I've just had a day of uplifting on my new glory fitted with new XT disk brakes. The feel is amazing and there is plenty of power and everyone raves about never having problems with them fading or pumping up. I however found the rear brake pumped up pretty badly on a steep, reasonably short descent. Basically far more easily than I would expect and the problem will be far worse in the alps. For the record, the front seems to be fine, the rotors are 8 inches superstar ones (not shimano) and someone else was running the same brakes and has never had any problems.

Am I expecting too much from what are affectively xc brakes? Would they benefit from a re-bleed (as I said they feel fine with plenty of power)? Any thoughts? Perhaps I should stop being a pussy and brake less?

No, you're not expecting too much and I wouldn't say they are XC only brakes. I've got XTs on my DH bike, never had a problem even racing in the Alps, and on my Meta where they handled the Passportes no problem at all.

Definitely sounds like air in the brakes, a re bleed should sort then out.

Thats what I thought but does air cause pump up? I thought it only caused the lever to pull to the bars. I never usually had problems with disks, always fit and forget so I don't really have experience of the problems that can occur.

sorry for noob question, but what is brake pump?

The fluid in the brake gets hot and expands meaning the lever can't pull back as far as is should and the power fades. You then get ridiculous fatigue in that arm from pulling the lever harder!

I thought air in the system just made a brake go all spongy and pull back to the bar, but perhaps it can cause my problem too?

Sorry, yup, air makes them spongy not pump. Expanding fluid makes them pump, especially moisture in DOT fluid but Shimanos are mineral oil.

I've never had my XTs pump, despite hitting some big hills with them.

if the lever feels soft but when pumped several times gets harder (oo-er) then it's air in the system.

My xt's are perfectly bled but some times i'll go for the brakes and get just about nothing! Apparently, unless you bleed them off the bike, then they can trap air in a pocket in the caliper?!

My xts fell a little like they get pumped if you squeeze and let go of the lever several times in fast succession. This happens when the brakes a cold and not riding, so it's not real brake pump. It also goes away as soon as you release the lever for more than say half a second, so I guess it's just something to do with the lever/reservoir design.
Anyway, this means that they occasionally feel like they are getting pumped when riding if I happen to be applying and releasing the brake rapidly, but it disappears as soon as the brake is released properly for a short time.

ah bum, I was hoping it was something that could be solved with a proper bleed. At the moment its an unacceptable amount of pump as the tracks were relatively short (triscombe in the quantocks) and my left arm ended up like jelly. I think I'm going to turn into one of those crabs with one huge claw.

Not keen on changing the brakes, especially since so many people rave about them and never have problems.

not sure about the quick succession braking thing, it would take 30 seconds to a minute for the brake to cool and get back to normal.

bleed them off the bike so the caliper is hanging vertically, when you bleed the XTs on the bike, you can get air trapped at the top of the caliper.

Also worth trying zip tying the levers pulled in over night to encourage anything in the system to come back into the reservoirs.

OK cool thanks for the info. I guess to save the hassle of taking my whole brake off the bike I can just remove the caliper and ziptie it to my frame in a vertical position right?

After I've held the levers in overnight, do I need take the reservoir cap off and top up with fluid?

yeah you can just take the caliper off and make sure it stays vertical.

You can take the caps off and top them up, more so just to check than anything – modern brakes have a breather, not closed system like a hope C2, so the should let air out of the reservoir.

if you've got the levers zip tied to the bar, you can tap the caliper and the hose with the rubber end of a hammer gently to help dislodge any air in the system.

The cause of brake pump is the fluid expanding past the reservoir diaphragm's capacity for expansion, the fluid then has nowhere to go except for pushing the caliper pistons out, giving that pumped up feeling.Two obvious causes are that you're either braking way too much(probably dragging them, rather than in short bursts), or you have a bit too much fluid in the system(try levelling the lever and removing the top cap/diaphragm to let the excess out).

What size discs do you have fitted? if your running little discs then the heat they generate wont be helping.

I liked my XT's but they did pump when I used to run 160mm discs, changed to 203's and they were fine even on long runs.

Dragging brakes too much Orr too much fluid or sintered pads

Just let a bit of fluid out if it is getting hot and expanding may need a little more space in the master cylinder

that used to happen with the old hope closed systems

I was using the brakes a lot but those trails are steeeeeeep! Certainly no scope for using the brakes on and off. I still wouldn't have expected them to pump up that bad. My juicy 5 never had any problems with a whole season in whistler and those descents are far longer. I'll try letting a bit of fluid out. Is it just a case of opening the reservoir and any excess will work its way out?

I've had it before with my Shimano, getting the air out always sorts it. To be honest I'd assumed compressing the air made it warm, the warm air expands.

I can't see it being the oil expanding, the whole point of the hydraulic fluid is the fact is doesn't expand/compress under pressure.

Dot oil is supposed to be "degassed" when bleeding brakes. I'd assume not doing this would lead to a similar pumping up the brakes situation. I'm not sure if the same is true for mineral oil.

tim, its a simple fix… a good bleed will sort the xt's out, had the same prob with my saints… it is just the air in the brake getting hot and expanding…

BTW i was the one on the other (old style) glory… legs ache a bit today… did you stay on for the cream tea's at duncan's house (the driver's) after the riding.

you should try gawton next… fewer tracks… but a spot on venue, couple ggod size jumps there too… give me a shout if you are going down that way.

neil.

My XTs have given me all sorts of interesting experiences over the last few months and I tried bleeding them over and over but all that would do is give temporary relief. However, bleeding them properly, using the dedicated Shimano kit with the syringe etc has left them feeling absolutely amazing and completely consistent. All the weirdness was down to air.

