Drum and barrel pumps for transferring oil, fuel and chemicals from containers; hydraulic hand pumps for driving cylinders, presses, jacks and crimping tools; and hydrostatic hand test pumps for raising a system to test pressure. They share the reciprocating principle but little else.

For control and independence. Pressure is applied in increments the operator can feel, which matters when seating a component or approaching a test pressure without overshoot, and the pump works during a power failure, in a hazardous area, or anywhere remote with nothing to connect to.

One that moves a large volume per stroke at low pressure to take up travel quickly, then switches automatically to a small high-pressure displacement once resistance builds. It saves a great many strokes on any job with slack to take up, and is usually worth the price difference.

It determines how many strokes a job takes. A large displacement fills a cylinder quickly but requires more force per stroke at pressure; a small displacement is easier to pump but slower. Matching it to the cylinder volume and the pressure needed is what makes a pump pleasant or exhausting to use.

Substantial ones - lever advantage combined with a small plunger area generates the pressures presses and jacks need. The rating is stated for the pump and must be matched to the tool and hose it drives, since the weakest rated component in the circuit governs the safe working pressure.

Considerably. Drum pumps are used for oils, fuels, solvents and chemicals with very different chemistries, and a seal or body material that suits one may be attacked by another. Specify the pump against the actual fluid rather than by container type, and never reuse one across incompatible fluids.

Very little - seal and check valve replacement when delivery becomes slow or pressure will not hold, and keeping the reservoir clean on hydraulic types. Failure to hold pressure is almost always a check valve not seating, often on a piece of debris, rather than a worn pump.