I found this thread here on the later forum, sounds similar to the problem in this thread. Near the end of the thread a post by the fellow with the problem reflects that he checked pump output to lead him to discover the pump housing was cracked where it could leak back in to the resovoir itself thus reducing pressure and volume. If possible I would suggest checking the pressure coming from the pump, without the hydra boost it may not have been enough to notice.
Power Steering Pump / Hydra Boost / Brakes
