Blown seal

[Daddy Dom]

I accidentally overfilled my front master cylinder and brake fluid is weeping from the bellows. I'm guessing the pressure has blown the seal. Can anyone confirm this is a thing?
Cheers, DD

[Steve Brown]

I accidentally overfilled my front master cylinder and brake fluid is weeping from the bellows. I'm guessing the pressure has blown the seal. Can anyone confirm this is a thing?
Cheers, DD

If it's just the overspill getting past the bellows why not use some paper tissue to soak up the excess, clean it up with brake cleaner and try again? It shouldn't have done any harm.

[Daddy Dom]

Sadly it's not overspill, it's a proper leak.
DD

[Steve Brown]

Sadly it's not overspill, it's a proper leak.
DD

OK, are we talking about the fluid reservoir here? Only that shouldn't be under any pressure. If you only overfilled it then it would make a mess when you refit the bellows and cap, but that is just spillage and why I suggested to clean it off and reduce the fluid level to normal. If the fluid leaks when you operate the brake that's another matter and unlikely to have been caused by too much in the reservoir. Did you reduce the level yet?

[MickeyMoto]

Blown seal? Reminds me of an old school days joke...

[Daddy Dom]

Did you reduce the level yet?

Yes, and a tiny bit had seeped out of from under the lid but nothing like what is collecting under the brake lever. I've ordered a new seal kit but these ones are only 18 months old.

[Butch]

Don’t understand. Over filled reservoir is not going to increase pressure and make seals ‘blow’ (weren’t penguins involved?). If this is all it is then as noted here it just needs some of the excess mopping up.

[72degrees]

Did you reduce the level yet?

Yes, and a tiny bit had seeped out of from under the lid but nothing like what is collecting under the brake lever. I've ordered a new seal kit but these ones are only 18 months old.

Are you sure it's not weeping from the hydraulic line banjo?

[3potjohn]

Maybe there's a tiny piece of grit or something causing a poor seal coincidental on refilling the fluid? I had an issue with my front brake after fitting a new seal and it was only some time later I found another already jammed inside.
good luck.
John

[MickeyMoto]

Could be the cylinder is old and knackered or scored and does not seal any more?

[Daddy Dom]

Don’t understand. Over filled reservoir is not going to increase pressure and make seals ‘blow’ (weren’t penguins involved?). If this is all it is then as noted here it just needs some of the excess mopping up.

Well TBH, I'm the one who does not understand and need everybody answering to be very patient as despite long-term ownership, I am not mechanically gifted. And you seem to know how a master cylinder works So, there's no leak from the banjos otherwise fluid would be dripping down the hoses. It's not, it's coming from the bellows.

So, could I ask, if I did the damage by overfilling (assuming there wasn't a foreign object), how can it not exit under pressure from the seals end? Where else is there for the fluid to exit if there's too much in there? (I'm discounting the lid.)
What happens? Surely I'm not the first person to overfill a m/c? How high up the window do you lot fill yours?
Cheers, DD

[72degrees]

Well TBH, I'm the one who does not understand and need everybody answering to be very patient as despite long-term ownership, I am not mechanically gifted. And you seem to know how a master cylinder works So, there's no leak from the banjos otherwise fluid would be dripping down the hoses. It's not, it's coming from the bellows.

Good point well made. In my experience, after a particularly messy setting up, filling and bleeding of a brake line/cylinder brake fluid seems to reach the parts...

Then I suppose you may not even have disturbed the banjo when fitting the new seals?

I'm still finding it doubtful that overfilling the reservoir could have blown the seals. More likely bore damage as has been suggested.

On the 250 I've occasionally filled it so high that when you put the bellows and the diaphragm back in a bit of fluid is displaced before I screw the cap back on and so have to mop up. No issues with the seals popping.

Did you replace just the seals or put a new piston in?

[Daddy Dom]

I'll check it our properly when the new seals arrive.

When I spotted the leak I took everything apart, cleaned it all, then reassembled. So same piston, same seals, same leak. Same penguin. Next time I will do it inside under bright light wearing white gloves.

[72degrees]

Was it a new piston when you repaired it the first time? When I've had to do the M/Cs on the 250s I've just bought a complete piston and seals kit. Actually, I don't suppose the pistons wear as such, but perhaps the bore does.

Fingers crossed another rebuild sorts it out OK.

[Daddy Dom]

Well I've been monitoring any drips and - blow me (or the penguin) - the leak has stopped.

So, I'm baffled but do remember during bleeding that I got the "mannequin pis" brake fluid coming out of the reservoir, so maybe some DOT 4 ended up in a crevice and has all leaked out now. Maybe it was all your collective breath-holding. Maybe it was because the new seals arrived.

I do know that the front brake just saved my hide from the crazy woman who pulled out in front of me. That's an odd moment, isn't it, when your back wheel comes up beside you then disappears again!