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My brake cylinder is failing. The pedel is soft and will go to the floor but I can pump the pedel to get it to work. I added a remote brake booster which also has a cylinder controlled by brake fluid from the master unit. How can I tell where the problem is coming from or which unit is failing

Jeff Becker


The remote brake booster has no reservoir of its own. When it works you get power boost. When it doesn't work you get no power boost, but the brakes still work in non-power mode.

When your brake pedal sinks to the floor while you have you foot on it, that is internal leakage past the primary seal cup back into the reservoir. I have had similar problems ever since I did a complete hydraulics rebuild in 1998. First symptoms were brackish fluid with rubber bits in it every 6 months, and the master cylinder leaked externally after 18 months.

After the next rebuild new rubber everything) the fluid continued to turn brackish even when flushed clean every 6 months. Then the brake side primary seal failed with the symptoms you describe. After repacking I had the same failure of the clutch side primary seal after less than two years. The next clutch primary seal lasted 14 months, and the following one only 8 months. Then a few more repeat failures of the clutch side primary seal only lasting a few weeks at a time. The packing kits were mostly from Moss Motors, one from Victoria British, but I think they came from the same source Made In England.

More recently I finally bought some generic 7/8" diameter hydraulic cups from NAPA. When I put one of those in the clutch side of the master cylinder the clutch hydraulic problem was banished. Now I'm waiting for something else to fail, because the fluid is still turning dark and brackish in 6 months time. As usual, modern rubber parts suck. Anyone know where I can buy a good master cylinder packing kit?
Barney Gaylord

That is exactly why I decided to replace my MC this winter, I have had similar issues with black crud in the fluid, and decided to fix it the right way rather than with new seals of an unknown quality. I have always used Si fluid too.

dominic clancy

If a new master cylinder had good seals, then a packing kit from the same manufacturer should be good as well, shouldn't it?
Barney Gaylord

In my experience, if the fluid turns dark it is bad fluid.

People have argued this since time began, but Castrol GTLMA does not do this in any car I've used it in. I've flushed and put it in numerous cars I got that had black nasty fluid and they then stayed clean and worked for years after, without rebuilds or other fiddling.
I've never yet had any rubber parts that dissolved in this fluid, admitting that I have not used a lot of repair kits in recent years. I used to be able to predict system condition by what brand fluid was in the can in the trunk.

Note that Castrol has recently changed the packaging, so it does not say "GTLMA" on it anymore, but it is DOT4 and I believe the only fluid Castrol make. I HOPE it is still the same juice!

FR Millmore

I will mention a few more things, the fluid is silicone, it was a new master cly,it was rebuilt in 2001, new pipes. From what Barney states, I need to order new seals from Moss and I don't have to worry about the remote booster. I have no black in master cyliner. It is a low mileage car not used every day like Barney's. Hope this helps. Is there anything else I need to know that may help?. Good hearing from you Dominic.


This thread was discussed on 03/03/2013

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