Most pumps die because they got over heated and cooked the final valving. It's not the effort of pumping that drives people to use SCUBA tanks so much as the TIME it takes to pump-fill correctly; you have to add in a lot of cool down time to keep a pump happy and lasting long.
There is a final little "one way" valve system as the last stage of air being pushed out of the pump. An o-ring and a little metal valve...tiny things....which let the hot-high pressure air pass out of the pump, but no air to backflow in from the gun). The pump gets hot, those tiny parts get hot, and the o-ring just "cooks".
Have seen three recommendations for pumping.
1. Pump for 5 min. and let cool down for 15min. before pumping again. Problem with this is that some people pump too fast, and can mangage to generate a lot of heat in those 5min.
2. Pump for 50BAR and let cool down (lets call it a 10min. cool down). PRoblem here is that 50BAR put into a little air chamber (like an AA s200 or Discovery) happens pretty quickly, while 50BAR in a big talon/Condor tank takes a lot of pumps.
3. (been working for my pumps for years): 5 min. cool down after each 50-60 strokes. This is my choice, and I'll vent the line after each set of 50-60 strokes to bleed out moisture. May take 3 or 4 sessions to fill an average sized gun, so you won't get it done in less than 1/2 an hour.
Pumps also sometimes gunk-up, Whatever lube used sometimes turns to gray-goo and just gums up the works; again it's that final valving that gunks up and the air wants to back-flow into the pump (you'll see the pump handle self rise from the bottom stroke or lock-up from that back pressure and refuse to pump at all). Only real cure is a cleaning and reassembly (but if you have it apart, putting in new o-rings is just too easy not to do).
The normal death of a pump would be from old age. they run with no real metal-to-metal contact, everything that moves is on o-ring seals, so the wear is slow if they polished the internal metal tubes correctly. O-rings will wear, and will eventually leak, so the pump just starts putting out less and less air at each stroke until it gets a seal-rebuild.
