it wont be master cyl if system is diagonally split, more likely to be load sensing valve at fault-when you say lock up -this valve adjusts pressure to rear brakes according to weight in van preventing lock up when little or no weight in vehicle. may want adjusting as they tend to wear on linkage, or could be siezed [if you get someone to press pedal whilst you are holding linkage you should feel movement- if so it's not siezed.]
k, July 2009
