If your Weihrauch HW 50 has lost its power often the piston seal and/or mainspring are worn out.
When replacing the seal make sure that the cilinder tube is clean and free from oil and grease.

If you have an old model HW50 with a leather seal, and you want to replace it with the new model seal, you will need the adapter 8621 and screw 8960 as well.