citroen dispatch brakes locking on

[angrydispatchowner]

hi all, i have a 2008 citroen dispatch 2.0 hdi 120. 126k on clock.
there seems to be a brake problem, when driving the van for say 5 miles from cold it will come to a stop unless the accelerator is pinned to the floor.
on doing this the brake pedal has loads of travel, all 4 brakes are locked as this has been tested on a ramp, to release the brakes i have to open the bonnet and pull the vacuum assist pipe off, this automatically releases the brakes.
the servo linkages have been greased and has loads of free movement.
my question is could the vaccum pump be producing too much vaccum? thus puling the diaphram forward activating the master cylinder? or could the servo spring just be weak and not able to hold its own?
at my wits end with it now, any help advice would be much appreciated!
cheers malc