Hi all!
I never push the loco under any circumstances. Just open the Steam valve slightly, shifting the reverser to one direction, closing the steam valve, reverser to the opposite. In combination with not too much water in the boiler this should be enough.
Even the double acting oscillating cylinders from Regner´s Lumberjack make no problems when priming is done that way.
Having now a Roundhouse Millie with slip eccentric valve gear I do it the same way but I don´t turn the wheels directly for reversing. I just put the loco on track and give it a half turn in the opposite direction and then it comes back on the blocks.
Hope this helps.
regards
Klaus