[gustaf]

If you had sand on the piston, there are probably some grooves that are causing it to hang up. This is a very precisely machined part and the slightest imperfection will cause problems with binding.

When I rebuilt mine (Call me crazy too, but I rebuilt my FD a year ago as well and it works great!!) the first piston and cylinder I used had a small burr that made the piston catch very slightly. Once it moved from it's resting place, it would not return to rest and would flood the motor. I got a different piston and cylinder that operated very smoothly and things worked great.