Piston seals are located between the piston and the cylinder wall. On the outside, the seal moves linearly and tightly against the stationary cylinder wall. A hydraulic piston seal is the seal mounted in the piston as a fluid seal. A pneumatic piston seal is the seal mounted in the piston against gas leakage.They prevent leakage and ensure that the oil/gas is kept in place. With this oil the pressure is built up on one (single acting) or two sides (double acting) to allow the piston rod to move back and forth.