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MG MGB Technical - Brake pressure failure switch

My brake failure light has never come on, even when using the emergency brake. I solved the emergency brake switch problem then went on to the master cylinder pressure switch. First thing I noticed was that the electrical lead was unplugged. I plugged it in & the brake warning light was activated. I pulled the switch, bled the brakes as per Paul Hunt,and reinstalled the switch which included the little plastic wrench like thing [see pictures]which acted like a large washer or shim. The brake warning light was still on. No one I know had ever seen the little plastic wrench like thing but after some thought and comparison to another late model MGB master cylinder I reinstalled the switch without the plastic shim and the light went out. The previous owner had the brakes rebuilt by a budget brake shop in LA CA. I bet they did not know to centre the piston.
My question is, what is the little plastic wrench like thing?
It fits nicely into the top grove of the switch just under the electrical connection.
I have two pictures, this picture shows the wrench separated from the switch, the next message will show how the little wrench thing was attached to the switch


Here is the second picture which shows the switch and little wrench as they were removed from the master cylinder.
I hope someone can identify this piece of plastic. It could come from any car or thing but a bit odd that the open end fits the top grove of the switch so well.


The switch you have is possibly a generic one & the extra bit allows it to work with other M/Cs. Personally,I've never seen anything like it before. Barrie E
B Egerton

The switch itself if available from Moss, part # 181-955
for MGBs 1974 & on. There are 2 different switches depending on year and brake master cylinder configuration.It screws into the center bottom of the brake master cylinder on a 79 B with a servo. It is almost impossible to see unless you looking for it.
I am wondering if the plastic thing is a bracket to support wiring when it is slipped into the upper ring of the switch, however, that is not the best solution to support anything. I am hoping someone recognizes the darn thing.

Never saw such, and I have looked at an awful lot of these, including when they were new. I say forget about it (but I would like to know what it is and where it came from!). The wiring is or was supported by the boot which sort of snaps in to the groove.

Fletcher R Millmore

Morris, I also have never seen that setup with the "wrench" on an MG. It's possible it doesn't belong where it was located and not letting the switch pin drop down enough to be actuated by the shuttle. Here's a valve assembly off of a 68 Ford, which is very close to the design on a "B". It will give you a good idea just how the valve/switch works. Also you can test the switch with an ohm meter while it's out. With the pin in, you should get resistance, with the pin out, or relaxed, there should be no resistance.

Paul Jennings

This thread was discussed between 27/02/2010 and 02/03/2010

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