Harbor Freight Leak-Down tester for engine - How to use?

My friends and I are fixing a car to race low-budget style. Found a used engine today and looked at it, everything looked great but the new leak down tester I bought from Harbor Freight gave strange results. We used 100psi input, but the regulator starts at zero. At about 20psi the right gauge that shows the pressure in the cylinder was at "zero" which on that gauge means "max" and is as far as it goes, but the instructions said to use 100psi. Anyway, heard air coming from various places. Later read a post somewhere where guys were complaining about this tool and indicated that the right gauge instead of being 100psi is only 15psi. Anyway, wondering if our engine is OK internally, the rest of it checked out ok. May have to get a better tester I guess and do like at 100psi like they say and see what the drop is. Any hints? Realize I need to have the crank secured at 100psi.