your ABS pump runs all the time, and it doesn't build
pressure, the problem could be restricted flow of brake
fluid to the pump. Remove the feed hose, (low pressure), at
the pump and see if fluid flows out of it freely. If not,
there is a restriction in the reservoir. Remove the
reservoir and clean out. You may not be able to clean it
sufficiently. If that is the case, replace it and the seals
for it in the top of the booster.
STC2780 -Reservoir for 1995-2002 Range Rover 4.0/4.6
RTC6741 - Reservoir for 1990-1995 Range Rover Classic
RTC6490G - Seals for reservoir to booster for all years.