alum radiators

I usually check the rad for pressure by going three to four pounds more than the factory spec. 16# on the cap, I'll take it to 19# or 20#. I'll pressure check an entire system just a few pounds over, too.

One inexpensive way to check a rad is the use of a bicycle inner tube. Cut it in the middle, opposite the valve stem, clamp both ends to the radiator. Fill with water. Put your air compressor to the valve stem and put air in it. (Of course, you'll have to plug off the overflow nipple, but that's not hard with a piece of hose, a couple of clamps and a bolt.) Let it sit for a little while and see if it continues to hold pressure, checking over for leaks.