A pressure regulator has a pressure chamber connected to an inlet flow area which receives liquid flow. As liquid flows into a forward chamber of the pressure chamber, pressure is exerted on the forward face of the piston, which in turn causes the piston to slide longitudinally through the pressure chamber. A bypass fluidically connects the forward pressure chamber and rear pressure chamber such that as the piston slides into the rear pressure chamber pressure increases at the rear pressure chamber, which in turn forces the piston to move forwardly, thus regulating flow out of the pressure chamber. A hand wheel is configured to turn a movable plate located inside the pressure chamber, with the movable plate configured to impede the piston from moving rearwardly. Pressure sensors and vents with a pressurized air source may be used to keep the pressure within a desired range.