Hi
That sounds like the master cylinder
if you pump the brakes with the engine off, the pedal should come up a bit and go hard,
if it goes down slowly then its the seals in the master cylinder, a good master cylinder and servo, should stay high and quite hard, once the engine is started the pedal should drop slightly
the diesels had problems with the servo and master cylinder been contaminated with engine oil, theres a non return valve between the servo and engine on the vacuum line that sticks open and lets oil vapour into the servo under boost, eventually the oil destroys the servo and master cylinder.
but your sounds like it might just be the master cylinder, servo can be checked once the master cylinder is off, to see if the servo has oil inside it, it should be clean and dry inside.
this was a recall for landrovers with the same engine
cheers
Joe