I have drums on the rear and discs up front. When cold all four wheels are free running. As soon as the brake system warms up: the brake peddle get hard and the front discs come on and stop the car.The drums are warm but not hot.
Usually when ever new brake pads are fitted it is crucial that where the pads are closely fitted into the calliper, that area is free of any build up of brake pad dust, rust or debris, this would allow the brakes pads to move together on to the disc when brakes are applied and once you let off on the brakes it goes back and returns to its normal position, but as brake pads are getting worn the cylinder piston in the calliper works its way outwards from the cylinder, keeping the pads close to the disc but under no real pressure. In your situation you have a similar problem and there seems to be too much dust, debris or rust and it requires brake pads removing, cleaning those areas thoroughly and fitting brake pads back, making sure they are free to move within. There is also another small matter you will want to check and that is, make sure that the small hole on the master cylinder cap is allowing the air in and it is not blocked as this allows the fluid to go down as the brakes are getting worn and also gives the atmospheric pressure it requires to function correctly.
I would get your brake booster checked out. Sounds like the problem might be there.
To add what Big John said, metal also expands when hot so there is little free space for moving parts to work correctly.
