Isolate the modulator
Use your hand pump to verify the modulator holds vacuum.

I don't recall if you replaced the filter, could it have fallen off?

The symptom sounds suspiciously like several recent units that had a broken spring in the valve body.

Serious varnish on the valves in the body can mimic this issue.

Failing gaskets between the valve body and transmission case can mimic this issue.

Loose bolts holding the valve body to transmission case can mimic this issue.

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