that is a hard decision, that is a big nice filter.. do you have any idea how it is cracking? is your pressure high... if the bottom one cracked and now the top one did the same thing I wonder if it was some kind of defect from the factory that is just now showing up..
pentair does not even have a filter that big, they have the 525 4 filter and you really don't have the room to go with a TR140 sand filter, that thing is huge....
Have you had any issues with the bottom piece since you replaced it?