682 inches in determining practical dimensions for the control valve 27 28 a first consideration is the effect of dynamic conditions on the brake system as explained above it is important for proper functioning that the control valve 27 shifts at the proper point upon brake application the shape of the brake displacement curve fig4 shows that the control valve shifting point can occur at a rather wide range of volumes if the brake pressure at shifting varies as it is likely to do under varying conditions of brake pressure application rate to allow for this the control valve 27 is arranged so that a rather large valve travel is needed to initiate porting changes an orifice limiting flow rate to the end of the valve then can produce a slight time delay in valve operation 34 tuning 34 of the system by adjustment of the orifice size and valve spring force can thus be accomplished to bring about desired valve operation up to this point in this description valve operation has been described primarily with increasing pressure operation for decreasing pressures is essentially the same except that the shifting point is not as important because braking is affected only by the minimum controllable brake pressure that will be obtained variation of shifting pressure also slightly affects the volume released and therefore the position of the primary piston.