Leather is an old fashioned sealing material but a lot of guys like to use it still to keep their classic air rifles in an original setup. Polyurethane is a far better material for breach and piston seals and normally puts the power output up due to better sealing and less friction. The old BSA and Webley flat cross section springs are very effective and give good performance. They are still available but you pay a little more for them.
Baz