poppet 164 disposed in the bore 162 is urged against the fluid pressure in the outlet port 154 by a spring 166 the spring 166 is compressed by relief valve knob 168 disposed in the bore 162 looking particularly at fig9 the metal relief valve knob 168 e g steel aluminum or brass compresses the spring 166 to urge the poppet 164 against the fluid and into a valve seat 158 extending into the outlet port 154 the poppet 164 and spring 166 prevents fluid having a pressure below a predetermined level from passing from the outlet port 154 through the bore 162 adjusting the spring compression by moving the relief valve knob 168 closer to or further away from the poppet 164 determines the fluid pressure level which will force the poppet 164 out of the seat 158 and allow fluid to escape through the boss 160 and into the atmosphere referring back to fig2 a the substantially cylindrical relief valve knob 168 has a top 170 a bottom 172 an outer surface 174 and an axial through bore 176 extending from the top 170 to the bottom 172 the through bore 176 provides a path for the fluid through the pressure relief valve 156 to the atmosphere preferably the knob top 170 is formed as a hex head for engagement with an allen wrench and the knob bottom 172 has an annular groove 178 shown in fig9 coaxial.