Renault Modus hissing

I bought a mk 2 Laguna the kids told me from day one there was a hissing, I never heard it, due to the fact I wear a hearing aid. A few weeks had gone by and my brake went solid just as I turned a corner nearly went into something that was on my side of the road. After that the breaks went fine. The kids still went on about this hissing. Another couple of near misses with the brakes seizing up. Then tonight the hissing has been loud enough for me to hear constant but when I put my foot on the brake it stops til I release the brake.