Next time you start the car, prior to starting, press the brake pedal hard and hold it there. Then start the car, you should feel the pedal drop as you gain servo assistance. Is this anything like you are feeling when holding on a hill? If so check the rear wheels for free rotation then using the foot brake with engine running..... safely jacked up and OUT OF GEAR of course! apply and check with peddle depressed to ascertain if you have breaking both sides and that they release and rotate unhindered when you release the brake peddle. If this all works fine then I would look to the master cylinder as the culprit. One last question does the brake peddle keep creeping towards the floor if you continue to hold pressure on it or does it reach a natural stop? I'm suspecting that it does! The brake master cylinder because of the way it works has a limited travel within its potential full range. This eventually causes cylinder wear within the usage range and when the wear gets to a certain level you start to get the fluid passing past the piston seal, this could either be due to wear in the barrel or more likely the piston seal.. If when you get this creep it stops at a certain point or slows radically then expect wear to possibly both areas whereas if it keep going then the seal is faulty. It might be possible to get a repair kit but frankly, just replace the cylinder complete.
