82 and 84 so that an operator may manually activate air flow control valves 58 or 59 alternatively control unit 60 may be an automatically controlled unit which adjusts settings of air flow control valves 58 or 59 in accordance with a numerical control system in either manual or automatic configuration air flow control valves 58 and 59 may be independently activated by control unit 60 to raise or lower the pressure in each or both of balance piston cavities 42 or 46 the pressure in one balance piston cavity may be raised by a certain pressure while the pressure in the other may be raised an identical amount a lesser amount a greater amount or may be lowered by any amount within the pressure range capability of the system during engine operation control unit 60 may operate air flow control valves 58 and 59 independently to control pressure levels in the respective balance piston cavities 42 and 46 air pressure inside balance piston cavity 46 is controlled via air pressure relief tube 47 by activation of air flow control valve 59 when air flow control valve 59 is closed the pressure inside balance piston cavity 46 is held at essentially the maximum pressure available from the high pressure compressor source 72 output to apply proportional pressure to the annular plate 43 as a generally downstream force represented by arrow 74 in fig3.