A servo valve comprising a housing having a supply pressure port, a control pressure port and an exhaust pressure port, a sleeve located within the housing comprising an axis, an inner surface defining a cavity, an outer surface in contact with the housing, and having a plurality of metering holes (26, 28, 30, 32) that penetrate through the sleeve from the inner surface to the outer surface such that each metering hole is in communication with the cavity and at least one of the supply pressure port and the exhaust pressure port, and a metering rod extending within the cavity for metering flow of pneumatic fluid through the metering holes, wherein the metering rod is coupled to a drive unit for axial movement within the cavity and is suspended within the cavity by a pair of spring seals (34, 36) each having a linear spring characteristic, each comprising a bellow spring, wherein the bellow springs each comprise an undulating profile in a radial direction of the servo valve, and being disposed at opposed ends of the cavity, wherein the spring seals sealingly engage with the metering rod to seal off the cavity, wherein the drive unit comprises a drive unit housing, a solenoid and an adjustable core, the adjustable core comprising a threaded first portion having a thread and a second portion, the thread of the first portion engaging a thread extending through the drive unit housing for adjustment of the relative position of the adjustable core with respect to the solenoid; wherein the drive unit comprises a plunger connected to the metering rod, the plunger being arranged to carry electromagnetic flux passing axially between the second portion of the adjustable core and the plunger, and wherein adjustment of the relative position of the adjustable core alters a gap distance between the plunger and the second portion of the adjustable core; wherein adjacent surfaces (74, 76) of the plunger and the second portion of the adjustable core are complementary in shape and extend parallel to each other along a direction which is inclined with respect to the axis; wherein the tip of the plunger projects into a recess in the adjustable core; and wherein the tip of the plunger is substantially frusto-conical in form, and wherein a receiving end of the adjustable core is provided with the recess, wherein said recess is substantially frusto-conical. wherein the drive unit comprises a plunger connected to the metering rod, the plunger being arranged to carry electromagnetic flux passing axially between the second portion of the adjustable core and the plunger, and wherein adjustment of the relative position of the adjustable core alters a gap distance between the plunger and the second portion of the adjustable core; wherein adjacent surfaces (74, 76) of the plunger and the second portion of the adjustable core are complementary in shape and extend parallel to each other along a direction which is inclined with respect to the axis; wherein the tip of the plunger projects into a recess in the adjustable core; and wherein the tip of the plunger is substantially frusto-conical in form, and wherein a receiving end of the adjustable core is provided with the recess, wherein said recess is substantially frusto-conical.