After WWII, aircraft still used reciprocating engines, and hydraulic systems operated at 1,500-2,000psi. Aircraft cabins were still cooled on the ground, using auxiliary units and engines required external starting equipment. Cabins were not pressurized, so aircraft flew at lower altitudes, and required the use of oxygen for the crew and the passengers in some operations. [Much has changed since then including the standardized test equipment we use today. This specialized GSE for the ramp and the shop came to exist largely through the efforts of the military and a few companies.]
During WWII, aircraft were being serviced in many ways. The USAF operated their aircraft mostly from fixed locations, making it easier to establish repair and testing activities. The US NAVY was operating from aircraft carriers and forward bases, making the maintenance and testing a more difficult operation, since electric power was not readily available. In such cases, the ramp or carrier test equipment was powered by use of gasoline engines, rather than electric motors.
The service made no attempt to standardize the types of test equipment that should be used. Most USAF aircraft were much larger and required different types of equipment, in terms of both flow (GPM) and operating pressure (psi).
Changes came rapidly with the introduction of the jet engine and turbo props. Equipment to support these new engine type aircraft were developed rapidly, by a few companies, and it only took a few more years for many major companies to enter including the airframe and engine manufacturers.
At about the same time, the CAA (now FAA) became more involved, and it also changed to meet the needs of these new type aircraft. Importantly, regulations evolved to further the need for aircraft safety both on the ground and also during flight.
With aircraft growing in size and weight, aircraft need to carry larger volumes of fuel (jet fuel). The handling of such fuels changed the manner in which aircraft were being refueled. Both speed and accuracy were essential, since down time at the gate did not produce any revenue.
Refuelers were developed, as well as refueling pits, permanently located near the underside of the wings. Safety was important, so both refuelers and pits were carefully engineered to ensure "no shock" when the nozzle suddenly shut down as the wing tanks were filled, so the wing would not be damaged shock alleviators (accumulators) were used. And the aircraft were grounded during refueling to prevent "sparking."
Carrying more passengers required ways to quickly handle the passengers during loading and when the flight was terminated or making an intermediary stop. The result was the development of passenger loading units, either portable or permanently connected to the terminal (walkways).
Coupled to this requirement, manufactures developed these passenger loading units to include starting units, air conditioning and in some cases, lavatory servicing units.
With the requirement that ground checks were also needed for the hydraulic systems, portable units, "mules," powered by electric motors, gas engines or diesel engines were used to drive the hydraulic pumps. These HPUs (mules) were either single, dual or triple systems, depending on the type aircraft to be serviced.
Hydraulic pressures ranged up to 5,000psi, and it was essential to maintain proper operating temperature and also the cleanliness of the hydraulic fluid by removing both particulate and water. Filters were installed rated either in terms of absolute or normal ratings. High pressure filters were rated at 3u absolute with Dp indicators and return filters usually rated at either 10 or 25u nominal.
These portable hydraulic test units (mules) were equipped with pressure gages, flow meters, temperature gages, hourmeters and were offered in both analog and digital configurations.