If I pressurize my system to 15 psi, it bleeds down slowly and I can hear a slight hiss from the throttle body side of the intake manifold, but it’s hard to pinpoint and I’m not sure whether this is bleeding down fast enough to count as a leak? I assume that the system can’t hold pressure perfectly, even with no “boost leaks.” Does this look like a problem leak to fix?

 

doesnt look like it to me... typically i dont worry about what pressure it will hold.. i just blow about 40 psi in there and then listen for a leak
if you hear one... run your hand over where you hear it and see if you can feel it.

your biss screw will allow air past the butterfly and then it just flows thru the motor

 

I was just doing this and after finding all the obvious leaks I was left with the same from the TB and about a 1 psi a second drop...

According to most sources I could find around 1 psi a second is normal

 

If you hear hissing from the TB, there is a seal that will leak if damaged. I would start there. I had one leaking in my old evo 8 that i had to replace.

 

When I started doing leak testing on my Evo 8 the main things I had to replace was the seals around the throttle body shaft and my injector top/bottom seals. When doing a boost leak test, move the throttle open/closed by hand, wiggle it and press in a few directions. If you find any leaking around it, I would replace the seals. This is a very common seal that leaks. There is a video out there that shows replacing them. Be careful when inserting them as the metal can deform.
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