how can i fix my brakes?

my brakes work fine on normal driving, but at low speeds in trafic my pedal goes hard, but not always. my vehicle is a citroen dispatch 1.9 diesel 53 plate taxi. ive had sugestions vacum told it was ok. someone sugested load sensor but i cant see how this would be related,as it works on load, and it does this with no passengers in. help! i dont want to go through an exercise or replacing alll the bits at a large cost. does anyone have any test and eliminate sugestions.