easiest way if you are a novice is to get a bucket next to a radiator open the bleed valve and catch a stream of water in the bucket. One bucket should sufficiently drop the pressure, may even need a top up after that. Leak is probably from prv (pressure release valve) which opens at 3 bar to release excess pressure to outside of the house. If prv is accessable use this to release pressure small red and round i think on vaillant has a black lever in the middle which you lift to blow off pressure
rawplumb, February 2010
