Cat does not use a compression check to diagnose engine problems. They recommend a leak down test. I made a leakdown tester from an air blowgun and 1/4" pipe fittings. Tee in a pressure gauge. Set the cyinder at TDC and put it in gear or lock the engine somehow. Pressurize the cylinder through the injector hole. I used the tip off of a gas compression gauge which fit on a 1/4" pipe nipple. You can easily hold it against 100 psi.
Use your compressor to presurize the cylinder to your max pressure. Shut off the air and see if it will hold. If air leaks try to locate where it is going. Intake or exaust manifold indicates valve. Into crankcase, piston or rings. Or into the dreaded cooling system may mean cracked head.