What kind of brake pads do you have exactly I have seen problems on a wide range of vehicles where the brake pads need to be hot in order to work properly and city driving low speed braking they have this issue. If the brakes are meant for a more performance application they won't keep their set when u brake them in. And in a sense need to be broken in each time u drive. And also if u have a stuck piston that is dragging the brake pads they could be over heating because what you're describing is like brake Fade not necessarily loss in pressure.