Age old question about water absorption in brake fluid

Sheppy
Sheppy Posts: 140
edited March 2016 in MTB workshop & tech
Apologies for raising this old topic again, having done a search I can see it's caused a few arguments but never answered the question I have!

So my question, my wife has an old (probably about 10 years old now?) Giant Terrago with MPH brakes on it. Every now and both the brake levers get very "pumped up" as in it's like the hydraulic fluid is over full. Brakes are slightly on and brake lever is already partially engaged.

So, sounds like a simple case of overfilled fluid but I have never touched the brake system on this bike as it is casual use on bridleways/tracks only. Still original pads and fluid. In the past I have been very lazy and just loosened the screw on the reservoir and squeezed the brake to remove some fluid, tightened it up and it's been fine for about a year.

So I'm *guessing* it may be the much talked about water absorption in the brake system (bike is kept in a very damp shed if that's relevant?). As I've been doing this a while it may be nearly 50% water by now :D Any thoughts on this being a huge issue? Or can I go on being a lazy sh*t? Any other thoughts welcome.

Comments

  • keef66
    keef66 Posts: 13,123
    Only have experience of hydraulic brakes on cars. On those they generally recommend changing fluid every 2 years. Main reason being water absorbtion. Which is a bad thing for 2 reasons:

    Corrosion of brake lines / components
    Under repeated braking there's a risk of water boiling, creating bubbles of vapour and rendering the brakes ineffective.

    I suspect you can go on being a lazy swine...
  • cooldad
    cooldad Posts: 32,599
    Car systems are not sealed like bikes, so water is not a big problem, but if my brakes were ten years old I would change the fluid anyway.
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  • Sheppy
    Sheppy Posts: 140
    Hmm, maybe I will look into changing the fluid. I have a Avid bleed kit, it would probably be too much to Hope (excuse the pun) that it would fit on the MPH's...
    Or maybe I'll just carry on being lazy, the Mrs doesn't exactly break any land speed records on the bike when she's out on it, she can always put her feet down like when we were kids to stop :D
  • The Rookie
    The Rookie Posts: 27,812
    I have bled some MPH's once a long time ago, I only had an Avid kit or a Shimano (older syringe with hose) type, not sure how I did it after al, this time but I did.

    I personally don't believe water is getting in, after all if water can get in, has can even more so and brake fluid can get out.......

    Interestingly I have now found a car with a sealed system and diaphragm like an MTB, but only the one, my daughters 53 plate Daihatsu, the other 4 cars in the household are all vented.
    Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.
  • lpretro1
    lpretro1 Posts: 237
    Problem with Giant mph brakes is they are the old fashioned 'close' system type so fluid expands under heat (can just be stood in the sun or in a hot shed) and pushes the fluid against the pistons and moves them out. To compensate the mph have a thumb wheel on the master cylinder to adjust the capacity a bit - BUT these usually seize up very quickly. You'd be best off buying a pair of bog standard Shimano hydraulic brakes and fitting those than messing about with the mphs on the problem won't go away. New 'open' system brakes all have a diaphragm in the mc to allow for heat expansion

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