An other tip is that the primer bulbs are usually designed to be squeezed in a vertical upright position with an arrow showing the up direction as well as the fuel flow direction. This allows gravity to assist the internal check valve in the squeeze bulb in working optimally. If you were to install the primer bulb backwards you would of course also have problems.
If the tank and its vent are breathing properly, the primer bulb and its internal check valve are installed correctly and in good working order then you will usually find that somewhere along the fuel line and its various connections air is being sucked in. Your fuel line does not have to leak fuel out for air to be sucked in. As others have said, fuel line/hose degrades with age and can cause problems but it is not expensive to replace. If the vent and primer are good over the years I have found the problem is often a bad connection. I run two fuel lines off my main tank and each one has 8 connections between the tank, fuel cut off valve (emergency fire and leak control), fuel filter/water separator, primer bulb and the motor connection. Make sure that each connection is tight and most importantly use only the most expensive, all stainless marine grade hose claps that provide smooth even compression and don't cut into the fuel line. The cheap crap clamps from places like Canadian Tire designed for cars and water hose should not see the inside of your boat even if they are 10 times cheaper.