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Simple way to bleed air only without getting fluid out?

Discussion in 'Maintenance and Servicing' started by chokpa, Sep 28, 2011.

  1. Hey Fellas

    Low sided the bike 2 months ago and since then had around a 2 week pause in riding. Since starting again I feel the brakes are really spongy, and coincidentally its the same side that i stacked it on. I figure in the two weeks its sat there there is a broken seal somewhere which is leaking air.

    That will need to get identified and fixed.

    However, in the mean time is it possible to only bleed air through the nipple without cycling the fluid as well? I don't have any tubing on hand to do the bleed and was hoping to keep it as simple as possible.

  2. I've bled brakes without tubing. It's messy though. Besides, if there's a knackered seal, air will get in instantly or even while you're doing the bleeding so there's no point until you identify and fix the underlying problem.
  3. I'm assuming its a slow leak because I rode it for a while after the crash and things were fine.

    Is it possible to only bleed the air out of the line?
  4. Actually just had a look at it and all the connections look good. Not even sc****d.

    My brake level did get pretty bent though. Maybe the fault lies there..
  5. Depends where the air is. If it's in the caliper, if you arrange things so that the bleed nipple is the highest point of the caliper for a while you might be lucky and have the air collect as a single bubble which then comes out first when you open the nipple and gently squeeze the lever. Or you might not be. Given a following wind you'd only lose a teaspoonful of fluid.

    If it's in the line, you might get an improvement by cable tying or gaffer taping the lever as far back against the bar as it will go and leaving the whole shebang overnight. This can allow bubbles in the line to rise to the top of the system and vent through the master cylinder fluid reservoir. For this to work there must be no high or low points in the line, just a continuous upward direction all the way from caliper to handlebar.
  6. Thanks Pat

    Little confused over height. The nipple is set on the caliper. Is it possible to make it any higher than it already is? Or do you mean have a curved brake line which actually rises above the nipple?

    I don't entirely understand what is meant by high or low points in this context (apart from cable kinks). The system is rigid and as far as I can tell, cannot be modified.
  7. Think about it ..... air rises. Nipple at the highest point....lifting front wheel.
  8. When I say that the bleed nipple must be the highest point on the caliper, I mean that there must be no part of the caliper that contains fluid that is higher than the nipple. Normally they're set pretty close to this but if the front wheel is turned one way or the other or if the bike is on a slope there is the possibility of small parts of the caliper bores being higher than the escape route for the air. Not usually a problem if fluid is being pumped through the system in quantity (though it can be) but an issue if you're trying to get away with just opening the nipple and squeezing gently until fluid appears. To make sure, the caliper might need to have one of its bolts removed and the other loosened to allow it to be swung into a position where the bleed nipple is unequivocally at the top. As I said, once you've got this set, you need to leave things for an hour or two to let microbubbles in the fluid consolidate into a single bubble immediately below the bleed nipple if you're to have any hope of success.

    As far as high and low points are concerned, what I'm getting at is that you need to think like an air bubble. Air bubbles will rise through the fluid but they won't travel downwards. For the fasten-the-lever-back trick to work, you need to follow the brake line from the connection to the caliper up to the master cylinder and work out whether a bubble travelling from one to the other would have to move downwards (or even just horizontally) at any point. That's downwards in an absolute sense, not downstream in the pipe. Splitters where the single line from the master cylinder divides into two lines to go to twin calipers are a favourite for this. If it does, the trick probably won't work because any bubbles from downstream of that point will just collect there and not travel onwards and upwards to the master cylinder.

    If your brake lever has been bent, examine both it and the handlebar clamp on which it pivots for cracks or other damage. What you're perceiving as "sponginess" could be something flexing that shouldn't and, believe me, you don't want your brake lever snapping off at an exciting moment :shock:.
    • Like Like x 1
  9. Thanks Pat. Funnily enough my bleed nipple is on so tight I can budge it at all...

    I've tied the lever back and i'll see what comes of it tomorrow.

    Fingers crossed and thank you very much!
  10. Have a look at a few 'How to' clips on youtube.

    Bleeding the brakes properly will involve some fluid escaping - it's unavoidable.

    Try and get a bit of clear plastic hose of the right inside diameter and a jar. It isn't strictly necessary but it makes for a neater job. Any brake fluid that spills on the bike, rinse off with plain water as soon as convenient. Don't wipe it off, rinse it off and fairly quickly.

    For what it's worth, I very much doubt there's anything wrong with your seals. Air has managed to get into the system while the bike was on its side. That happens. There's nothing wrong with it except a little air bubble or two.

    Get a ring spanner of the right size (close is no good - get the right one) and make sure it's long enough to have decent leverage. Watch the videos - bleed the brakes. Fixed. Betcha.
    • Like Like x 1
  11. On a bike that's been down, as in a prang, it's not hard to see how air can get into a brake line.

    If the lever was pressed while the bike was down, either during the actual slide down the road or by the rider lifting the bike while the bike was horizontal, prior to the actual lift, it's possible to pump air into the line.

    Normal bleed procedure will clear it, though I'm Mystified as to why anyone would want to not lose any fluid. Brake fluid is VERY easily contaminated by water, and any change of old for new, is a good thing.