After 3 years of use 2 of my 3 caps for the 7.5 gallon Flo-Fast jugs showed cracks like the one in the following picture.
I contacted Flo-Fast and they said that they had a batch around those 3 years ago that had some size deviations that led to cracks like these. They were sending replacements free of charge. They haven't arrived yet but I had already ordered replacement caps from a vendor (I am pretty impatient when it comes to exchanging emails. If a company doesn't answer within 24 hours I assume they don't care about emails) before we got some successful email exchange going. The caps I received are clearly made from a different material (I would guess it's fiberglass and not polyethylene or styrene like the old ones), so maybe it was not so much a size issue but a problem with the material used.
My jugs got exposed to a lot of temperature difference while closed and that might have added to these stress cracks.
Anyway, now you know that it might be a good idea to take a closer look at these caps when you notice a pronounced gas smell around them.
Update: The 3 replacement caps from Flo-Fast arrived over the weekend and they look exactly like the ones previously ordered from a car supply shop. The material looks like it is fiber-reinforced and seems to be a lot sturdier than the original caps that came with the jugs. We'll see how they hold up.
