I keep getting a sticky piston in the rear calliper of my Formula Oros. It's a reoccuring thing. I do what you should, get sticky piston out by holding the other piston in and pressing the brake, lube it with fork juice, push back in and repeat. However, doing this numerous times still doesn't cure the problem.
What do I have to do? Do I need to strip the calliper down and put new pistons in? That sounds like a ball ache 
fork juice may not be the right thing. I don't know if it is compatible with the seals in brakes but I doubt it. Use the correct fluid or red rubber grease. you may have a corroded piston in which case remove it and clean it - but have new seals to hand as you might damage a seal putting it back or the seal may have gone / swelled up.
Try teh right lube or try new seals
It's just silicon spray- won't have any ill effect, but won't fix the problem either. Strip and clean (or get a shop to do it- it's not hard, but it can be a pain in the arris). Might need new seals and piston, might not.
Strip , clean, replace stuff and rebuild.... pain in the @ss but will solve it. (sorry just fully read the post above) but yes that's right!
yeh, Fork Juice is jut silcon spray, like Northwind said. I'll try again with the some red rubber grease and if that doesn't fix it I'll buy some new pistons and seals and have a go at taking it apart and putting the new ones in.
Cheers
Use brake fluid to lube the seal/piston.
I'm sure I've just used some Dot 5.1 to lube it before. I'll have a go with the Dot, if it doesn't work I'll get some red rubber grease and if that fails then new pistons and seals put together with a touch of red rubber grease.
Based on my experience recurring sticky pistons can be caused by a few things
Dirt trapped between calliper and piston
Temporary fix - pump the piston out as far as you dare and clean/lube with a cotton bud and brake fluid/silicon spray/fork juice/ etc. Refit reset brakes
Permanent fix – strip clean fit calliper rebuild kit.
Hardened seals on the piston
Temporary fix - pump the piston out as far as you dare and clean/lube with a cotton bud and brake fluid/silicon spray/fork juice/ etc. Refit reset brakes
Permanent fix – strip clean fit calliper rebuild kit.
Calliper Corrosion
Temporary fix - strip clean then very very fine wet and dry to remove the corrosion rebuild the brakes but expect the exposed aluminium alloy to corrode really fast
Permanent fix –new brakes
Top Tips
Before you start;
Read the manual
Watch useful videos on U-Tube or Manufacturers (Formula) website
Have the right tools/bleed kit/fluids/cleaning materials, etc
And make sure you have plenty of; Penitence, Time, Space, Light and in my case Coffee
EDIT
Enjoy 
Just lubing it now's very unlikely to fix it- might delay having to fix it properly a little but it's preventative rather than repairy.
@ spooky b329
Use brake fluid to lube the seal/piston.
DOT fluid is not a lubricant
yes, it will assist in the inital re-installation of the piston against the seal inside the brake caliper halves
but it doesn't do anything to address the issue of seal drag on the pistons as the DOT actually causes the seal to swell slightly (this helps seal the piston bore)
what you actually want (and is used / recommended by brake manufacturers in their factories and during servicing) is high temperature silicon grease (AVID sell this under their Pitstop label, or just use RS or automotive DOT rebuild grease)
you want to use this sparingly on the seals before re-installing the pistons, and then wipe any excess on the piston surface with rubbing alcohol before rebuilding the caliper
makes a massive different to a disc brake, will make the pistons feel silky smooth and prevent long term issues with piston imbalance 
Had sticky pistons on one of my Avid Juicy 5s. Bought a new piston kit and some red rubber grease and replaced them. I'm pretty crap with anything that can't be installed with allen keys and/or a hammer, and I didn't have any problems. Feels quite nice now. Think the other one needs doing at some point.
had a sticky piston on my maguras iirc. the tech site suggested just cycling them and that worked ( touches wood )
iirc it was because the bike had not been used for a while so i guess the lubrication had dried up.
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