If your brake pedal goes to the floor when the brakes are hot, you've got air in the brake lines.
As a minimum, you need to have your brake lines purged of air. Also, if you haven't replaced the brake fluid in several years (or ever), you may wish to have the brake system flushed, refilled with new brake fluid, and purged of air.
The brake fluid may not actually need to be replaced. However, I mention this because brake fluid is hygroscopic (i.e., it absorbs water); and if it's old and been exposed to the air, it may hold water, which will turn to steam when your brakes get hot. This steam will have the same effect as air in the lines, making your brake pedal spongy and/or allowing it to go to the floor with little or no resistance.
In any case, this is a serious condition, which needs immediate attention.