as above nearly sh!t meself a few times this week due to the brakes not doing as they should

mostly on a wet day or day after its worse but heres whats happening,
press the stop pedal

and everythings ok unless i happen to hit any sort of bump in the road or go over anything metal like a grid cover etc and the brakes loose power and takes twice as long to stop. also get abit of a grating sound like im crunching gears is the best way to describe it. had new front pads this time year so they're still alright so im at a bit of a loss with my
limited non-existent mechanical knowledge.
sounds a bit daft i know but this is the only time it happens,any ideas ppl
cheers keith