Hi guys, have been having a bit of bother with a stiff front wheel for a while now - has been on and off. I'm looking at the piston and am wondering if it's meant to sit flush with the caliper or if it's meant to rise above the caliper by about 2-3mm. 

Cheers, Louis. Happy riding  

Nope only time it should be flush is when you first put it in after you have put new pads in. After that it sits proud, try CAREFULLY to push the piston out a short distance, use an air line if you have one, put something inside the caliper so the piston doesn't come right out, old brake pads are ideal for this, and check the state of the piston to see if it's rusty, if not then that's probably not your problem. Have you checked the front wheel bearings?

Cheers for that slice  have changed the brake piston, seals, pads and fluid. When I took the old pads out they were getting worn unevenly - could this be over tightening the front axle (pulling the forks in closer together maybe?) It's a ybr 125.. Would dodgy wheel bearings come on and off (stiffening the wheel up every day or two?

Uneven pad wear is usually down to the sliding pins sticking in the caliper body. I'd clean them up and give them a light coat of silicone grease.

As above does sound like pad wear, good idea to free up the pins, give them a sand down with wet and dry then grease them, see if that makes any difference.

Cheers guys appreciate the replys. Will let you know how I get on. If the slider pins are sticking, this could cause sticking brakes couldn't it? 

Yes, that could cause sticky brakes

Right, so pretty happy today as I sorted the brake problem - I'd bought pads that were ever so slightly bigger than the stock pads. The piston pushed it forward but it didn't spring back (got wedged in). Shaped into the stock pad shape we were good to go. Also when I bled the brakes I didn't give the lever a few final pulls to push the piston all the way out, resulting in a lot of lever play.