I have a 2007 Doblo Hitop with a bootjump. The vehicle drives well except for an occasional hiccup. Every once in a while , when braking, there is a crunching/grinding noise and it seems as though the brakes are not engaging , causing momentary drift forward. I’ve had all brake pads, discs etc replaced, new bump stops and shock absorbers fitted by a reliable mechanic. After all of that the same problem has appeared again. Can anyone advise of other possible causes ?