My Briggs and Stratton mower won’t start. Only used it twice. I may have over filled the gas tank and flooded it. Not sure. Gas is leaking from the air filter.
Overfilling the gas tank won't cause a mower to not start. Overfilling the gas tank however can lead to problems due to heat expansion so you should remove the excess fuel before leaving the mower in the sunshine.
Engine flooding happens not by overfilling but rather letting too much gas into the engine. This may happen if you press the priming bulb too many times (assuming your engine has priming bulb and not choke), or do multiple weak starting attempts repeatedly with choke on (assuming your engine gas choke and not priming bulb). The solution is to use priming bulb sparingly and if equipped with choke, do one start attempt with choke on, few attempts with choke off, one attempt with choke on, etc. until the engine starts.
I also have an old mower with Briggs&Stratton engine that's hard to start. How I managed to start it is as follows:
Put only high quality small engine gasoline (alkylate gasoline) there. If alkylate gasoline isn't sold in your area, drain or use all fuel from the tank before the winter so bad gas won't make starting harder.
Always start the mower not over grass but on some flat area where the blades aren't touching anything. Uncut grass acts as a brake, making engine starting hard.
Press the priming bulb three times (my engine is equipped with a priming bulb not a choke -- if your engine is equipped with a choke, put the choke valve on)
Pull the cord until you feel little bit of resistance. The resistance depends on the position of the engine, but in some positions it's quite small. It's a mistake to pull until you feel maximal resistance because that consumes part of the pull cord travel, and if you let the cord be pulled back and try pulling again with the engine in the maximal resistance position, starting is harder.
Once you have found the position of the pull cord where you feel a little bit of resistance, yank the pull cord very fast. Try to make it start the first time. The first time is always the best possibility to start the engine because your muscles get fatigued and further starting attempts won't be as strong, and also you may not know how many times you need to press the priming bulb if the engine doesn't start the first time.
If you can't make the engine start after you've followed these instructions, you need to somehow decide how to operate the choke / priming bulb on subsequent starting attempts. With choke, it's a good idea to do first pull with choke on, three next pulls with choke off, then again a pull with choke on, then again 3 pulls with choke off, etc. With priming bulb, pressing it occasionally once may be a good idea after multiple starting attempts.
If you still can't start the engine, maybe the engine is worn or maybe your hands are too weak for this engine model. In either case, spraying some starter fluid to the air filter could help in starting.