Ditto on Chris Mullet's response. The piece of wood between the pistons is very critical because if you crank the pressure up to 100 - 140 psi and they move, they will come out with a bang. There is a good chance that one piston will pop first and if you don't have the wood to restrain it/ slow it down, you have the opportunity for disaster. Once one piston pops, you will have to push it back in and restrain it with the piece of wood using some clamps so that you can pop the opposite piston.

There was no problem getting the front calier pistons to pop out on my 140. It was a no-go for both of my Girling back calipers. On one caliper I was able to get one piston out. On the other caliper both pistons were firmly stuck in place even applying air at 140 psi. For some reason the rears seem to be much more problematic than the fronts and rebuilds (at least for the Girlings) seem to be in short supply. I was able to source one from Rock Auto and the other from Iroll motors.