Not familiar with meriden, but some hammer guns never had sprung pins, sometimes it there is room you can retro fit springs but often there is no room because of the design of the pins
Using a heavy grease on the pins an stop them moving around so much, if not already present you can use a round needle file to file angles into the extractor that push the pins back, some times the angles are present but do not function correctly if they are worn or altered in some way.
If you still have issues you could take a bit of length (matter of 2mm)off the end of the extractor guide rod to prevent the extractor lifting before its well clear of the pins but this really is not necessary.
