There are a few different sizes from the vacuum pump to the bottom fitting. Also a tee and cap. Check these carefully, there must be at least 18" Hg at the modulator. Any leak will be a problem.
I would pull the vacuum lines off and get exact replacements from your local auto parts store.
Remember, TOP fitting on the modulator goes to the Transmission. T to T, easy mnemonic. That rectangular block is only there to make it idiot proof.
The vacuum regulators can fail, either hard or soft shift. Hook a vacuum gauge to the top fitting and run the engine. Should be 18" at idle and dropping off with a quick stab on the accelerator. If not, replace and adjust per the thread listed above.