Hissing brakes on 2000 peugeot 306 2.0 HDI?

Hi there i have recently noticed that there is a hissing sound coming from the area around my brake pedal the sound is there almost constantly with the engine running and seems to go when i depress the brake then come back when i take my foot off i have checked other forums and people have experienced similar with 206s and other 306s a couple had been to the garage and the mechanics werent to sure what the problem was. Please help