noticed on the porsche web site that before buying a car it should be leak down tested to confirm engine is good.i made my own tester but does anyone know the correct test procedure?here is what i know.warm engine,put each cylinder to be tested to top dead centre so that inlet and exhaust valves are closed.set regulator to 100psi and turn air on.after this im not sure.i then turn air off and time the pressure drop for 1 minute.can anyone help out?