While it is running for 2-3 seconds, does the airflow meter vane respond? If not, it may be gummy or you may have an air leak (alternate air path from a ruptured air hose).

Another thought - you say line input pressure at the FPR is adequate, and the return line opens at about 90 psi, but there is no flow in that line. This is a contradiction. Input pressure must be below 90 psi, or there would be flow back to the tank. Have you checked the accumulator too?

Steve

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'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7