Hi Bill and all,
its a hard one to wrap your brain around
if oil or any liquid is displaced by a positive displacement pump against a closed circuit the result is the same as a mechanical block of the pump, a dead stop
in our engines oil pressure leaks off through the bearing clearances and of course the pressure relief valve
if it didnt the pump would lock and probably the drive fail !!
Take Bills hydraulic ram, the ram will move under hydraulic pressure, if theres air in the cylinder this will be pressurised up to whats needed to lift or move whats loading the ram, Air operated rams are also used in industry
On most hydraulic control systems a pressure vessel is included in the circuit to smoothen the actuation.
There are a couple of ways to dampen the movement of a pressure gauge, some are filled with oil or glycerine
or a needle valve is put before the gauge, restricting the flow, this dampens out the fluctuations in readings
(carb balancing gauges or gauges on piston compressor heads)
HTH
John O R