We used to make OEM radiators,the most demanding customer specification for radiator durability was pressure cycling from 0 to 2.5 bar with glycol at 130 centigrade for70.000 cycles,x bar-2sigma. With a batch of 7 taken from production. The expansion bottle should ideally have similar durability to the radiator. The toughest customers were VW and Honda! It would be interesting to know if you or your supplier is doing a durability test or just a simple pressure test.