Mysterious hydraulic self-destruction


I’ve got a bit of a mechanical riddle on my '95.5 honda passport 4wd. It has a hydraulic clutch, and I’ve had to replace the master and slave cylinders three times each. They seem to be lasting about a year without losing fluid, then the pedal starts to go soft and before long the seals blow. The system is extremely simple, and according to several mechanics and the dealership problems are rare. None had seen repeated failure like this. So now the plot thickens a bit. Last time I replaced the master cylinder, it came with a new reservoir and everything. When the pedal started getting soft ~2 months after installation, I could pump it back up for a week or two, but that eventually quit working. If I take the cap off the reservoir, however, and pump the pedal, it’s good to go for a day or two. Any thoughts on what could have been causing the previous cylinders to fail or why removing the cap would allow this one to function(it does have a small ventilation hole)?