in the cruse et al patent referred to above the only force initially acting to maintain input shaft 24 in its original position is the force of spring 28 so that by adjusting the spring 28 the initial bias force required to begin booster operation can be adjusted to meet the particular needs of the situation as stated earlier in the cruse et al patent the initial input force must overcome not only the spring action but also must work against the pressure of the working fluid and must overcome both in order to begin booster action the present invention therefore allows for a much greater control of bias force and a minimization of the bias force in the event that there is a failure of pressurized fluid either due to a leak in the system or failure of the supply there is sufficient mechanical linkage between the input and output shafts such that the output shaft can be moved strictly by the force applied to the input shaft without power boost as the input shaft is pulled with no pressurized fluid the regulating piston will compress spring 28 until piston 22 contacts the sleeve 29 which in turn will provide a force against the capping plug 26 to drag the input shaft and regulating piston to the left moving with it the main piston 10 and in turn the output shaft 30 in this mode since there is no power boost the working device attached to the output shaft 30 will be moved only if the input force is sufficient to overcome the resistance of the working device however since the output shaft can be moved in the absence of working fluid the design provides a fail safe system this is useful in the situation where for.