By-pass is usually associated with and unloader or regulator valve. Both of these valves have a by-pass port that when pressure in the system is greater than the operating pressure the valve piston will move away from the seat area and allows the fluid to be redirected either back to the pump inlet, feed tank or to atmosphere. This by-pass port has a large diameter hole to keep by-pass pressures low.
Positive displacement pumps are always trying to move a fluid as long as they are running. If the discharge flow is stopped and the fluid has nowhere to go something in the system will fail (split hose or fitting, broken connecting rod(s) or stretched manifold on the pump).
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