Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Brake master cylinder/dirty fluid

Replaced the brake master cylinder on my 69 gt 2-3 years ago, now it feels like it's going out again. firm brake, fades at a stop, getting worse by the day. i checked the fluid, and it's pretty black. would changing the fluid maybe solve the problem?

yeah, i know it's a long shot, but thought i'd ask.

don g

Brake fluids containing Polyglycol ethers are regarded as DOT 3, DOT 4, and DOT 5.1. These brake fluids are hygroscopic, meaning they have an ability to mix with water and still perform adequately. However, as they become saturated with water, they turn brown, then almost black. Your fade problems is the result of the water boiling, creating gas bubbles that compress when you hit the brakes. Brake fluid should be changed once a year.

Just to help you choose a good brake fluid, be aware that the term “Wet Boiling Point” refers to the minimum temperature at which a brake fluids will begin to boil when the brake system contains 3% water by volume of the system. The term “Dry Boiling Point” refers to the temperature at which brake fluid will boil with no water present in the system. Amongst the brake fluids presently available there are three possible candidates. The first, DOT 3, is a poor choice for high performance driving due to its low dry boiling point of 401 degrees Fahrenheit (205 degrees Celsius) and a wet boiling point of 284 degrees Fahrenheit (14 degrees degrees Celsius) and is now generally considered to be obsolete. Be advised that American DOT 3 brake fluid is formulated differently than British DOT 3 and is incompatible with the aging Original Equipment natural rubber seals that were used throughout the brake system. It will slowly but surely dissolve them! However, after all of these years the original seals have probably long ago been replaced with seals that are made of more modern materials. The second candidate, DOT 4, is much better with a dry boiling point of 446 degrees Fahrenheit (230 degrees Celsius) and a wet boiling point of 311 degrees Fahrenheit (155 degrees Celsius). Of the different brands of DOT 4 brake fluid on the market today, Castrol LMA synthetic with its dry boiling point of 509 degrees Fahrenheit (265 degrees Celsius) and a wet boiling point of 329 degrees Fahrenheit (165 degrees Celsius) appears to be the best available.
Stephen Strange

Thanks, Stephen. I'll replace the fluid and see what happens!
don g

2-3 years is a very short time for the fluid to go black.
PaulH Solihull

This thread was discussed on 27/08/2010

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.