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MG MGB Technical - 1964 MGB Replacing Brake Light Switch

Hello there!
I need to immediately replace the Brake Light Switch of my 1964 mgb roadster. It seems to be an easy operation, but, is there any special recommendation I must take into account before doing it at home?
L Munoz

I've just snatch fitted mine by myself without problems a few times - another BBSer has suggested having a helper gently pressing the brake pedal at time of swapping to stop air getting in.

Preparation would be checking you have the correct sizes of tools to fit the existing and new switch (should be the same size, 7/8" deep socket) and perhaps some pliers to hold fitting below, you'd possibly also want to check that your existing switch will loosen before bringing in a helper.
Nigel Atkins

As Nigel says, you should be able to change the switch without loss of brake fluid or without air getting into the system. I have done it several times. Do the job as quickly as you can to minimise the chance of an air bubble getting in. Be aware that the new switches sold these days are often badly made and don't last very long before they fail.
Mike Howlett

Easy as you say

Unscrew the filler cap off your master cylinder
Place a piece of plastic bag on top of the reservoir and screw the cap back on, this makes the reservoir air tight and stops the fluid siphoning out
Have your new switch close by and ready to go
Disconnect the wiring from the old switch and screw it out making sure you hold the base so as not to twist the brake pipes, screw the new one in as quick as you can to minimise any fluid loss, connect the wiring and remove the piece of plastic from under the cap-----------job done,
Wash any fluid spilt on the paint with water immediately to stop it eating into the paint
There will be no need to bleed the brakes as the piece of plastic under the cap keeps the system full of fluid during the job

willy
William Revit

Oh, with such a large sized socket do bear in mind that the thread to the switch is quite small so don't over-tighten with the larger sized handle to that size socket, I use a handle with (t)bar hole at the top and tighten with just using the very centre of the bar and fingers.

ETA: better and more comprehensive advice from Willy there, totally agree with all
Nigel Atkins

Changing is easy with minimal loss as described. Use some good grips to hold the fitting firmly to prevent it from moving and stressing the pipes, and wedge some absorbent paper round the fitting just in case of a dribble. However mine came undone surprisingly easily and didn't lose any fluid.

If you can't get NOS most replacement switches seem to be rubbish and have been causing complaints for years. My original switch started getting erratic after about 40 years, I changed it, then less than 2 years later I noticed it was needing more pedal pressure before they came on. Changed the switch again but this time fitted a relay, and five years later it is fine. http://www.mgb-stuff.org.uk/electricstext2.htm#blrelay

However last year I fitted a micro-switch operated directly by the pedal, similar to rubber bumper models, but mounted under the shelf in the cabin. That operates as soon as I start to move the pedal and lights the lamps much earlier than the hydraulic pressure system http://www.mgb-stuff.org.uk/electricstext2.htm#blconv

I've left the previous system in place as 'belt and braces'.
paulh4

Yes, I've done this in the last few weeks. I was careful to support the base whilst I loosened the old switch. I then picked up my new switch in my right hand. I used my left hand to unscrew and remove the old switch, then a nanosecond later screwed in the new switch.
Gordo

I did exactly the same, forgotten until you mentioned it!
paulh4

My OE brake light switch worked fine for 45 years. I took advice from here and tried a Chinese copy with a spark suppression circuit added when it failed. That lasted one year before going high impedance. I then fitted another Chinese effort and a relay to baby it and that has worked fine for the last 4 years. I put a spark suppression diode across the coil
Take the advice on here and its a 5 minute job to swap the switch.
Stan Best

Just did this on my 67. Just unscrewed the old one and put the new one (or in this case, I took out the bad new one and replaced it with an old one I had in my spares). No dripping or air gets into the system.
Bruce Cunha

This thread was discussed between 24/05/2019 and 03/06/2019

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