I would definitely bleed them properly and I reckon they'll be spot on.

Hi Neil, good to meet you yesterday, was an awesome days riding. We went to Brendan's track at the end and that was a new definition of steep the whole way down! I had to stop and let my rear brake cool down. We did go for cream teas after, was a great way to finish the day. I got Jim's (that was his name right?!) number in case I head down to Gawton in the near future.

So back to the brakes, sounds like I could do with a re-bleed. However, that requires buying some fluid and a kit of some sort. For now I'm thinking if I unbolt the caliper, fix it so it is vertical, tie the lever in over night to allow any air trapped in the caliper to rise to the reservoir and then take the cap off and tap see if I can get any air out. Is this worth doing before faffing around with bleeding?

Thanks again for all the replies.

Cheers,

Tim

that works if you're lucky…

the calliper is in 2 halves, the hose attaches to 1 half, if the air is in the other half of the calliper, you'll need to fiddle about with it a bit.

bleed them properly; buy some fresh oil, and put the bottle in the freezer over night before you use it to get any dissolved gas to condense out*

(*this really works, but it's for brake perverts only – the difference is marginal)

if you don't know how to bleed brakes, make friends with someone who does, buy them beer curry and cake.

tim… jim… thats his name… emails in the profile… mail me if you are down this way… was a good day…

as for the brake… like ahwiles said… I have had some success with the pump lever / back to the bar overnight… but it is a ball ache… and you cant beat a fresh bleed… a old motorcrossers trick: get a bit of bunsen burner pipe (the orange stuff.. get a long length, stick a bolt in the end making an air tight seal, then cut a slit into the pipe at the end furthurthest from the bike, then stick that in a pot… put that other end on the bleed nipple… open it up, pour the new fluid into the reservoir, and keep pumping the lever while topping up the res'… the slit lets the fluid out but closes over once the lever is back to the bar not letting the air back in…

simple quick and effective…

neil

Ok I'll do a proper bleed at some point. The shimano instructions say add oil in the reservoir and let it come out the nipple on the calipers until there are no more bubbles visible in the pipe. Someone told me I should be pushing oil in through the nipple with a syringe and letting it poor out the reservoir. What should I do? Seems sensible to follow the shimano instructions but I am aware of better ways to bleed avid brakes that differ from the avid instructions?!

Neil, that sounds like a good idea, might give that a go. Cheers

ignore shimano on this.

buy a 50ml syringe off ebay (you can't get these things in boots), and some windscreen washer hose.

get some mineral oil based brake fluid, i got mine from halfords, it says 'specialist mineral oil' on the bottle, and has some pictures of brake pads – apparently citroens use it or something…

take the brake pads out, put them in a different room, close the door – you ARE NOT ALLOWED TO OPEN THE DOOR UNTIL YOU'VE FINISHED AND HAVE WASHED EVERTHING.

push about 4inches of washer hose on the end of the syringe – and fill the syringe with oil.

push the end of the hose onto the bleed nipple, take off the brake lever top cap and rubber seal, loosen the bleed nipple with a 7mm spanner 1/4 turn and start pushing oil through the system.

this can get messy, have the lever clamped to something at about shoulder height, hold the caliper / syringe at about waist height – but have a stool or something to rest them on.

repeat 3times from here:

after about 10mls have gone through, close the nipple, squeeze the lever 3 times, holding the caliper with the hose/nipple at the top, push the pistons back in.

loosen the nipple, pull back on the syringe a little – a bubble may come out of the nipple – this air was hiding behind the piston on the nipple side of the caliper. push another 10mls of oil through.

you may see a bubble appear at the lever – this air was hiding behind the piston on the hose side of the caliper.

go back up to the repeat instruction ^^^

push enough oil through to fill the reservoir at the top of the lever, put the rubber diaphragm/seal back on, replace the top cap, and screws, these don't need to be super tight.

clean everything with an old t-shirt. wash everthing with warm water and washing up liquid, rinse well with more warm water.

you are now allowed back into the room where you put the brake pads.

top tips there ahwiles, never heard off freezing the oil before.

Thanks, thats some good instruction there. I assume that I need to put something betweeen the pistons when I squeeze the lever so that I can get them apart again?

Turns out Bike in Bristol where I bought the brakes are going to re-bleed them for me FOC which I think is tip top service! Hopefully they'll be able to do me a pucker bleed.

I assume that I need to put something betweeen the pistons

not really – by letting the pistons pump out you will fill the callipers with oil, any air will rise to the top, and get squeezed out* when you push the pistons back in.

*smaller bubbles need more encouragement – this is why i suggest repeating the process about 3 times.

bleeding brakes is a bit of fiddle, and the results are no better than the person doing the fiddling. i doubt that a shop would go through all the faff i described above, which is why i bleed my own brakes.

and why i choose shimano – mineral oil is more or less harmless to your skin, DOT fluid is evil stuff.

i always bleed with the yellow spacers in the pistons.

if you need a set i think i have spares. mail in profile.

Hi thomthumb thanks but I have the spacers I think. If there is nothing between the pistons when they go together, will they come apart by themselves? If not, I can't see it being at easy job getting them apart without having a gap to get a screwdriver in there.

I'll speak to the shop to see how they do it.

thanks for all the help.

cheers, Tim

Duplicate post

if you only squeeze the lever 3 times, you won't pump the pistons out very far, but you will have put enough oil behind the pistons to encourage any trapped air to escape.

if you use the spacer – the pisons don't pump out – which means you can't use the action of pushing them back in to purge the bubbles out.