For Mootho, and others who would like to know more. Here is a link to the History of paintball gear, as presented by paintball x3. Gear History 1981-2000
I preferred nelson based marker back in the late 80's due to lighter weight, better air efficiency, and user accessibility vs the sheridan based pgp, which needed a proprietary valve tool and had a lead gasket that needed to be replaced every time the valve had to be pulled.
My only complaint about my early nelson based pumps was frequent cup seal failure, broken #4 and #6 power tubes, and losing/stripping the brass field strip screws, usually the ones that secured the pump arms to the hammer.