under the varying pressure of the fluid source valve body 12 further includes a single outlet port 18 through the side wall thereof a piston 20 is disposed within chamber 14 piston 20 is also of a generally cylindrical configuration with an annulus 22 being defined between the skirt portion of piston 20 and the inside wall surface of chamber 14 piston 20 is sized to permit fluid flow around its periphery through the annulus the primary flow path through valve mechanism 10 is however through an axial flow path through the interior of piston 20 more particularly piston 20 has an opening 24 in the piston head to permit the introduction of fluid entering chamber inlet 16 into the hollow interior of piston 20 the flow entering piston 20 through opening 24 exits through the open skirt bottom area 26 it will be appreciated that the flow capacity through opening 24 in piston 20 is of a lesser capacity than the flow capacity of inlet 16 accordingly upon the introduction of fluid to chamber inlet 16 piston 20 is axially displaced downstream within chamber 14 by the pressure at inlet 16 means such as spring 28 is provided for yieldably opposing axial movement of piston 20 in the downstream direction the skirt portion of piston 20 acts as a shutter to close a portion of outlet port 18 to fluid flow in response to the fluid pressure acting on the head portion of piston 20 to regulate fluid flow through the chamber outlet port accordingly as the pressure of the fluid source increases a greater force is applied to the head.