If the brake pedal pressure to stop the van is ok it could just be the vacuum electrical switch on the servo u/s. If the vacuum is not building up could also be the vacuum pump of sucking air in the system pipe work.
MK 8 L4 H3 Motorsport campervan Past camper vans 1974 mk1 v4 with 2.0 pinto fitted 1986 mk3 2.5 di swb 1990 190 lwb 2.5 di 1998 100 lwb 2.5 di 2006 350 jumbo 135 tdci