type indicated generally at 16 in the female coupling end portion again with reference to the male coupling end portion an internal surface of revolution is also provided with an inwardly facing annular groove 10c that is preferably fitted with an o ring this o ring in the groove 10c is adapted to seal against the external cylindrical surface of plug 12 turning now to a description of the pressure cap or closure of the present invention fig2 shows a preferred embodiment wherein a ball retaining plug 20 is provided with an outer annular wall 20a and with an inner portion 20b that cooperate to define an annular slot 20c therebetween ball elements 24 are provided in openings defined for this purpose in the outer annular wall 20a of this plug and these ball elements can be positioned as shown in fig2 so that the ball retaining plug and the pressure cap itself can be inserted on the annular end portion of the male fluid coupling the annular end portion 10a of the fluid coupling 10 is received in the slot 20c as the pressure cap is so positioned on the end of the male fluid coupling portion the 34 built in 34 plug 12 will be displaced axially inwardly by the end wall 20d of the central portion of the plug 20 once the parts have been so positioned an outer ball locking sleeve 26 threadably received on the plug portion 20e as shown in fig2 can be rotated in order to achieve axial displacement of the.