Brake fluid - going up!

[5150]

Hi all,

Apologies for what is probably a ridiculous question (and for asking it after not posting much on teh .org), but here we go....

Is brake fluid affected by humidity? During the ridiculously sticky, hot weather we had a couple of weeks ago, I noticed that the fluid level in my front reservoir has gone up above the 'Upper' line (it was in the middle before), and the rear level has risen a bit, too. Is it possible that it's been absorbing moisture from the air?

If not, is there any other reason why the fluid level would rise?

What can I do about it? I don't want to just have at it with a syringe, because then there'd be less (possibly contaminated) fluid in the system.

I've never had to bleed brakes before, but if that's what needs doing, I'd like to try rather than just go crying to the dealer.

As far as I can tell, braking performance (both front and rear) is unaffected.

Thanks!

It does absorb moisture, but the brake system is a closed system, and no water vapour should be able to get in. Hot weather wouldnt cause that much expansion of a liquid. Quite odd...someone im sure will be able to explain this phenomena!

If your fluid has absorbed moisture from somewhere I would definitely bleed the brakes through with new fluid. More importantly you need to find out how the moisture is getting in, is your reservoir properly sealed?

[5150]

Thanks for the replies, guys!

The bike had its 11k service about three weeks ago, so I'd certainly hope it's all been sealed up properly. I wiggled things, and everything appears to be tight and snug....

I can't see any leaks or fluid leaking out from anywhere, either.

did they overfill it when they did the service, ie topped it up.

[5150]

Yep, it was all fine after the service - it just seemed to shoot up over the course of a few days.

[5150]

An update for those interested:

I think the brakes need bleeding. Braking power has diminished slightly since my last post, and upon jiggling the bike a little on a hunch, I saw a small air bubble in the front reservoir.

Could a big bubble somewhere else in the system cause the fluid level to rise? It'd certainly make the brakes feel spongy, from what I can gather....

Sorry for all the n00b questions!

Originally Posted by 5150

An update for those interested:

I think the brakes need bleeding. Braking power has diminished slightly since my last post, and upon jiggling the bike a little on a hunch, I saw a small air bubble in the front reservoir.

Could a big bubble somewhere else in the system cause the fluid level to rise? It'd certainly make the brakes feel spongy, from what I can gather....

Sorry for all the n00b questions!

The reservoir is not filled to the brim and then sealed. Air in the reservoir will just be air above the fluid mixing with the fluid because you 'jiggled' it!
It'll only be a sealed system between the pistons in the master and slave, which you can't see as the lines are not transparent.
Did the service you had done include some work on the calipers? Every time I've had a service they always clean up the calipers and apply liberal amounts of copperslip etc to the pins that hold the pads in place. Maybe that's what they did and the pads are moving back a bit more now when you release the lever. If they topped up the reservoir to max, then now the calipers have freed up a bit, it could be reading slightly over?
All of the above could be total nonsense though, if you don't know what our doing with your brakes, I'd get somebody who does to look at them?

[5150]

D'oh. That's a good point.

I don't want to start taking things apart when I don't know what I'm doing, either - so I think I'll admit defeat on this occasion and take it to the garage.

I'll make sure I find out what was wrong, though, and post it up here for anyone else who has the same problem.

Cheers, all!

When bleeding the brakes, I got a 50 cc syringe from the local vet, put a short length of PU tube on the end and started to draw the fluid through the nipple until bubbles stopped. Topping up the reservoir as i done it.

When doing the front, it's only when i do the second caliper that the lever goes stiff again and the air bubbles totally disappear.

The wee one way valve kits work but i find them slow and I refuse to pay for a mitivac.

If i remember right we allow for something like 5% volume expansion per 38 degrees c. Unlikely to be that I think.