2 things come to mind here
1- main or return filter clogged up?
2- main relief valve spring broken?
might want to check that

-pull the filters ( i would also check the filter in the tank also)
-remove the relief valve and physically check it.

one more thing...
if this is a sectional valve worse case scenario is that it has a blown seal inside and is bypassing internally but i'd check the filters and main pressure relief valve first before taking the assembly apart.