That's how I did mine(s13)... The left-side piston took a long time to turn, but it slowly made progress. The one on the right side was done quickly. Just a few turns and it was all the way in.
My advice to you is to just keep turning.. that's all I can come up with.

Also, make sure that the rubber piston boot doesn't interfere, I think it might make things difficult if it gets stuck somewhere.