no[little] power brakes assist

15 inches is PLENTY enough vacuum to operate power brakes. Inspect the vacuum line and check valve. Start the engine and let it idle a minute or so. Then grab the check valve and unplug it from the brake booster. If you hear air rushing into the booster, the booster, check valve and hose are all good. If not, then it is not getting vacuum somehow. At that point, inspect the vacuum line closely for cracks, remove the check valve and verify that you can pull air through it from where the hose plugs in. If you cannot, it's bad. If you can and you've not found a crack in the hose then the booster is at fault.