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Brake Pressure Failure Switch

Posted by Chowjusky 
Moss Motors
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Chowjusky Avatar
Noel Saunders
Los Angeles, USA   usa
As some of you might be aware it's a b@#$* trying to get hold of the brass assembly brake pressure failure switch for a 69 GT because Moss has stopped making them. My question is, would something like this make suitable replacement or should I continue my quest?

NASpecMGB Avatar
Desmond Mullen
Durham, NC, USA   usa
Are you talking about the "brake check switch" on the left side of the dash? Those things are the butt of many jokes ("it's an ejector seat switch!"winking smiley and of little-to-no actual use (IMHO).

Sudden brake pressure failure is pretty uncommon in MGBs as far as I know. As long as your brake hydraulics are in good shape (and the rubber bits not too old, and the fluid nice and clean) then it's very very very unlikely to happen.

Is your goal to "improve safety" or to "restore the vehicle to original spec/function"? The answer to that may dictate your choices and options.

Good luck with it!

-Desmond Mullen

applebj8 Avatar
Jon Appleby
Lake Wylie, SC, USA   usa
1971 MG MGB
I made some notes from when I had trouble with mine 2 or 3 years ago:

Almost forgot. My brake check light hasn't worked for years. I broke down and got a new one from Moss and it still doesn't work. I've unscrewed the plastic sender before but not knowing what I'm looking for I just stared at it and screwed it back in.

It is a multifunction device. 1) It tells you if the pressure on one circuit is greater than the other by way of a moving piston in the block on the fender. 2) Pushing the button inside on the dash should make the light glow. This is the test function. Neither function is a very reliable indicator of problems. I suggest ignoring it but if you MUST get it to work, it will take some troubleshooting. Are you up to it? You will need a voltmeter.

Voltage feeds the test switch via a purple wire. Check for voltage at the purple wire. If voltage is present, then move on to one of the wires hooked to the sensor on the block that your brake lines tie into. If you do not have voltage here, then go back and check the light bulb in the test switch. If you do have voltage at the sensor, ground that wire with a jumper and the dash light should come on. The sensor is not really a sensor at all; it is merely a contact that goes to ground inside the block when the piston in the block moves from its centered position. The test switch does the same thing when you press the button; it allows the circuit to go to ground, completing the circuit, making the light glow.

If the bulb is bad, the light will not glow
If the test switch is not grounded (black wire) the light will not work
If someone has cut the bridge wire inside the connector at the block, the light will not work on "test" but may work when there is a hydraulic problem

There has been a lot of confusion on how the system works in the past. Hope this helps, but for all the good the switch does, you may be just as well off ignoring it…

Update July 2007:
I just received a new switch from LBC Co. along
with some other items like a voltage stabilizer and
coolant temperature sensor. The reason I'm
writing is to let you know that if you're simply
after the switch (Moss # 141-725) what they sent
me was a Ford Motor Company part.

It's an exact replacement: C8AZ-2B264-A from a '69

See here:

I paid $28 plus shipping so I'm thinking it would
be worth checking at the local FoMoCo dealership
to compare price.

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alex77 Avatar
alejandro alcoser
monte alto, Texas, USA   usa
1969 Triumph TR6
1977 MG MGB "Harley Quinn"
1979 Triumph TR7
The switch on your link is a hydraulicaly (is that a word?) activated swich, like for some brake lights. (The B uses a mechanical switch). The brake failure switch is a mechanical switch. Yes, its screwed into the brass assembly with the brake lines, but there is a piston inside the assembly, between the front and rear lines. When there is an im-balance, the piston moves, pushing a plunger in the switch. There should be no fluid where the switch is screwed in, unless the internal o-rings are leaking.

So, the swich on your link is not the same as the switch in the brass assembly

Ralph 7h Avatar
Ralph Siebenhaar
Viersen, Germany   deu
May be Ralph Nader can explain the sense of this federal rule that came on in the late 60's and has been canceled in the late 80's again.
Its value has been explaned by Desmond perfectly. This whole mimic is perfect non sense!
The only advantage of the whole system it the check switch on the dashboard that can be rewired easily to act as a usefull oil pressure warning light.

ex-tyke Avatar
Graham Creswick
Chatham, Ontario, Canada   can
1976 MG MGB
There is a side benefit to the "brake check" light on the dash, that being to indicate when your handbrake is applied - at least it does on mine!

Chowjusky Avatar
Noel Saunders
Los Angeles, USA   usa
This is great info, thanks guys. As I am not doing a showcar resto, I am not bothered whether it works or not. I guess the best option is to look into what I can use the switch for.

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NASpecMGB Avatar
Desmond Mullen
Durham, NC, USA   usa
ex-tyke Wrote:
Quote: There is a side benefit to the "brake check" light on the dash, that being to indicate when your handbrake is applied - at least it does on mine!

Hey Graham, your "Brake" light is a totally different ball of wax than what Noel is talking about. Yours is a simple microswitch on the parking brake and has nothing to do with the hydraulic system or with the silly "brake check" switch found on late-60s/early-70s MGBs.


-Desmond Mullen

gsj28 Avatar
Gary Jackson
Tracy, California, USA   usa
1954 MG TF
1959 MG MGA 1500
1965 MG MGB
1965 Pontiac GTO    & more
You can use a bolt in place of the switch.

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Simon Austin Avatar
Surrey, BC, Canada   can
I just rebuilt the brake system on the V8 after losing the rear brakes and then having the fronts lock up due to heat. That damn switch never did tell me I had a pressure differential but the mushy brake pedal sure did.

I've since removed the brass distribution valve and plumbed the system so the front and rear systems are completely independent of each other.

I understand the theory of the distribution valve and the safety legislation requiring a "brake-failure" warning system but I think I've only read of one person on here who said they had the light on the dash actually come on due to a system failure.

Ray Avatar
Ray Silver Member
Ray Huckleberry
Westminster, MA, USA   usa
1973 MG MGB GT
1976 MG MGB "MB"
USe mine as an emergency brake light.

Ray Huckleberry
76B CB Conversion
Westminster, MA 01473

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