[s-cars] What constitutes a leak?

[Tony Guttmann]

Having gone the RS2 turbo/manifold, Lucas injectors, Forge b/p valve, Samco,
Stromung (equivalent) exhaust route, I'm still having problems with
excessively rich mixtures--hence air leak suspicions. This has precluded
proper chip tuning.

Disconnected big hose from MAF outlet, fitted adaptor and inflated to
15 psi. Blew off adaptor and nearly took out my eye. Reconnected adaptor, got assistant
to hold adaptor in place to reinforce hose clamp, and repeated. Have also
plugged hose where it enters top of rocker cover (breather pipe), and also 
plugged inlet to throttle body. (Also replaced oil filler cap, and O-ring
on dipstick, though this is irrelevant to what I am about to say). Inflated
system to 20 psi. As I watch pressure gauge, it slowly drops, over a minute
or so to 4-5 psi. But surely, I think, some leakage through the turbo must occur?

I check by tying plastic bags around exhaust pipe outlet, and sure enough,
they slowly inflate as the pressure drops. So it slowly leaks out through
the (new) RS2 turbo.

My question, ladies and gentlemen, is do I have a leak? My mechanic, who was
doing all the work, claims that (a) expecting a turbo to be leak proof is
unrealistic (he put it more graphically), and that (b) such a slow leakage
would not significantly affect air/fuel ratios.

I told him that my impression, from the collective wisdom of this august
group, is that under the conditions I have described, 20 psi should hold.
Bullshit claims he, it's not the @#$%^ space shuttle! I shall ask the guru-hood
I reply. So here's the question:

What constitutes leak-free? I've already spent more than 12 months of my time
and my tuner's time (by remote control--we are 20,000km apart) trying to get
a decent chip set for my installation, thus far without success. Am I, or
more importantly (on this occasion) is my car leak free under the above
conditions?

And yes I have replaced the OX sensor, the ISV is leak free (100%), the
plugs and plug boots are new, and the whole car has only done 55,000 miles
(new cam belt, RS2 6-speed gearbox, Koni sport shocks, blah, blah, blah). 

I eagerly await the collective wisdom. My thanks to anyone who deigns to respond.

Tony Guttmann
'94 UrS4, '94 80 2.6 quattro (the daughter's)
'96 328i convertible (the wife's)
2004 Kona single speed and Giant OCR Compact (the son's) + 1 dead Volvo 244