pressure on the piston face 510a the sealing action prevents fluid flow through the body of the valve i e into main chamber 528 at the other smaller diameter end of the valve body 520 a commercially available rolling diaphragm 521 is attached to the end of the spool 515 to form a 34 zero friction 34 seal and to centre the spool 515 in the bore 520a this arrangement effectively contains the fluid within the body of the shut off valve assembly 500 at this end while allowing free movement of the spool 515 in reaction to the pressure and force differentials which may exist in the valve body 520 an outlet poppet valve 560 receives the fluid flow from the outlet port 502 of the shut off valve assembly 500 and discharges it into the tank not shown either directly or through a diffusion conduit of some kind not shown the function of this valve is to maintain a reasonably constant pressure differential between the interior of the shut off valve assembly 500 and the discharge to the tank not shown in its construction it resembles a conventional check valve with a spring 518 loaded poppet 517 and guide 519 which can be.