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MG Car Repair/MGB (Canadian Spec.) brake failure light


My brake pressure failure switch on my 73B has failed itself and is leaking. I am wondering if I have to replace it since the interior brake pressure light has never worked in the 10 years I have owned the B. Can I just remove the old switch and install a plug?

MGB Brake failure unit
MGB Brake failure unit  
Hi Andre
The brake pressure failure unit is a very simple device.  It compares the pressures in the two independent circuits and if they are different, then it operates a switch that turns on a warning light.  However, if the unit fails due to worn out hydraulic seals, then brake fluid can leak out through the switch.  Sealing the switch hole will not fix the problem, because the two brake circuits would be joined together into a single circuit, which would be dangerous.

I attach the relevant page from the workshop manual.  You can see that it is very easy to take the unit apart and replace the seals.

The warning light has a manually operated switch.  The only function of the switch is to test that the bulb is working.  It does not test that the failure unit is working.

I recommend that you buy the correct parts from Moss Motors, strip and rebuild the pressure failure unit, and fit a new bulb in the test lamp.  

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Barrie Jones


MG Sports Cars from 1949 to 1980 - Including MGB, MGBGT V8, TF1500, TF, TD and modern Midgets. I also specialise in SU carburettors, Lucas wiring, suspension, steering & brakes.


I have owned my 1955 TF1500 since 1966. Technical specialist for the TD and TF with the MG Car Club T Register. Also owned 20 MGBs and currently own an MGBGT V8. Written 3 books on MGs and produced a DVD on how to strip and rebuild the TD/TF gearbox.

MG Car Club.

Barrie's Notes on the 1953-55 MG TF (author). /=====/ Barrie's Notes on the MGB (author). /=====/ Barrie's Notes on the MGB GT V8 (author). /=====/ The Essential Buyer's Guide to the MG TD, TF and TF1500. (author)

M Phil, C Eng, BSc (Eng).

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