Probable cause of the brake condition you describe is a master cylinder piston leak. The effect is to allow brake fluid to pass behind the piston thus losing the pressure differential between front and rear that is necessary for brake function.

You may want to get a master cylinder rebuild kit as well as your rear brake pads. Also, if you have never changed your brake fluid (should be done every two years) you may have condensate contamination of the fluid. Under the heat produced by heavy braking, moisture in the fluid can turn to steam which is compressible and can offer a similar effect.