Brake help

[PEIFamily]

I thought my primary tank sensor died as my primary gauge quit reading, I picked up another new sensor yesterday and was the same. I was driving today and went to stop, barely had any brakes, had to step on the pedal. Inspected the brake cans and all are operating. Checked my tank drain, the secondary has tons of pressure but primary has a fraction of the secondary. Truck was fully aired up. I am guessing a check valve? What is the best way to trace? Thanks