This cross section view shows a typical bore sealing cylinder fitted with the DEL recovery system which is shown in green. This recovery system prevents hydraulic oil from leaking onto the ground like you find on typical hoists. Bore Sealing vs. Displacement Sealing Greater lift capacity. No bleeding required. Tubes may rust if seals are too agressive.
Tubes are under oil and protected.
Air is drawn into the hoist as it lowers. No outside air is drawn into the hoist upon lowering. Sealing surface is internal and well protected. Sealing surface is exposed and vulnerable with the ram extended. Sealing surface is hard to inspect and requires honing to improve.