Lawnmower Prime Bulb Not Working

I have a 4 OHP Quattro Briggs & Stratton engine on my lawnmower, and when I use the prime bulb to try to get the engine started the priming doesn't work. I take off the top of the sort of rectangular metal box that holds the filter, (and the filter appears to be clean), and pour a little gasoline into the engine and the engine starts up. Can someone tell me how to get the prime system working again? Thanks,
JD
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Read the section entitled "A primer on priming" at:
http://repairfaq.ece.drexel.edu/sam/lmfaq.htm
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The tiny tubing running to and from the primer tends to break down, loosen up or fall off.
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Thanks for your response, but I have a followup -- How do I fix it? Do I buy a new tube or do I just clean what is there or what? Also, is there any trick to getting to the tubing or is it obvious where I am going. Haven't worked with this type of problem before.
JD
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The site I offered before has instructions for cleaning/repairing both B&S and Tecumseh engines.
Look for the sections on cleaning the carburetor since this will most likely require you to interact with the primer bulb and tube.
http://repairfaq.ece.drexel.edu/sam/lmfaq.htm
FYI - I had never worked on a lawn mower engine before last weekend, but by using the instructions at this site I was able to replace the flywheel key, which required making a tool to remove the flywheel. All the info I needed was found at that site.
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