Just wanted to post this so that others will find it.
MTD will tell you that the primer bulb on small MTD yard tools (in this case, it was a 4-stroke Troy-Bilt trimmer) is not available as a separate part. They will then sell you an entire replacement carburetor. Due to ethanol in the gasoline, the rubber used in the primer bulb hardens and cracks sometimes after just a season or two.
Well, they're right, and they're wrong. Sure, MTD won't sell you the primer bulb, but every Home Depot will sell you, for under $12, a kit containing several different "universal" bulbs, typically with the Ryobi/Toro names mentioned. One of the bulbs in this kit is a precise fit for the bulb on the Walbro carbs they use on their trimmers and such. Buy it, unscrew the retaining ring around the base of the bulb (two small Phillips screws), replace the bulb, and put it back together. There you go. And you just saved at least $30. Buy several kits and put them away; you'll need them.
In other news, Walbro small-engine carbs are essentially unrepairable in other ways. If you get a clogged jet, you might as well throw the carb away. No amount of cleaning, blowing-out with compressed air, or swearing, will get the thing to run again, ever. Forget it.
Sears must have figured this out, because when I ordered a replacement carb for a lawn tractor with a 16HP Briggs single-cylinder engine, the replacement wasn't a Walbro, it was a Nikki, and was better-engineered than the original Walbro. It was, alas, $144.