Hard brake pedal problem, help ??

[tonyotoole1]

Hi, i have recently purchased a 1.9 TDI PD 130 Highline. The car is a cracker, however the brakes are pretty strange...When i use the brake normally it is not a problem, however, if i use it for a prolonged period - down a hill - the brake pedal becomes very hard to press, and the braking performance is not so good I read a paragraph about it could be something to do with a vacuum pipe that is leaking or burst...Does anyone have any idea's or do they know where i can find this pipe? Any help would be greatly appreciated. thx