Brake pedal sticks

I have a 68 Plymouth Valiant with a slant 6 in it. I notice since the car has been sitting for a year or so that now when I push on the brake pedal, the brake pedal sticks and I can put my foot behind it and release it. The brakes do work and the pedal doesn't go to the floor. So I don't think the brakes are bad. Driving it the brakes feel fine when they are applied. But it's just the brake pedal sticking that concerns me.

Anyone ever experienced this issue and how to fix it?

Here is what kind of Brake booster it has under the hood.