I was having a spot of bother with what I though was a sticky piston so pumped the pistons out a bit to clean them. The outboard piston came out but wouldn't push back in (so I opened the resevoir to see if that would help) but the piston stayed out (the inboard one is flush). Anyway, I tried the lever a little more and popped the piston too far out and lost the oil.

So, can I push the piston back in and bleed (I've got the oil) or have I wrecked the caliper/seals? It's a Shimano LX caliper and I can get a replacement, but I'd rather not. Help would be appreciated as I was intending to ride tomorrow.

push it back in and bleed it. what have you got to lose??? 20p in fluid.

What have I got to lose? Well if it's not safe quite a lot. I wanted to know if it really is just a case of pushing it back in or whether I need to replace the caliper.

when you bleed it and compress the lever so the pads touch the disk squeeze as hard as you can and if no oil leaks out it should be fine.

if your not confident in you home mechanics the take it to your LBS.

http://www.parktool.com/repair/readhowto.asp?id=126

Well I need to learn to bleed the system, I reckon I'll be ok. The piston pushed back in with a solid click, so I've give it a go. I'm still a tad worried about safety though, am I likely to have damaged the caliper?

you wont have damaged it!! how do they fit pistons in the 1st place??

bleed the system then put some thing solid between the pistons and press the lever to test for leakage. if there is none then i would say you will be fine.

Not a safety issue its a performance issue. If you can't get the air out then it'll be spongy or if you have damaged the caliper body/piston interface is may leak behind the pad or may not move well – that should be worst case scenario 🙂

Wish me luck then. Having managed to completely build the bike from scratch (I'm a bit of a numpty, but learning) I steered clear of hydraulics for just this sort of reason, I guess I need to bit the bullet, if I managed it once I'll be fine. The piston didn't pop out, it stayed in place but leaked oil from one area, so hopefully everything will be fine.

On a related issue, CRC are selling LX calipers for £19.99. They're list as rear 160mm IS. I'm assuming that, like my old Avid mechanicals, the calipers are the same and it's the adapters that count. Could I simply fit a rear 160mm caliper to my current front 180mm adapter and use that?

Probably,but I can bleed my Hopes,and I'm definitely cack-handed.Remember it's not a race,and it's very difficult to break anything.If you're getting stressed,take a break,and get some good instructions (Park tools usually good) and follow them.
Ian

Is the lever going stiff all I'm after? I've primed the system, closed the bleed nipple and pumped the lever, it's gone stiff. The Shimano instructions then say to open and close the bleed nipple rapidly with the lever depressed to clear air from the caliper, but when I do this I seem draw oil and air back into the system from the bleed tube and the lever goes spongy again. Bubbles have stopped rising into the resevoir but if I muck about too much I'm going to run out of oil.

do as you have done. till lever is stiff. then go for a test ride if its pap then you have a problem that needs sorting out.

its quite hard to tell you what to do via the net. best off finding a mate who has the "know" to teach you what to do.

all you should do is pump fluid in the nipple till it comes out at the lever. do the nipple up. leave lever pulled back to bars for 5 mins to get rid of bubbles. if it feels a bit soft put a little fluid in the lever and pump it a bit but slowly.

When I built up my XT brakes from scratch I thought I'd bled them properly, got the lever really stiff and everything. Following morning they pulled straight back to the bar, not impressed. Another bleed(ing) session ensued and since then they've been excellent.

Well, thank's to you all for the help. After an initial panic that I'd wrecked it (OCD doesn't help I'm afraid) it seems to be ok. The brake works, there's no sign of leakage, the lever's stiff and has the same amount of pull as the rear – so fingers crossed. Apart from trying to hoke out an air bubble in the resevoir that turned out to be a bit of metal that I assume was meant to be there it was reasonable straight forward. I'll give it a careful ride tomorrow and see how we get on. Anyway, hopefully that's another bugbear dealt with, I tend to be rather nervous of my ability to get something right, but once I've done it I'm much happier. So that's a complete bike build and hydraulic brake bleeding!

well done and congrats!!

use the park tools website (that i linked to) for any info as its very useful.

also head to halfords and get some mineral oil for car brakes/citroen suspension. its much cheaper and you get a lot more than the shimano pink stuff.

Yes, that was one of the things I meant to ask, what exactly is the Citroen fluid I need, since it's mineral oil I need I don't want to end up putting DOT fluid in a really wrecking things. I'd also be interested if anyone can confirm that Shimano calipers are interchangable (front/rear different rotr sizes) with the appropriate adapter – just in case I need one.

The Park Tool site is great, I use it a lot along with my copy of Zinn and the Art of Mountain Bike Maintenance. One of the things that's been confusing me about bleeding the brakes is that there seem to be several different ways of doing it, I stuck to the Shimano one which seems to have worked (although I must confess to sticking the bleed tube on the nipple, opening the resevoir and then turning the hose bolt rather than the nipple and wondering why the fluid level didn't drop!).

Hydraulic fettling is a bit of a big step.Once taken you do wonder a little what was stopping you before (apart from the frustration etc.).

I had to jump in at the deep end last year when the kind guys from here donated a full XT groupset including a box of bits which when put together were in fact brakes. Got me to get over my fear of hydraulics though.

it is just mineral oil.

citroen used it in there suspension in there BX i think. (the one that went up a down)

go in to halfords or motor spares shop and ask for mineral oil.

if you ever need a quick fix then you can use sunflower oil.

I've just noticed, whilst browsing the Halfords site that their own brand mineral oil has weights quoted (like suspension oil), 15W, 10W etc. and it doesn't mention that it's for hydraulic braking systems. Do I need a specific weight or would it be safer just to stick with BikeHut mineral oil for brakes?

http://www.halfords.com/webapp/wcs/stores/servlet/SearchCmd?srch=brake+fluid&x=0&y=0&action=search&storeId=10001&catalogId=10151&langId=-1

the stuff at the top LHM Fluid. when you go in the shop ask one of the guys on the parts desk. dont tell him its for your bike as he will try and sell you the more expensive stuff.

its all the same just in a diffrent bottle

shaun