My old Lawnmower got to the point where the priming button was no longer soft and pliable enough to return to the out position after pressing once. Of course that means that priming the carburetor was impossible without taking the air cleaner off and pouring a few drops of gas in. Dangerous and annoying.
I found the part at Amazon.com. When it came in it was very easy to replace.
You can view this YouTube video that will show you just how to do it.
It only takes about five minutes. No need to buy a new mower.