ya i was going to say its your booster,
TEST:
start car,
pump brake once or twice so you know there is pressure, then pump and hold the pressure on the last one,
shut the car off,
hold for 30+ seconds and if you have the same amount of pressure your brake system should be fine(meaning no significant leaks and your master cylinder and brake booster are functioning properly.) BUT CHECK EVERY CONNECTION AND MAKE SURE THERE ARE NO LEAKS!!! Have a mechanic help obviously...
NOW...
No leaks and your pedal stayed firm, if your car still has a sticky brake or pulls to one side constantly you probably have a stuck caliper (or rear drum cylinder). An easy way to see which caliper it is, is to get an inexpensive heat gun and test to see if one rotor is blazing hot, severe cases will blue the metal.
If your pedal SINKS its indication that your master brake cylinders seal has cracked and is leaking.
If your pedal STARTS TO RISE OR BUILD PRESSURE its a good indication that your brake vacuum booster is shot.
I went into such detail because... EVERYONE SHOULD KNOW THIS!!! Its not common knowledge but man oh man people its your fundamental basics of a car, learn how to change your oil, tranny fluid, brakes, and make sure that your ball-joints tie-rods and wheel-bearings are ok to drive on. it takes maybe 15 min a month to give your own vehicle an inspection, LEARN HOW.
hope this helps everyone..."I don't care if your pos runs, i just want your pos to be able to stop."