Brake booster check valve grommet Vacuum Leak - Stalling

Hi everyone,
I thought i'd ask for some help tracking down a vacuum leak. My duster stalls when i put it in gear. It was intermittent but its getting more and more common. I tested for a vacuum leak with carb cleaner but didn't get anything. I unattached the brake booster vacuum line and plugged it and the idle seems to be good now. I got a new check valve and it worked for a while but i kept getting random stalls that got worse and worse. I suspect its the nature of the grommet. The hole in the booster is an inch in diameter, but all the valves i've found are smaller than this. Their solution seems to be a nesting set of grommets that I suspect leak. They appear to just sit on top of each other with the smaller diameter grommet being held in place by the vacuum. its easy to yank off though even when the engine is running. does anyone have experience with these things? am I installing them wrong? Does anyone have a good check valve source with the correct grommet? They never list the booster hole diameter on any of the sites. Thank!