[DanEC]

I had a similar experience with my surge tank that I bought used. The prior owner said he had pressure tested it and it was tight but upon installing it I had a slght weep around the soldered flange when hot. I took it down to the radiator shop to see what they could do with it and when I went back they said they had a real hard time finding the seep because it would hold pressure just fine when they aired it back up. After several repeated tests they finally did spot a very small seep and soldered it up. So apparently pressure testing these tanks isn't always a sure thing.