Good call, the gray ones are horrible and they will split. The bluish green ones are the cap to have.btw check your pressure cap on the expansion tank we tend to forget about that one
As far as the petcock goes we have had several members that broke the whole socket out of the tank when trying to remove it so I just avoid it altogether. I did have to put a new rad in one of the cars - I think it was the '94 - when the upper hose nipple cracked.
...Lee