I overhauled the DE filter and found no tears etc. But I had three grid fingers with slight miudpoint weave damage which I wrapped shut with electrical tape a few years ago. So this time I decided to replace said fingers (3) and now I get no blowback. (at least not yet) There is no way so much dirty DE could have snuck under the tape. I am at a loss to explain this issue any further. Although there is one thing I learned in life, when you can't seem to figure out the problem with a mechanical device, overhauling it seems to do the trick.