Brake pedal remains firm

Hi guys, I was driving my 1995 disco around today and noticed that if I'm in slow moving traffic and press the brake the pedal remains solid for a while afterwards, like it does when the engine is turned off. It's ok after a short while but braking power reduced drastically in slow moving traffic. Anyone have any ideas?

Cheers

poor vac pump or servo or leak in pipe inbetween

Cheers for the reply, will get on the ramps this weekend take a closer look! Merry Xmas peeps