Let's start with the basics. How large is the pump in hp? How about the plumbing in I.D? How old are the cartridges?
I have worked on these filters since the first 300sq. ft. models came out in the very early 90's. The manifold is the same in all of them. When ever there is 30 or more psi showing on the gage with nothing downstream ( a clog, non-booster pressure cleaner, etc.) to cause this pressure, the manifold will collapse as yours has done. Maybe not immediately, but it will happen.
You do need a new manifold. Don't replace it until the cause of the problem is found. You probably need new cartridges as well as there are most likely small cracks in the end where it sits on the manifold.
Since the manifold is cracked, your cartridges may not have been cleaned as often as they needed, your pump is too large, or both, are the best-guess candidate for what you see.
If the bands that surround the cartridges are broken or the fabric seems "hairy" you need new cartridges. If the pump is anything over 1 hp it is too large and needs to be downsized. Just my opinion, but it has worked for the past 16 years.
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