I have a 05 400Ex and when I went to replace the rear brake pads, I couldn't get the piston to compress so the pads won't fit. Any advice is appreciated!

 

I don't work on EXs but have never failed on calipers with a G clamp and a socket. The piston can "**** over" and jam easily though so pressure must be even on the piston. If it does jam, take the clamp off and press the brake lever, this un-jams it.