Hey guys

I've just dug the XT660R out after a few months of sitting unused. After cleaning and lubing the chain I headed off round the block to check the bike was OK, only to find that the front brakes are almost non-existent. Lever pressure at the bar seems normal but the bike just doesn't want to stop. The XT has always been rubbish on the brakes, and scares the hell out of me when I come back to it from a modern bike, but this is something else! It's never been capable of an emergency stop, but now it barely stops at all.

Maybe the pads are contaminated? Is there anything I should do to clean the disc, or the pads? Or rough them up? Or check caliper operation?

My money would be on the pistons being stuck in the caliper through lack or use/movement, or the slider pins are seized solid, specially if the lever still feels taught. The brake fluid may have picked up some moisture during the lay up too, so for the sake of a few quids worth of brake fluid, a flush and bleed would be a good plan after you've freed up the pistons/sliders.

If you think the brakes were a bit dodgy before, it's quite likely that the pistons and sliders were partly corroded already?

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Thanks.

Before you posted I had taken the pads out, given them a clean and a rub on some wet and dry paper, and then cleaned the caliper with brake cleaner. The pistons looked like new, although I don't have a brake pad spreader and I couldn't move the pistons by hand. Reassembling everything, there was definitely more feel in the brakes, although still not on the same scale as most other bikes I've ridden. After a 3 minute ride round the block with a bit of braking, the disc was almost too hot to touch. I don't know if this is normal, or whether it's indicative of the pistons not retracting properly and keeping light pressure on the disc. Maybe this is what glazes the pads?

Should I be able to move the pistons by hand without a screwdriver or pliers? How do I know if they're a bit sticky?

If the discs are too hot to touch after a short ride, then the pads are binding on the discs. This could be the pistons, but could equally be the caliper sliding pins being stuck as I mentioned earlier. It will also, as you say, end up glazing the pads.

You really need to take the caliper off and try and get some movement back into the sliding part of the caliper. You can try lubricating the sliding pins and working the two parts of the caliper back and forth, but you may need to strip it to clean everything up.

It is possible to move the pistons back by hand, but not all that easy. generally lever them back, but they shouldn't take excessive force. Check that the reservoir isn't full to the brim though because the fluid needs space to get back in there. If it's brimmed that pistons won't push back because they'll be nowhere for the fluid to go.

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At the risk of asking a really dumb question...

There's no spring or anything to actually move the pads away from the disc when the pistons retract slightly. Even if the pistons are moving smoothly, is it not possible for the pads to remain in light contact with the disc anyway??

Yes. There is no return spring as such, so the discs do very lightly touch the disc, but not so as you'd notice and certainly not enough to make the discs hot. This is why even the slightest amount of corrosion on the pistons or sliders will hinder the pads' return after the brakes have been applied. There is a certain amount of springiness in the disc (and as its floating) which will help return things back to an equilibrium. Other than that you are reliant on a lot of components sliding freely.

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Thanks again Pleiades.

It sounds like I need to get myself as piston spreader for the weekend and check the pistons are moving freely.

You keep mentioning the sliders. Are you talking, in the 660R's case, about the single pin that keeps the pads in place?

No, the pad retaining pin is something different - it's only function is to hold the pads in place.

The sliding pins are the two in the diagram below shaded yellow. The two rubber boots circled red are what keeps dust, dirt and moisture out and stops them corroding. Often the boots perish and cause the sliding pins to corrode and get sticky. It is these sliding pins that need cleaning up to make sure that the two halves of the caliper can slide freely.

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Pleiades, is spot on as it will be either the pistons sticking (less likely as some pistons are moving hence the heat but you might have one or more stuck) or the caliper slides seizing. Either way, you really need the caliper torn down and given a good clean, grease (slides) and rekit if required or simply reassembled if the seals and boots are OK.

If you're not that confident, although the job isn't hard, its not something you should stumble along just in case. Do you have someone close that could give you a hand for an hour or so?

BTW, it doesn't take much to semi-seize a piston and you will push them out due to the hydraulic advantage you get on application but it is only the piston seal that retracts the piston when you let go of the lever, hence it applies poorly and drags on release. Its a square section seal (as opposed to an oring) and as the pressure builds up behind the piston it moves forward and the seal distorts. When you let go, the distorted seal is what pulls the piston back to a released position. This is also why disc brakes stay adjusted...if the pads wear, the piston has to move out further to grip the disc and consequently has to move through the seal to a new adjusted position.



Additionally, this action is what makes a warped disc a dangerous situation. While riding and flexing the wheel the warped disk will push the piston back into the seal, so on first application of the brake...it will take alot of lever movement to apply the brake and increase application time. In a bad case it may take two pumps of the pedal or lever to actually grip the disc.

Thank you both very much.

With your help, reading til 1am and a bunch of youtube videos, I finally understand what a floating caliper is and how it works.

I just went back out to the bike, took the caliper off and had another fiddle. With a little bit of force I was able to press the pistons back in with my thumbs, move them out by pumping the lever, and press them all the way home again. Good stuff. I then tried tried sliding the caliper on the slide pins. Nothing. A couple of seconds of light tugging and it suddenly freed off and was sliding smoothly again. A quick bit of reassembly and I could see the slight caliper movement when I used the lever. That wasn't visible before.

A quick spin round the block and braking was restored. Not like a real bike, but as good or better than my XT has ever been.

Back on the drive, I heaved the bike back on the centre stand and was able to spin the front wheel freely with my foot.

The only niggling doubt is whether the disc should still be so hot. I'm literally only doing half a mile at 20mph with regular fistfuls of front brake, so maybe that is normal heat generation, but i certainly can't hold a knuckle on the front disc. I'll try again later with a more normal run and see if the disc air-cools. I'll also take the GS out this weekend and see how hot the front discs gets.

Thanks again guys. Great forum.