I've just attempted to install new front brake pads for the first time on this bike (GasGas 270TXT) and I think the pistons are siezed. After removing the old pads, I can't push the pistons far enough back into the calipers to install the new pads, let alone get the caliper back onto the disk.

Am I missing a trick here? On my previous bikes I could just push the pistons back into the calipers with a screwdriver, install the pads and off we go. The curious thing is, if I reinstall the old pads, the brake works fine.

I'm befuddled.

It would be possible for it to be too full of fluid and you are hydraulicly locking it when trying to push it back in far enough for the new pads.

Remove the cover and see if some spills over.

I am assuming that it is moving back and forth fine if the old pads are working and that it is going in some, or I would suggest that the master is incorrectly adjusted.

this used to happen to me when i had the techno what i done was got a set of pliers and forced it in or if that didnt work let the fluid out then re bleed it bleeding only takes 5 seconds

Hydraulic lock sounds about right. I'd leave the old pads on while pushing the pistons back though, this saves trashing the pistons with the screwdriver or whatever else your using.

Sorted it. Thanks guys.

It was a bit of a "Doh!" actually: As Alan B suggested, it was the master cylinder adjustment. As soon as I loosed it off, the pistons slid in reasonably easily.

I use a very small G clamp to press the pistons back in - saves on scratching anything and you get a nice even pressure.