I think this may be the reason that some of us are surprised that so many have problems with Paterson tanks. The screw top versions (System III) were replaced far more than 25 years ago with the tupperware-lid style (System-IV). The sealing washers on the ancient tanks stand very little chance of still being pliable enough to work, if indeed they are still present - it was a grey, slide-in sort of ring if I remember rightly.
My advice would be to find the newer tank version, but don't forget to keep the reel(s) from the old one as a spare because they are the same size. The new reels have mouldings to grip the central-core of the light trap so if the old reels slide around use an elastic band, or an original plastic clip from the old tank, to keep them in place.