2001 Volkswagen Passat 'Y' reg Passat brake pedal needs to

My son owns a 2001 reg VW Passat saloon 1984cc manual. He experienced high fuel use when having driven the car 5 to 7 miles and it had warmed up. He could smell the brakes when he checked and found them very hot, so the brakes are locking on. To releasethe brakes the pedal has to be pulled up about an inch and a half then the car drives great and the brake lights go off until you brake again and the same thing happens. He took the car to Kwik fit in his area and they advised changing the pads front and rear and also the discs (rotas?) On the front, this was done and there was no inprovement. He took car back and the fobbed him off with a story and did a free CO2 test and said that the Catylic Convertor could be braking up? My son is not Mechanical, so I advised taking the car back and demanding refits or rechecks of the full system, my son did not do as told. I ended examing the vehicle after 4 months and found the battery dead and this brake pedal problem. A new battery was fitted and the car is running well except for the need to pull the brake pedal up after use! I have checked the vaccum pipes to the servo unit and around the engine the non-return valve is okay and the brakes work fine. One of the guys from kwik fit said there was a recall on this car for the servo unit air release valve, but I have checked this and VW say no, no recall for this car. Is there something I've missed on the servo unit side?