Originally posted by WydRyd
My understanding is that if air is leaking back out through the intake, it's intake valve, if it's leaking out the exhaust, it's exhaust valve and if it's leaking back out through oil filler, then it's piston rings.
Maybe some of the more experienced would chime in to verify this
This is correct - the breather (or oil fill) will hiss under leakdown test of the rings are not sealing, and the intake and/or exhaust will hiss if a valve is the problem. A couple of things to be aware of - the best way to do a leakdown is on a warm motor - this means running it up to operating temp then testing it after ~15-20min cool down. Basically, it's get the air cleaner, intercooler, and smog hoses (driver's side) out of the way, and get to it.
You need to be sure that the cylinder you're testing is at TDC to get good data, and that the car is in gear and the wheels are well blocked - 100psi will move the piston down, and it will roll the car over your toes. Also, with a Porsche, they have a bad habit of dislodging carbon whenever you pull a plug, which then usually finds its way into the exhaust port. So, if you get a good exhaust hiss, it's wise to either go drive it a few miles and check it again, or if the motor is out of the car, hit the exhaust valve with a rubber mallet while it's pressurized to see if you can dislodge the carbon.