Did some testing with well water this morning. At zero pressure there is some flow from the permeate line, about 10% so it's not plumbed backwards. With clear tubing at the bottom of each housing I could see flow (and bubbles) from the permeate side of all 5 of the housings. I also tested each individual housing and get a little bit of flow from the permeate side at zero pressure. With the whole system together the most I can get is 68 pounds of pressure on the gauge between the 5 micron filter and the first housing. This is with the screw on the pump head at max.

This system is in my basement and has never been subject to freezing temps so I don't suspect a cracked housing, although anything is possible. I'll replace all the o-rings this afternoon and give it another try.

Thanks for the great advice.