More pedal effort required, but pedal does not sink or show signs of leakage? Possible bad booster.
Try this first: With engine off, push the pedal a few times to make sure there's no boost left in the booster. Now with foot pushing firmly on the pedal, start the engine. Does the pedal sink a bit and then stop? That means your booster is at least working. Listen for hissing or other odd noises when you push the pedal, and also for changes in engine speed. Either of these can signify a leaky booster.
Another quick test, but one that requires that you be somewhere out of traffic and out of danger, is to get going and then stab the brake fairly hard several times in quick succession. If you're getting enough boost, the brakes should feel pretty much the same each time. If not, you'll find that the pedal effort goes up after the first one or two.
Another test for vacuum leaks: Stop the vehicle, and take your foot off the brake before shutting down the engine. Now go do something for an hour or so. Come back to the vehicle, and without starting the engine, push the brake pedal a few times. If the booster is holding vacuum, you should hear it whoosh a little, and feel some boost for at least a couple of pedal pushes, before the pedal firms up. A good booster should hold vacuum for much longer than that.
If the booster is good, make sure there isn't a leaky hose from the manifold, and that the check valve is not cracked or broken.
If everything with the booster looks good, you should make sure that your front calipers are not getting rusted and stiff, and that your rear brakes are adjusted correctly. Probably not a contributor, but since the rear self-adjusters are a farce and a joke, it doesn't hurt to check anyway.