Overheating disc brakes resulting in lever pump?

the4thdeadmonk

OK, i've yet to experience the whole "maintenance free" aspect of disc brakes, but with my shimano XTs i think i'm getting close.

They feel fantastic, solid levers, no squeal, loads of power and no drag/rub.

However, on longer descents they tend to pump out, so that the lever gets further and further away from the handlebars. It doesn't seem to result in the brakes rubbing, just in my arms getting more fatigued from trying to grasp a brake lever thats further away.

I'm not prepared to change my braking style, so how can i fix the problem?

Organic pads might resist the heat build up but i ride in pretty much exclusively gritty and muddy conditions, so that rules them out.

Bigger rotors might help but i don't want the extra power or weight (well, its the looks i object to actually!)

Can i squeeze a little fluid out of the bleed nipple, thus emptying the reservoir a little and giving more room for fluid expansion? Can try this tomorrow anyway, just interested if anyone else knows a solution.

Cheers!

supersonic

Unless the brake is overfilled, its not advisable to lower the oil levels as it can effect the performance in normal conditions. Brakes are more prone to pumping out if they need bleeding. It could be you are simply exceeding the capabilties of the brake.

Sir HC

Bleed the brakes, make sure there is no air in the system.

One thing that can happen, is you bleed the brakes with worn pads, then fit new pads, resulting in their being two much fluid in the system.

the4thdeadmonk

Hmm... cheers guys. although everyone else has offered the same advice, i really don't think bleeding is the answer, they've been bled twice now.

I'll maybe try once more using the official shimano method instead of just forcing the fluid in from the bottom with a syringe, but if the levers are solid surely that suggests that the brake doesn't need bled?

supersonic

Its just that the problem is more noticable at very high fluid temps. But as you say, may not be that at all! Larger rotors do provide a larger heat radiator, and vented rotors are supposed to work well, but you don't seem too keen. If a mechanical problem cannot be found, then its going to have to be new brakes I'm afraid.

dave_hill

I'm mystified by the levers pumping out. Usually, long descents cause the exact opposite to happen - as the fluid heats up bubbles start to appear in it which can be compressed. This causes the lever to feel spongy and eventually it will pull right back to the bar with no effect!

Why are you so unwilling to change anything? Usually if you have a problem you need to change something to sort it! :?

Do you cadence brake or do you drag your brakes? Cadence braking can improve your control and lets your brakes cool more effectively.

The difference in weight between bigger and smaller discs is minimal. Granted it is rotating weight but I swapped my 160mm and 180mm discs for 203mm and noticed no difference whatsoever to the way the bike rides. It did solve my brakes overheating though and gives me a lot more modulated braking ability for longer.

the4thdeadmonk

Its a fair point, the main reason i don't want larger discs is that i like the principal of nice small discs on my nice, neat XC bike. Ridiculous eh?

What is cadence braking? I drag my brakes a lot because it suits my riding, plus if i say so myself it doesn't seem to slow me down too much

I'll probably go for the bigger rotors then, followed by trying BB7 mechanicals if that doesn't solve it, i'd be willing to sacrifice a bit of performance for consistency and simplicity!

Splasher

WTF? Boiling brakes in the UK.

Cadence braking was a technique developed for allowing a car to continue to steer under heavy braking and is what ABS is based on. I would have thought it would be a ridiculous thing to do on a bike.

When you bled your brakes, did you push the pistons right back in and wedge them there? If not you've overfilled your system.

And finally, this might sound stupid too, but are you sure it's lever pump and not forearm pump? Are your levers measurably further from the bars at bite point or do they just feel that way on your tired hands?

the4thdeadmonk

Its an interesting point! I'm pretty familiar with how arm pump feels (two seasons DH racing with rigid forks...) so i don't think its that, but of course, the levers do seem to pump out at the same point of the descent that my arms would begin to pump, hmm...

That said, my V-brakes never seemed to pump out, and if anything, they caused even more arm pump, so i think that maybe rules it out.

When i bled the brakes i used the yellow block shimano provide for the purpose, although it slots in quite loosely, perhaps it allows the pistons to move out slightly..

One more bleed then perhaps its time for 180 rotors!

Splasher

Why not try one more bleed and some organic pads? Cheaper and lighter than 180 rotors.

the4thdeadmonk

Yeah, but past experiences with organic pads (on hope brakes) led me to conclude that they were rubbish in the wet. I'm prepared to believe shimano pads might be better, but at least i know where i stand with bigger rotors.

Awkward b*gger eh?