How to tell if your vacuum transmission modulator is bad on a 1993 mustang?
Pull the hose to the modulator and see if it has fluid in it. If so, replace the modulator.
modulators are cheap enough. I would replace it if you don't trust it.
You can pull the vacuum line, put suction on one end and hold your finger over the other end and see if it holds suction. If it does not, then the hose llikely has a crack in it. Replace that hose.