Your leakdown number sound just fine. The leakage is also not in the heads. The air you feel coming from the valve cover is just finding it's way past the piston rings (normal) into the oil pan, and up through the oil drain holes in the heads.
If you were leaking past a valve you would hear it through the throttle body if it were an intake valve, or through a tail pipe if it were an exhaust valve.
My leakdown numbers were also 13-15 percent.
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1988 Mustang GT
stock bottom 302, C4
10.9 @ 128 with 8 psi of Novi 1000 boost!----STOLEN
1989 Mustang notchback
Warmed up 306 with a Procharger.
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