It could be that if you're getting "remanufactured" pumps from the neighborhood auto parts store, they basicaly clean and repaint them and put them back out for sale. It could be poor rebuilds... I have experienced this with starters. finally I went to GM and bought a NEW starter and never had to replace it the entire time I owned the car. ( it was a Cadillac Fleetwood with the 350cid Olds diesel .. they were notoriously hard on starters ) Or... since you have HydroBoost brakes, it could be something connected with that, that is causing your troubles.
1997 National Seabreeze 133 Limited
1993 Mazda Miata
2005 Basset Hound, "Fred"