I-R P175 builds air OK but won't kick back in again
I-R P175 builds air OK initialy, then idles down as it should when you stop using air.
However, it won't kick back in unless you pull out 'start-up' valve on dash, let bleed off, then push valve back in.
Something is sticking or plugged that won't let it drain down to allow itself to start up again. Changed unlaoder valve on top of tank,, no improvement. HELP!!