I bought one recently, just a cheap Sealey one. Compression test said it was low on #3. I thought it was just a cylinder head gasket. The thing with a leakdown test is you can narrow down where your issue is when you locate where the air is escaping from, for example exhaust or inlet valve, gasket, rings or bore.
We had air escaping from the breather, 100% leakdown so I thought maybe we'd nipped a ring ? Fault is below.

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'74 Challenger..9.46 @ 145.9 1/4, 6.001 @ 118 1/8 so far. 4023lb !!! # N/A, Marsh performance 655ci, Indy Maxx, T/R, Indy 600-13 X's, Street legal, pump gas, full interior, Cal-Tracs, mufflers, 3:73's and real 10.5 radials.
9.51 @ 142.4 1/4, 6.003 @ 114 1/8 with our old mule KB, 572-13, 580 wedge.
RHD '68 Barracuda Fastback 323ci street/strip. Best ET 13.88 @ 99.03