Since the pressure of chamber is different compared to the air pressure, the oil flow through the gap between the plunger and the cylinder is shown in Figure 1. The oil flowing between the plunger and the cylinder is a laminar flow.
Figure 1 Oil flow and through the leakage gap
As shown in Figure 1, the variation of oil speed is fully
depended on the pressure difference between the chamber and atmosphere, and it
is not affected by the plunger speed. The oil flow rate between the gap of
plunger and cylinder, 
can be written as
where, 
is the coefficient of
the viscosity of the oil.