A cracked primer bulb. You will need to replace it.
You don't. They are not difficult to replace.
Don't know that a leaking primer bulb can be repaired. Most likely, you have to replace it. You don't want to forget the routing of the fuel lines!! You might want to make a drawing of the routing before you remove the primer bulb.
Remove the filter cover. unhook the two fuel lines behind the primer bulb. Remove the two screws (inset star) that hold the carburetor, then gently rotate the filter base to expose the back of the primer bulb push the two retaining tabs and remove the primer taking notice which fuel lines go into it, one is longer than the other. Replace in reverse order and replace gaskets if damaged. Paul
A detailed description is available in related links.
Check with Ebay, I found one there.
No gasoline is reaching the primer bulb.
You either have a bad bulb or a bad fuel line between the bulb base and the gas tank. If there is the slightest hole or crack in the bulb it will not work. They are easy to replace.
www.odref.com/poulan/pdf/snapper/all/s31sst.pdf
There does not appear to be a primer bulb on the Stihl BR400 that I can see in the exploded parts.
It is simply held in by plastic tabs on the primer bulb. All you have to do is pry behind it and easily pop it out with a flat screw driver.The tabs break easily but replacment primer bulbs are available.http://store.chainsawr.com/
The primer bulb probably needs to be replaced unless either one of the fuel hoses leading in and out of the primer bulb bracket is clogged. Be sure you mark how each fuel line goes before you remove the bulb so you can replace them correctly. Hmmm. Thinking more about it: If the primer worked properly before disassembly, chances are you crossed up the fuel lines when re-installing. Try switching them.