Bare "plug" near brake master cylinder

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Dave Wheatley
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Re: Bare "plug" near brake master cylinder

Post by Dave Wheatley » Thu Apr 23, 2020 6:30 pm

Thanks for that Paul and Ian.

That looks the answer.

It's been like it for 18 months, and the brakes appear to work OK.

A bit of investigating tomorrow I think.

Assuming the hole where it fits has been blocked off, what do you think the result would be?
1978 MGB GT
1931 Austin 7 RM
1955 Standard 10
1972 Ford Cortina
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Ian Fozzard
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Re: Bare "plug" near brake master cylinder

Post by Ian Fozzard » Thu Apr 23, 2020 6:36 pm

I can only guess at this, but I would think that without this sensor you would have a brake fluid leak and no brakes, so I would think the hole will be plugged somehow (unless of course the brake system has been reworked to remove this feature).
The only result of not having this working would be a lack of any notification of brake failure! Just the same as earlier Bs which only had single line brakes as well. First indication in that case is a pedal which goes to the floor and absolutely no retardation!!!!

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Paul Scott
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Re: Bare "plug" near brake master cylinder

Post by Paul Scott » Thu Apr 23, 2020 6:54 pm

Looking at the video from Moss if you had a leak on back or front a light inside the car would iluminate notifying you of an issues.

Have fun locating your blocked off hole. Most likely on the master cylinder as per John Twist video.

Good Luck with it Dave

Regards
Paul :D
1975 MGB Roadster
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Dave Wheatley
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Re: Bare "plug" near brake master cylinder

Post by Dave Wheatley » Thu Apr 23, 2020 7:00 pm

Thanks Paul and Ian.

More investigating tomorrow. Will report back.

Paul, your link to the Moss catalogue is password protected, but a search for the part number revealed all.
1978 MGB GT
1931 Austin 7 RM
1955 Standard 10
1972 Ford Cortina
No modern!

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Paul Scott
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Re: Bare "plug" near brake master cylinder

Post by Paul Scott » Thu Apr 23, 2020 7:09 pm

The Catalogue should not be password protected.

It's from David Manners Group , just tried to find a link but it does not have any images.

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Paul
1975 MGB Roadster
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Dave Wheatley
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Re: Bare "plug" near brake master cylinder

Post by Dave Wheatley » Thu Apr 23, 2020 7:54 pm

Couldn't wait! The threaded bit of the plastic sensor has broken off which is why the sensor was just lying there doing nothing. The threaded bit is still in the master cylinder and might be removable with an old fashioned easyout. I don't think that the hole is open to the brake fluid, and the sensor is mechanical in operation as per the Moss video. I can't think that a plastic part like thatwould be compatible with brake fluid and pressure.
1978 MGB GT
1931 Austin 7 RM
1955 Standard 10
1972 Ford Cortina
No modern!

Ian Fozzard
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Re: Bare "plug" near brake master cylinder

Post by Ian Fozzard » Thu Apr 23, 2020 8:36 pm

That's good Dave, progress!!
I'm not so sure about the connection with the brake fluid though? Perhaps there is a two way valve which only moves if there is some sort of pressure differential. The valve may have seals which act as membranes sealing off the brake fluid
Unless there is some sort of membrane I would have thought the sensor part would have to be in contact with the fluid.

You can probably tell I have nothing like this on my BGT!!

Ian F

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Charles Farran
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Re: Bare "plug" near brake master cylinder

Post by Charles Farran » Fri Apr 24, 2020 8:45 am

Definitely the brake failure switch on the later dual cicuit cars. It fits under the brake master cylinder. They are not the easiest to set right. (The workshop / repair manual indicates the approach. See also the John Twist utube video on it which demonstrates the principal. If yuu need to replace the switch they are readily available. You need to be careful pulling the plug / socket on the lead off it as you may have the pinns corroded such that they break. I was unable to source the connector at the end of the wires. Managed to get a second hand one from Welsh MG who chopped it off an old harness from a car they had broken up.
Cheers, Charles
1980 Roadster

Ian Fozzard
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Re: Bare "plug" near brake master cylinder

Post by Ian Fozzard » Fri Apr 24, 2020 8:51 am

Thanks Charles. Can you advise on how this functions? I am assuming that it measures some pressure difference in the fluid? Would there be a leak of brake fluid when replacing the switch?

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Charles Farran
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Re: Bare "plug" near brake master cylinder

Post by Charles Farran » Fri Apr 24, 2020 8:57 am

Provided the seals in the second barrel of the brake master cylinder hold you shouldn't get any leaks though the threaded hole where the switch locates.Some owners ( certainly in the U.S). dispense with the switch & replace it with a bolt/ threaded * to blank it off - many take the view that it is worse than useless as ,provided you check your levels regularly before using the car, if there is a leak during a journey the light comes on at the same time as the brakes fail! I am not recommending you do this!
Cheers, Charles
1980 Roadster

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