It's not that you have to KEEP them compressed. Once you compress them, they stay pretty much where they are. It's compressing them in the first place that's a real b****.
Unlike the fronts, which push straight in, the rear pistons actually ROTATE in a clockwise motion when compressing. So even if you push straight in with all your might, they won't budge. They are like giant screws. But there are two ways to do it. The first is to simply open the bleed valve and then push them in (make sure to have a tube attached to the valve to drain properly). I have used a pair of needle nose pliers (opened) to rotate the piston while pushing it in, but you have to be careful you don't slip and put it through your hand.
The other is to get a special tool that compresses and rotates the piston at the same time. The tool is cheap. I'm just too lazy to bother with it, even though I'm sure it would save me a lot of time.
