Brake Light Switch - Fluid Leak

I have long since replaced mine with the mechanical switch that works flawlessly but still have the garbage switch plumbed in the front brake circuit. All has been fine until I lost brake pressure recently. I thought there was a leak at the MC or Servo (replaced 18mos ago), but low and behold it was leaking at the brake switch, actually coming out of the top where the wire connectors are. So not only do these switches not work, they are also prone to leaking. Quality at its poorest. At this point I would just like to put something in there to block it off rather than just putting another switch in there. Does anyone know of a cap you an just put there or do I need to just buy another crappy switch to plug it. Thanks for any info.


How does it seal at the plug to cylinder? Washer, gasket or tapered thread ?

Remove it and visit your local hardware store. With a great deal of luck it is pipe thread of the variety in your location UK and USA pipe threads differ!!

Make that leaky switch into a plug. Drill out the center. Tap to the available pipe. . Or to NF fine., Fit a bolt with sealer and a gasket…


I don’t know if I would have ever looked at the old switch (I too have changed to a mechanical switch). Thanks for the tip!

Looking back I should have saved the original as it did not leak. I guess I never thought these would leak, but evidently they are quite cheaply made. I went through a couple of iterations of new ones and finally switched (no pun) to the mechanical one.

Its probably something like 1/8”bspt thread at a guess?

Hello All,
A little Off Topic, but worthy of a small rant relating to leaking brake systems.

Start of Rant

The brake level warning system arrangement is flawed in my opinion. Having worked in the world of CNC machines and automation in general for a long time, monitoring anything related to safety was always fail safe. An Emergency Stop circuit for example is normally closed and is actuated when the circuit is made open. This is important, as a broken wire will cause an open circuit and if the Emergency circuit were to be actuated by a closed circuit, it will never happen with a broken wire; hence it fails safe.

The brake fluid level relies on an open circuit to indicate that its in a safe range, closing the circuit to illuminate the warning light when the fluid level drops to a dangerous level. Accordingly, a broken, or disconnected wire in the circuit will render the warning system ineffective. Hence, it fails unsafe.

Rant Finished.



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Agreed. Unfortunately, the warning is a dash light, which inherently must be open to indicate safe and closed to warn of trouble. So there’s nothing that can be done to make that warning circuit failsafe. You could make the level sensor normally closed, but that’d just mean you’d have to have a relay or some such to control the warning light.

The best option is to make the level sensor as reliable as possible, which probably means replacing the cork float nonsense with the SNG stainless steel float arrangement. And testing it periodically.

Excellent plan! I have done similar fixes before. Remember that if you seal the plug up with an O-ring, it’d have to be a special O-ring; regular nitrile is not compatible with brake fluid. Better yet, just screw a bolt and nut through the center of that switch base and seal it with a copper washer under the screw head.

It’s a 1/8" tapered NPT fitting. You should be able to buy an appropriate plug.

M10 1mm will also fit adequately as the thread pitch is similar (25 vs 27 tpi) and the taper deals with any slight difference in diameter.I have taken to using metric threaded switches that fit 1960’s VW Beetles and Kombis. These seem to work without leaking.

Those #%&^**! switches fail so often I just got tired of changing them. I fabricated a mount and put a mechanical switch in… and never looked back (yes, pun intended!)


Wherever space and time interact, there is information, and wherever information can be ordered into knowledge, and knowledge can be applied, there is intelligence.
Pavel Mirsky, mid 21st Century Russian General

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I’ve never had one fail completely. I put in a mechanical switch to more quickly activate the brake lights.
The OEM switches often take too much hydraulic pressure to activate the lights. The mechanical switch activates as soon as you touch the brake pedal.

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Well John, I guess it’s all in the definition…

I call it failure when I am stepping on the brakes and the car behind me is “surprised” and the lights won’t come on. And yes, it takes more and more pressure over time.


Wherever space and time interact, there is information, and wherever information can be ordered into knowledge, and knowledge can be applied, there is intelligence.
Pavel Mirsky, mid 21st Century Russian General

True, I was thinking of complete, total, utter failure. failure to work even if you pushed the pedal hard enough to give you shin splints. :laughing:

Well I have. I reckon we must have changed out 20 or so for customers.

What happens is that the internal sealing on the switches is inadequate, allowing brake fluid to get past the rubber diaphragm into the air space around the copper disc that makes the contact.

As the fluid is incompressible, once it replaces the air the diaphragm can’t move when the system is pressurized and so the switch stops working.

So far, on my own cars, the VW switches work well and have lasted longer than the junk ones.

Lemme get this straight: You are deliberately cross-threading an incorrect thread into the hole?

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Yes. You are correct.

Even when mine worked it did need significant pressure to activate it. I just ordered a couple of the crappy switches from Welch, will just install a new one for now and try and find something else that works better for the long-term. I did ask Welsh if they had heard of these leaking and they said no so perhaps a “one off”?

Here is a quick update. I ordered two parts and received them last week. I installed the first one, and it leaked, really, so not a one off as I thought or hoped for. I installed the second one and it is ok…for now. My confidence level on this part is zero at this point. I recommend everyone reading this audit their installed brake switch (whether it is being used or not) and check for leaks (see picture showing the leak location). When these go bad you will severely lose braking performance.

I’d it possible that the union the switch screws into has a crack down the side that opens up when tightening the switch into position?

I’ve had my brake switch for years, never leaked and I did take it out and reinstall as part of the total overhaul.

I’m away from my shop so I can’t go a get a switch to see what the thread is. I’d recommend that you take the switch to your local hydraulic shop and ask for a plug for the fitting for the switch. Your might try your local car jobber for the same thing but a hydraulic store is a sure bet. Worth a couple of bucks at most.