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MG MGB GT V8 Factory Originals Technical - brake fluid leaking..?


I noticed that there was a few drips of brake/clutch fluid on the drivers mat of my car so had a look and there was some under the brake/clutch master cylinders. I tried to inspect to find out where it was coming from but you know how tight it is in there. I took the brake master cylinder out and the bottom had a coating of fluid on it but I cant find a hole/crack/rust. It doesnt appear to be coming out of the plunger and the pipe fitting on the back looks good. I have filled it and stood it on paper to see if it leaks over the next couple of days. I know this wont test for pressure but I would have thought that the only part that should be pressurised would be the piston.?.,

The thought that I had that I hope someone could answer is are the filler caps meant to seal? I have noticed that on at least one occasion that there was fluid on the top of the M/C and Clutch M/C and wondered if it was being forced up/out and running down.

Your help would be appreciated as I dont want anymore practice at getting these things in and out.
frank swinton


I wouldn't have thought that the filler caps are meant to seal. I would think that there must be an air inlet hole in the filler cap to replace the fluid in the reservoir as it falls due to normal usage - ie as the brake fluid is forced out, air needs to get into the reservoir to replace the fluid.

I have a similar problem - a very slight leak of fluid, evident by fluid seepage on the panel below the master cylinder - I just assumed that it was a slight weep from either a pipe union or a seal, and intended to remove the master cylinder, strip it down and replace the seals in the next few weeks.

I'd be interested to see if anyone else has any opinions.


The cap are intended to breath and there is a small hole to allow for this. Actually with a slotted screwdriver you can easily separate the cap. Also at this time any crunge could be cleaned out. I too have a slightly leaking clutch master cylinder and in a few weeks (as the car goes off the road for the winter) I will address the problem

Removing those cylinders on the early cars is no one's favourite job. I had mine resleeved, but alas the solder was faulty where the reservoir attaches to the cylinder causing major paint damage. I threw them out (literally) and bought new ones.

I always seem to have a small amount of fluid on the top of each reservoir, just a few drops and to help minimise this I keep the master cylinder level about 3/4" below the top. I am sure the vented caps are the cause of this.
Ian G Buckley

If it is overfilled and gets hot it will overflow onto the top of the MC and hence down the side. BMC cars used to have a foam collar round the (vented) plastic cap. Put a gunsons on it, that will soon show you if you have any leaks, mine pin-holed on the top. But if it has run down the pedal that usually means it has leaked past the piston seals.
Paul Hunt

I'm rebuilding a roadster, replaced all pipework and master cylinders with new copper seals and banjo's, i coulkdn't get a decent seal between the seals and the master cylinder where the banjo wnet in. Turned out to be a poorly machined master cylinder. Replaced with another new cylinder and seals, all ok. Tested it by standing on brake and put tissue paper under the master cylinder assembly through the large square grommet hole from inside the car.
Ian Sanders


...and a Gunsons is a ...... Hammer?
frank swinton

Well, I suppose you could regard it as a hammer exerting a constant force - Gunsons EeziBleed.
Paul Hunt

This thread was discussed between 30/09/2002 and 02/10/2002

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