He is right. If your modulator will not hold a vac, it is bad. What color vac modulator do you have? It must hold a vac to work properly.

On the original 1981 vac modulators, the cap was just a black rubber cap. You removed it, turned the T pin, and replaced the rubber cap. The rubber cap was to hold vac.

If your Mityvac will not pump up and hold a vac by placing it on the trans modulator nipple, the trans modulator is bad. Or maybe it was installed cockeyed.