Example implementations relate to a pressure regulator assembly for a closed fluid loop of a CDU. The pressure regulator assembly has a cylinder having an internal volume, and first and second hollow pistons slidably connected to the cylinder to split the internal volume into a first volume portion having cooling fluid, a second volume portion having driver fluid, and a third volume portion having compressible matter. The first volume portion is fluidically connected to the closed fluid loop. The first hollow piston is reciprocated by the compressible matter to maintain operating pressure of the cooling fluid in the closed fluid loop. The second hollow piston is driven by the driver fluid in response to predefined pressure drop of the cooling fluid during predefined time period, to inject additional cooling fluid from the first volume portion into the closed fluid loop to restore pressure level of cooling fluid to operating pressure.