Pressure testing a questionable tank could have disastrous consequences. Turn off your bilge pumps, get out of the engine compartment and get yourself a healthy insurance policy for diesel spill. 3-5lb. pressure through a few micron size holes will drive fuel through your skin. If the tank is already weak, failure could be catastrophic.
Aphorisms to live by .....
If you can live with the consequences, go for it - wg
Y'am what I y'am an' thats' all that y'am - Popeye