Replace the breech seal with a modern rubber one. Won't cost a lot and will make a big difference.

There is an 'oil' marking by the slot in the cylinder where the piston lives. Put half a dozen drops of thick engine motor oil in there and work the barrel up and down to allow it to spread in the cylinder. Do not use thin oil such as 3 in 1

Leave your tin of WD40 in the place where it normally resides. DO NOT use it on the internal working parts of the gun

My guess is the power is down because the cylinder has run dry of oil. The leather breech washer has become hard and distorted with age and that is preventing the barrel latch from holding the barrel down.

Seen these problems many times