I've had one of that construction (pre-88) fail stuck because the spring rusted in two, so I can imagine if the ball fell out it could fail open.

If you can't cobble up a pressure gauge, you're stuck with part swap diagnosis. It doesn't make sense a filter clog or insufficiency in pump pressure could be helped by closing down the return path if the regulator was indeed functioning correctly. No sense at all.

But the regulator could be OK, and the pressure OK too, if the AMM is reporting much less air than is actually passing into the engine, as a consequence of a variety of common troubles, so your richening of the mix by increasing the rail pressure might make it run.
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Art Benstein near Baltimore

Don't be irreplaceable. If you can't be replaced, you can't be promoted.