low pressure space or chamber 36 positioned in the front of the piston and a high pressure space 38 positioned in the rear of the piston a plug 40 having an axial through hole 48 is screw threadably received in the front of the extending portion 32 of the set screw member 30 preferably a separate plunger 42 extends axially and slidingly through the plug although it is to be appreciated that the plunger could be integrally connected to the piston thus the rear end of the free floating plunger contacts or is positioned in close proximity to the front end of the piston 28 and the front end of the plunger contacts or is positioned in close proximity to the valve body 24 second resilient means 43 such as a relatively strong second spring 44 is confined between the rear end of the plug 40 which is secured through the medium of the set screw member 30 to the valve housing 10 and the front end of the piston 28 so as to continually bias the piston rearwardly or to the right when viewing the drawing a fluid passage 46 is provided in the peripheral wall of the valve housing 10 the passage 46 is in fluid communication with a hole or port 33 in the rear part of the extending portion 32 and extends axially forwardly to communicate with the front opening 12 and then via.