sections 28 and 30 in this first position of operation the zero pressure angle portions 32 of the coupling sections 28 and 30 are in driving contact and there is no tendency for the differential unit to resist or limit its normal differential function thus full differential action is available as long as the coupling sections 28 and 30 are in the first position illustrated in fig1 and 2 if the relative rotation of two axle shafts exceeds the predetermined range which has been designed into the unit for maintaining the coupling sections 28 and 30 in their first position there is a sufficient pumping of hydraulic fluid agains the piston member 36 to force the piston member towards the left in fig1 until it makes contact with the coupling section 30 continued pumping of hydraulic fluid causes the piston member 36 to urge the coupling section 30 against braking rings 46 which are fixed to the differential case 12 and this action is assisted by movement of the positive pressure angle portions 34 of the coupling assembly into driving contact with each other as shown in fig3 braking discs 48 which are carried by the coupling section 30 may be provided to increase frictional contact between the coupling section 30 and the surfaces of the braking discs 46 once the coupling sections 28 and 30 are in their second position as shown in fig3 the positive pressure angle portions 34 of the coupling sections tend to urge the coupling assembly into tighter contact with the locking means of the differential unit until all differential.