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Broken Brake

Discussion in 'Bling and Appearance' started by pat65, Jan 20, 2007.

  1. My rear brake stopped stopping me the other day.
    There is no firmness in the lever at all. I have inspected it and there are no leaks, the reservior is full, the hoses are good etc.
    I'm guessing it may be a problem with the master cylinder?

    Any advice would be appreciated.
  2. Brakes

    Have you tried to bleed the brakes at all? Did the fluid run low at any time or has the bike fallen over? Sounds odd that it's happened all of a sudden. Could be master cylinder if none of the above has happened.
  3. Did the brake come good after a while? Perhaps after some time cooling down? Maybe it works, but still feels a little squishy?

    Did the pedal just swing away from your foot whith no back pressure at all, and swing a lot further than normal? Were you riding on a hot day, and using the rear brake quite a bit? Are you new to riding?

    If yes to most of the above, then you boiled the brake fluid. I've done it twice now. After the first time I had the fluid changed, in case it had absorbed too much water, which reduces the boiling temperature of the fluid. It obviously didn't help.

    Learn to use th rear brake less. Use the front brake. Are you skidding the rear tyre into corners as well? That would indicate too much rear brake.
  4. Your master cylinder is attached TO the resevior, so it's unlikely that there's something wrong with it, or the reservior wouldn't be full. If you can push the pedal right down as far as it can go with no resistance you may have had one or both of the pads fail, although you should be getting a grinding sound when you brake. I'd remove the back wheel and check the pads anyway; you COULD have had one of them pop out, for example...
  5. Could still be the master cylinder. shot seal can allow bleeding back into the reservoir without loss of fluid level....
    ...2 cents again
  6. Hold the lever/pedal down with some pressure against it. If the lever/pedal slowly drops or comes closer to teh end of it's travel, you have a master cylinder problem.
    If teh pedal/lever stays very high, and is quite hard, you might have a stuck piston caliper or the sliders may be seizing up.
    No firmnes indicates either air or moisture in the system. If there is moisture in the fluid, when heated, it can form steam bubbles, and cause a lack of braking.
    Can you have someone actuate the lever/pedal, and have a look at the caliper? Some slight movement should be noticeable when this is done.
    If this were my bike, I'd do a thorough bleed of teh hydraulics, and disassemble and clean the sliders on the calipers ,taking the opportunity to attempt to push back the pistons in the calipers, to check if they are stuck or not.
    Or if this is too tricky, I'd take it to someone.

    Regards, Andrew.
  7. ^^

    wot he said, because stopping is very important :!: :idea:
  8. Re: Brakes

    No, haven't tried to bleed them yet, fluid has never been low and the bike hasn't fallen over.

    Was a warm day but it wasnt a long ride, I did have a pillion though which requires more braking effort. I dont think I over use the rear brake.
    The brakes didn't come good either. :(

    I've checked the rear pads, plenty of meat on them & they seem fine.

    The only resistance on the pedal seems to be on the return spring. I can push it down with my hand, it doesn't drop slowly under pressure. The calipers don't seem to move either.

    That sounds more like it to me (although I am no expert).

    It was just weird that it happened all of a sudden with no evidence. Thank god it wasnt the front..

    I might try bleeding it first & then get the master cylinder checked out.

    Thanks for the advice :grin:
  9. Could be a master cylinder jammed. Could be a lack of brake pads and disc.
    Could be poor seals on the master cylinder allowing water into the new fluid.
    I've had all of these so far (usually on older bikes).
  10. Brake pads and disc are fine. I'm leaning towards the master cylinder.
  11. Well, tell us when you find out the actual cause. We are all intrigued now.
  12. Ok, I was water in the brake fluid. I took it somewhere to get looked at. They replaced the fluid and its all good now. :)

    Now, off to St kilda...
  13. Hmm, well that is very weird. If it was water in the fluid, it would mean that the brake fluid would boil at a much lower temperature. When that happened, there would be virtually no back pressure on the brake pedal, as all you would be doing is compressing the vapour in the Master Cylinder, brake lines, and/or caliper piston.

    However, the brake would work when cold, and it would recover after the failure when it cooled down, and the fluid condensed again. I can't imagine any sort of vapour lock preventing that from happening.

    I would keep an eye on it if I was you. Water in brake fluid can also mean rust, which could mean that the Master Cylinder piston could be jamming.
  14. I have found that water is much more compressible than brake
    fluid in this sort of system, and it doesn't need to boil to screw
    up your brakes if there is a LOT of water in the brake fluid.

    I learned this when I rode in a downpour with a worn-out master
    cylinder seal which let water in, and lost all my back brake.
    The brakes were _definitely_ cold then. I wasn't, I was sweatin
    cause I was in the middle of city traffic in a downpour with
    only a front brake riding on tram tracks. :roll:
  15. RoderickGI Thanks, it does seem a bit weird. I will keep an eye on it. :?
  16. Sorry Hotcam, water is compressed by only 0.00034% for one PSI of pressure. Yes, it is compressible, but a lot less than brake fluid. Silicone brake fluids specify a typical compressibility of less than 2%. (A typical value may be 1.8% at operating pressures of 2000 PSI.) Even if water compressibility has a linear relationship to pressure, which it wouldn't, water is less compressible than brake fluid at 0.68% at 2000 PSI.

    Your worn out Master Cylinder was the problem. Braking works by converting kinetic energy to heat, so even in the rain your brakes are getting hot. Local temperatures at the caliper piston could get quite high for short periods, causing any water in the system to vapourise, usually in small bubbles. These would cause brake failure. However, I suspect that you just couldn't build up pressure in your system as you had a leaking Master Cylinder, or air had gotten into the system. :!:
  17. Water in your brakes will cause what was happening. Oil and water don't mix. Tiny particles of air get trapped between the oil (brake fluid) and the water particles. This also will give the mix a milky colour. That air is what is being compressed, even when cold.

    Water usually gets into your brake system due to condensation in the resevoir. Although it may be other more serious causes. Regular brake fluid inspections/changes will help you keep on top of things.
  18. Well, actually, most people use DOT 4 Brake Fluids, which is usually Glycol based, and is Hygroscopic, which means it absorbs water. This is deliberate, so that any water that comes into contact with the fluid, such as water vapour in the reservoir, is held in solution and therefore is less likely to cause rust. Rust would be bad in a brake system.

    So oil and water does mix in this case. DOT 3 and DOT 5.1 Brake Fluids are also Hygroscopic. DOT 5 Brake Fluid is typically silicone based and is not Hygroscopic. In this case you are correct.

    As water is in solution in a DOT 3, 4 and 5.1 brake fluid it does not form an emulsion (the milky colour), and therefore it does not trap air around it. In a DOT5.1 brake fluid it is quite possible that air will be held in suspension with any water, particularly in an environment where the suspension is agitated, such as on a motorcycle. However, DOT 5 fluid doesn't absorb water, so this would not be a large effect.