Brake master cylinder leaking

[Johnnyhil]

I bought a brand new brake master cylinder for a project car but due to being busy it didn't get fitted for several months. Bleed the system and then the car sat again for several months on axel stands while doing other bits and pieces to it.

When I got it on its wheels I found the pedal was very soft and there was brake fluid dripping from between the master cylinder and servo. Unfortunately it's now out of warranty.

Took the master cylinder off and took it apart and as you would expect the seals look brand new. The cylinder had a few dark marks running in line with the length of the cylinder but nothing I can feel. I'm not sure if this is normal or not.

I bench bleed it and left it held by the vice with the bleed lines feeding back into the reservoir for the last week. I've pumped it several times a few times a day. So far there hasn't been a single drop leaking out of it.

Any ideas what the issue is?

[paul_c123]

Sounds like it leaks when the fluid is under some pressure, but you're unable to replicate the required pressure with your test.

[GreenV8S]

That sounds strange. Did you get a firm peddle while you were bleeding it?

[Johnnyhil]

paul_c123 said:

Sounds like it leaks when the fluid is under some pressure, but you're unable to replicate the required pressure with your test.

That makes sense.

I'll try using plugs in the outlets and see if I can get the pressure by hand. Failing that I'll put it back on the servo with the plugs in and try it.

Thank you

[Johnnyhil]

GreenV8S said:

That sounds strange. Did you get a firm peddle while you were bleeding it?

I can't remember exactly but I assume I must have because i ticked bleeding brakes off my list. It was several months ago unfortunately.

[GreenV8S]

Johnnyhil said:

I'll try using plugs in the outlets and see if I can get the pressure by hand. Failing that I'll put it back on the servo with the plugs in and try it.

The master cylinder would typically see several times your body weight applied to it under heavy braking. You can certainly apply that much using a vice or similar, but it's not very controllable and you could easily end up bursting the seals. A safer way would be to orient the m/c vertically and put a beam across it that you could tie down at one side and lean your weight on the other side. Presumably this will just confirm that yes, it's leaking. You could try replacing the seal, but since you say the current one looks OK I wouldn't be optimistic that will fix the problem. Most likely you'll end up replacing the m/c again.

[Johnnyhil]

GreenV8S said:

The master cylinder would typically see several times your body weight applied to it under heavy braking. You can certainly apply that much using a vice or similar, but it's not very controllable and you could easily end up bursting the seals. A safer way would be to orient the m/c vertically and put a beam across it that you could tie down at one side and lean your weight on the other side. Presumably this will just confirm that yes, it's leaking. You could try replacing the seal, but since you say the current one looks OK I wouldn't be optimistic that will fix the problem. Most likely you'll end up replacing the m/c again.

The vice was just to hold it while I pumped, I wasn't using to to apply any pressure.

I checked on it this morning and found a a few drops worth had leaked out past the piston and that is without any pressure. Definitely needs replaced.

Thank you all for your help