Did you check that the rear of the seal is smaller in diameter than the front , I found this to be a problem with the 22 mm Susie seal once fitted to the piston.
The seal seems fine, when I put it on the piston the rear of the seal was a smaller dia than the piston, only the lip was bigger.
Might want a tad off, but due to the rain & DIY commitments it's not been out for a good session to get tested / bedded in.