than filament bulbs.
G Britnell

My PBO had only just replaced the Hyd switch on mine ('70) as I bought it so I fitted a relay etc (No capacitor) and now the lights come on after a very small pedal movement, without the relay it was much slower
B Anderson

Hi Mike,
My experience so far with the Hella switch is that it operates under very little pressure and always on the initial movement of the brake pedal. I'm not saying that a mechanical setup is any better or worse, it's just that it seems like a lot of work when a relay and a quality low pressure switch works fine.


PG Crowell

This thread was discussed between 21/10/2012 and 28/10/2012

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