I wouldn't remove the hard vacuum line to the modulator. Instead, cap one end and draw a vacuum on the other end. If the vacuum holds- and the line is clear- (no kinks or obstructions, check for this by blowing through it or pull a vacuum unplugged, it shouldn't hold), it's OK.
Some modulator lines went to a fitting on the intake that had a small orifice that acted as a sort of delay valve. If you still are using one like this, be sure the orifice isn't clogged up w/carbon, etc., or the tranny might shift weird.