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MG Midget and Sprite Technical - Brake light pressure switch

Quick question guys, is it possible to replace the pressure switch, without having to bleed the brakes. Mine has stuck on.
1275 1974

Dave
Dave Barrow

yes.
Stretch a bit of cling film over the reservoir filler cap, secured with an eleastic band. R & R the switch. I would use a bit of PTFE tape on the switch thread.
GuyW

What Guy said, although I've never used PTFE tape on brake threads.

You may already know this, but the switches supplied these days are often of poor quality and generally don't last long unless you use a relay in the circuit to protect the switch contacts.
Mike Howlett

I don't know if they still are but the switches used to be a part that was often faulty from new so do test them when fitted. I've had one that only worked when I pressed the pedal fairly, such as when testing after fitting, but with my normal gentler use of the brake pedal the switch didn't work.

Have a wet rag under the union, and others with a watering can to hand just in case, and be prepared to undo the existing switch, snatch it out and replace with new prepared switch as quick as possible.
Nigel Atkins

Or replace with a Mini switch bolted to the pedal box, adjustable for sensitivity, and it doesn't need bleeding. Leave the existing one in place and use the wiring for the new switch.
This is the type and costs £3.23 (incl delivery)
http://www.ebay.co.uk/itm/CLASSIC-MINI-BRAKE-LIGHT-SWITCH-MECHANICAL-FITS-ON-THE-BRAKE-PEDAL-13H3735-/361148968563



Jeremy MkIII

Unless they've improved that type used to exploded itself apart meaning a hunt on the footwell mat for the various bits.

Things might have improved but when I used those they'd often only last weeks (if you were lucky).

I've got a feeling that I glued the two parts together on the last one.
Nigel Atkins

Thanks all, switch is on order from Moss. so hopefully it will be ok. Relay sounds like a good idea.

Dave
Dave Barrow

Are you referring to the brake pressure warning switch located on the right side of the engine bay or the switch that turns on the brake lights to warn following traffic which is mounted at the pedal assembly? The answers seem to be split between those 2 systems!?
Stan Kowznofski

Stan

Only the very last UK cars used the dual circuit brake system with the pressure loss warning switch, along with a mechanical brake light switch on the pedal box.

Most UK cars were single circuit with a hydraulic switch to turn on the brake lights.
Dave O'Neill 2

Nigel,
Rags and water is good advice if using DOT4 paint stripping fluid! But the reference I made to cling film is to prevent loss of fluid - and therefore no need to bleed afterwards, which is what Dave was asking about.

If you seal the reservoir cap then the fluid is held from dripping out by the vacuum created. The principle is that if no fluid drips out, then no air can be drawn in either. The old switch can then be undone and the new one fitted fairly swiftly and no need to bleed.

But if some air does get in, then the quick way is have someone press lightly on the brake whilst you slacken and retighten the switch by about 1/8th turn - just enough to let a little fluid weep out. And THEN the rag and water is a good idea!

Mike, re the PTFE. I also wouldn;t use PTFE elsewhere on brake pipe threads as the seals are always between pipe flares and cones, never the threads themselves. The brake pressure switch is the exception and I am not convinced that the threads on the switches available are that well made these days!

I also use a relay. The poor quality switches seem to work OK at first, but fail a year or so later unless a relay is used. Since I fitted the relay the switch has beeen ok - about 10 years now.
GuyW

Nigel,

the one I have on now was an ebay cheapie and has been in place for 10 years. It's mounted on the outside of pedal box (in the engine compartment) so maybe it doesn't have to cope with the same pressures as when mounted next to the pedal itself?
The only extra is a diode, fitted to stop speaker interference when the brake pedal was pressed.
Jeremy MkIII

Dave,
if you want or feel better with a relay then fine but you don't need one unless you have modern made poor quality brake light bulbs. Nowadays it seems you need to buy heavy duty bulb to get previously standard quality bulbs from 10+ years ago.

I've also seen quick response (can't remember what they're called) brake light bulbs, I've no idea if they're any good.

Once you find a hydraulic brake light switch that actually works they do seem to last many years, pr my last one has, so far, that'll make sure it stops working now I've put that.

Nigel Atkins

Guy,
sorry I was posting at cross purpose, I know what you mean by the method you describe as I think you may have informed me about it on one of my changes, thank you.

As brake fluid is such nasty stuff doing anything with it and I have water ready. I know the brake fluid shouldn't come out from the tee but what about from the fitted switch.

Also before or after the swap of the switch the reservoir might be topped up as a conjunct to the initial job so more risk of loose fluid on paint.

I've snatched swapped previous switches, with the risk of drips perhaps so have the rags protecting. I've probably also risked air but my reasoning was that the fluid would be coming out from the tee and the new switch isn't primed with fluid so there's already air in it. Perhaps I was lucky but I didn't need to bleed either.

IIRC I did have my very reluctant glamorous assistant press the brake pedal whilst I cracked open the new switch after I used your method just as a belt, braces and bit string and chewing gum check even though I probably didn't need to. It was this reluctance that suggested in future I continue the risk of snatch fitting by myself and risk bleeding (by myself) after if needed.

If you look at my Profile photo (on an appropriate size screen) you'll see the effect of brake fluid from trusting someone else and not checking myself.
Nigel Atkins

Jeremy,
you're right I was referring to the switch fitted just behind the pedal that causes itself to exploded.

Speaker interference(?), the only speaker interference I know of is the partnered verbal handbrake from the passenger seat. :)
Nigel Atkins

Unfortunately a diode won't work for that Nigel! :)
Jeremy MkIII

Some useful advice I got was not to use the nasty cheap self-destructing pedal switches and use universal motorcycle rear pedal switches instead. They are much more robust and there's always a way to get them to fit.
Greybeard

I chose a slightly different solution.I bought a switch from an industrial supply house called GRAINGERS. It is a momentary on switch and I think it came with 3/8 -24 threads. Similar to the one Jeremy mentioned but made of metal and more robust. This is on a US spec car but one could drill the brake box to fit.
J Bubela

Thinking about it more, maybe it was momentary OFF switch, It was a long time ago.
J Bubela

I fitted one like Jeremy shows (in the manner he describes) about 25 years ago, and it was secondhand then. Not exploded yet … although given it's age maybe it was made a little better than modern replacements.

In between the hydraulic one breaking and fitting the mechanical one, I had a momentary push button (with associated wiring) gaffer taped to the gear lever for thumb operation.
Very James Bond and ideal for surprising tailgaters without risking my rear end ;-)
G Hawkins

Just replaced the switch, it was as easy as everyone said, no loss of fluid, and still have a firm pedal.
The new switch looks to be good quality but time will tell.
Only grumble, why did they not put a spanner flat on the four way union, so you can undo the switch without having to resort to some method of not putting strain on the pipes.

Dave
Dave Barrow

This thread was discussed between 12/07/2018 and 16/07/2018

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