I love those. I've often had to jump on the top of pistons to get em to finish going down. My preferee method is jumping the down normal and using high speed down valve to really open it up to let oil through.
OP, I repacked a canton a few months ago. Simple single stage head, I've torn apart and repacked 50 plus cantons. This one was a roped hydro, blue jack. When I put it back together the same exact thing you ran into. There was an inch of void space with nothing in it in the head. I double checked against what came out, the same. Checked against the mechanical drawings, they showed it the same. Bearing ring, packing, wiper. But there was an inch of unused space. I called canton and they confirmed no issue. I put it back in service and it was all good
