A small head gasket leak/cracked head will not show up on a radiator pressure check. Radiator pressure tests only go to about 15 psi during the check. A tiny defect in a head gasket may experience 160 psi from the piston's compression, forcing the burnt gases into the coolant.
Since the 15 psi radiator check doesn't go anywhere near the compression psi range, it will show up as a false negative on the test.
http://www.blockchek.com/index.htm
You can buy the kit at your local Napa store.
This is the thing to use to check to see if you have a head gasket, crack etc. Neat stuff. It is basically a test tube with a one-way check valve at the bottom. You hold it in the radiator opening and fill the tube to the indicated level with this blue fluid. You then apply suction via the squeeze bulb which draws the vapors up through the one-way check valve from the radiator and up through the column of blue liquid. Any hydrocarbons existing in those vapors will turn the blue liquid to yellow indicating that there are combustion gasses in the cooling system. If it stays blue, that means no leaks.
I borrowed this same contraption from my mechanic buddy a few weeks ago when I thought my Charger was running hot and I saw air bubbles in the coolant. It reassured me that there was no problem.
Just for kicks, when the fluid wouldn't change color from the blue, I checked it by holding a capful near the running exhaust pipe and it immediately turned bright yellow.