Hmm, there is something not right about this... If the line going to the modulator leaks but then when you connect the mighty-vac directly to the modulator it does not leak then I would understand that you have a leak in the line. But, in your case, you put the mighty-vac on the line going to the modulator and it did not leak (I am assuming you put around 15hg of vacuum for about 5 minutes and it did not leak)? If the line did not leak, then your modulator is not leaking. Even though you said you put the mighty-vac directly on the modulator and it did leak, this must have been a leak around the hose going from the mighty-vac to the modulator. My question now is, what is wrong with your transmission, why do you want to change the modulator?

Thanks,
Adiel