power brake booster vacuum leak

[6spdYJ]

The YJ has developed a new issue. It's not major - yet... At idle I have a distinct vacuum leak directly in front of the firewall on the driver's side. It goes away as soon as I touch the brake pedal. When I turn the engine off I can hear it drawing down for a few seconds after the engine has stopped turning. At this point it isn't affecting any function of the engine or causing any hard pedal stopping.
Based on experience, could it just be a check valve stuck or would it more likely be time for an upgrade?