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Front brake has too high pressure!

Discussion in 'Technical and Troubleshooting Torque' started by jack_1313, Apr 23, 2011.

  1. Hi guys, I'm having a problem with the front brake on my 1982 CB250RS. Here's what happened.

    I had the caliper off the disk and, being a mechanical nooblet, accidently squeezed the lever, causing the brake pads to permanently press together. No problem, I thought. I opened up the brake fluid reservoir cap (thinking it might somehow help relieve pressure?) and prized the pads apart with a screwdriver just far enough to get the caliper over the brake disk and back on the base. I then replaced the cap and tried the lever, only to find that it was now very high pressure. That is, the slightest touch will activate the brakes, and a slight pull will give me heaps of braking force.

    The facts:
    The rubber seal that sits under the fluid reservoir cap has deformed and doesn't sit properly around the edges of the reservoir.
    I tried bleeding some fluid out of the brake caliper, thinking it would relieve the pressure. I first tried with the reservoir cap off, then on. In both instances, the lever would loose all pressure when the fluid was draining out with the drain hole open, but as soon as you closed the hole and released the lever, it would return to the same high-pressure state as before.

    My questions:
    1. What exactly has happened here? Why did pressing the pads further together than they normally go when there's a disk between them cause this problem? Am I right in thinking that this happened because I removed the cap and thus broke any suction caused by the air-lock?
    2. Am I right in thinking that the deformed seal is failing to prevent air from getting sucked into the reservoir, and that bleeding a small amount of fluid from the caliper with the reservoir cap would otherwise have fixed the problem?
    3. If not, how do I fix it?

    As you can tell, I don't really have a good understanding of how the system works, so you'll have to forgive me if my ideas are way-off. Thanks for any help you can offer!
  2. I suspect therein lies the cause of your problem. More than likely that one of the pistons was extended too far and is now stuck.

    Take the caliper off and see if you can in fact push the piston(s) back fully. If not then unfortunately it probably means rebuilding the caliper.
  3. OK Obviously one does not squeeze the brake lever while the callipsers are off as the pads may come together as you said and it may not be easy getting them apart.
    And if there is no pads in when you do it you will likely pop the piston right out.

    However having done it, what you then did, that is prise them apart, should have set things right again. So something else is happening. Could be a stuck piston as jd said but if so I would expect it to be dragging on the brake without the lever being operated. Is that happening?

    Also why did you have the callipers off? Were you doing something else to the brakes, like changing pads? If so it so, could be just the extra width of the new pad taking up some slack you preveiously had and a different grade of pad could lead to more bite.

    Unless it's a stuck piston we need more symptoms.
  4. You sure it's not just that your brakes are now working as they should, rather than as 30 year old brakesd typically do?
  5. .....
  6. Thanks for the help guys!

    I had the caliper off because I wanted to rub some of the dirst/dust of the brake pads to see if it would help stop my front brake from squealing so much. Needless to say, I didn't get around to doing that, though cleaning the disk did help a bit.

    Yeah, I've been riding the bike for the last few days and I'm very confident that it is not right. Firstly, it is clearly different from how it was before my slip-up. Secondly, pulling the lever gives heaps of braking pressure before the brake light switch is even activated. Thirdly, it is completely different to how the front brake lever feels on my 1997 Hornet (which just had a roadworthy), as well as any other bike I've ridden. There is zero free play in this lever - if I just touch the lever I'm braking.

    There is also contact between the brake pads and the disk when the brake is not applied. I can tell this because, with the bike on its centre, when I try to spin the front wheel it doesn't really (ie it turns but doesn't really spin), though whatever braking force this equates to must be small since the bike rides ok. Also, the disk squeaks while riding normally (ie not braking), indicating contact with the pads, although I did sometimes head this squeak before as well.

    I tried pushing the inner brake pad out today but it wouldn't move at all. If it did move back further that would mean space between the disk and pads when the brake is not applied, which seems odd to me because I don't remember ever seeing space between them when checking the pad condition before.

    Am I right to think that if the pad and piston can be pushed back further (ie isn't somehow jammed) then I could do so freely by draining all brake fluid?
  7. I think your problem is also known as "finally having decent brakes." Go play with the brakes on any modern sportsbike and you'll see what I mean. They should be rock hard, you just have to use a bit more finger finesse. So go practice emergency stops until you're confident you won't grab 'em and lock the front wheel in an emergency.
  8. ok, jack I believe you. That isn't right.

    My first thought would be that one of the pistons has jammed. The other is that perhaps there were spacers between the caliper and mount which you have misplaced, or perhaps there were not but you put a washer on there or something...

    ... hang on!

    Does it actually have a twin piston calliper? I would have thought it was a single piston sliding mount calliper, in which case the sliding pins are not sliding. Take it off again and check the basic assembly. Clean and lube the pins (lightly) with molly disulphide grease (hi temp) and reassemble. Slide the body back and forth in place and make sure it's working. Then re-bleed your brakes and clean up - meaning get anything oily off the disk now. BrakeClean is good. While you have the pads out, give them a bit of a rub on some cement, or coarse sandpaper if feel all technical. You could give the faces of the disk a quick rub with some fairly fine emery paper too, if you wanted. Make sure you do it evenly, the same all the way around.

    [edit] - Right! Much dicking around on youtube and places. It is a single piston sliding body calliper. It actually looks like you can break it open. If you look through the wheel from the right hand side, there are two recessed bolts that go in. They look like 10mm or 12mm. Those would be the ends of the sliding pins. You'd need a socket, a ratchet and an extension. ... Before you go that far - just give the thing a damn good wriggle around, maybe a bit of a tap in various directions with a rubber mallet or a piece of softwood drift and a hammer. If you can't free it up, then crack those two bolts while the calliper is still bolted on to the forks, (much easier, by the look of it) and then strip down the sliding pins, clean, lube, and reassemble. Take careful note of what came off in what order and how it went on and so on... A camera phone my be a good thing to have here...

    Another thought occurres to me... There is only one way the pads can go in - right? You couldn't have put them in arse-about-face, could you? That would explain a lot ...

    [2nd edit] Look, after thinking about this for a bit ... If you're not real sure about it, then take it to a shop (carefully!) and get a mechanic to have a look at it. It isn't the sort of thing you can afford to have not working. It might be a good time to throw a new set of pads in it anyway. Tell the bloke about your adventures and ask him to make sure the calliper is sliding normally.
  9. Could be as simple as lever adjustment.

    There will be an adjusting screw on the brake lever, back it off a little
    bit to give you a few mm of free play in the lever before the brakes come on.
  10. Not on a 30 year old budget Honda there won't be.

    I tend to concur that the sliding caliper may be seized. De-seizing such things used to be an annual chore on bikes of the era used in UK winters.