vacuum pump keeps breaking

I have a 2002 E350 Super Duty 7.3 PSD, and have a recurring problem with vacuum pumps. I am now on my fourth pump in a year and a half. I have checked all related components, brake booster, master cylinder, pads, discs, and can find no other problems in the system. Can the pumps be overworked? It seems that they work continuously anyway but I don"t know. Another, probably related problem, is a brake warning light that goes on and off for reasons I can't figure. Any help would be greatly appreciated, thank you.

PS. I know that the brake light goes on when vacuum gets low, but it also goes on when the vacuum is fine and a new pump has been installed.