In an in-line piston pump, a drive shaft and cylinder block are on the same centerline. Reciprocation of the pistons is caused by a swash plate that the pistons run against as a cylinder block rotates. A drive shaft turns a cylinder block, which carries the pistons around a shaft. The piston shoes slide against a swash plate and are held against it by a shoe plate. A swash plate's angle causes the cylinders to reciprocate in their bores. At the point where a piston begins to retract, an opening in the end of a bore slides over an inlet slot in a valve plate, and oil is drawn into a bore through somewhat less than half a revolution. There is a solid area in a valve plate as a piston becomes fully retracted. As a piston begins to extend, an opening in a cylinder barrel moves over an outlet slot, and oil is forced out a pressure port.