The hissing is most likely coming from where the white plastic elbow fitting pushes into the booster. If either the seal dries out or the fitting cracks, it will leak. The hose in the picture looks more like heater hose than the rigid vacuum hose. If that's the case, the constant flexing of the hose will lead to harder brakes as well. The check valve just helps keep vacuum pressure in the booster at all times, even under low vacuum situations and or repeated braking. Many of the TR8s with big cams develop low manifold vacuum pressure, which result in a very hard pedal the second time you step down on the brakes if enough time doesn't go by between pushes. Makes repeated braking in stop and go traffic difficult. A buddy of mine just installed a vacuum pump on his car that just creates vacuum for the braking system. Things may be a little different on the FI cars from what I'm used to.