Abstract: A metering valve (10) having a bi-directional motor (11) with a linearly-displaceable output shaft (21) in which the shaft (21) is separated from but cooperates with a bi-directionally movable piston (16) to provide a variable flow aperture (102) in a secondary flow path (12,52,74,78,76,102,100,48) extending axially through the piston (16). Flow along a primary flow path (12,18,48,14) is varied by movement of the piston (16) in response to an imbalance of opposing hydraulic forces. The imbalance in forces results from varying the foresaid flow aperture (102) by displacing the output shaft (21). However, the flow aperture (102) is only transitorily varied by displacement of the shaft (21) since the valve (10) is designed to provide a follow-up servo function by which the piston (16) is kept at a substantially constant distance from the shaft (21).