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Does the engine run fine after it starts? Most common problem with primer bulb losing prime is an air leak in the fuel line or primer bulb. Air leaks don't always leak fuel. If there is an air leak between the primer bulb and gas tank there is no pressure, only vacuum. If it is not a major leak it might not leak without pressure but it will allow air to enter the fuel line. If it is a very minor air leak it could allow air to replace the liquid in the fuel line over time, but so little air once it is primer again that the fuel pump can over come it with volume of fuel.
Primer bulbs do not stay firm, as they are when initially pumped up, after the engine is running. There are check valves in the primer bulb that hold pressure in the primer bulb and throughout the fuel line to the engine. Once the engine starts the fuel pump creates vacuum drawing fuel into the engine diminishing this pressure and releasing the check valves. At this point the primer bulb is just another segment of fuel hose allowing fuel to pass through it as the fuel pump demands. The primer bulb functions much better when the outlet side is higher than the inlet side. This position makes it easier for the check valves in the primer bulb to seat and release as needed. If the primer bulb is functioning properly and is positioned with the outlet side higher than the inlet it should not be necessary to cover the end of the fuel hose with your thumb to get it to prime.
If the engine is hard to start after being fully primed there could be a problem with the fuel system on the engine. Some engines are hard to prime once air has entered the system, especially fuel injected models.
The fuel filter in the tank could be plugged, a fuel line gone bad, or the primer bulb has cracked, gotten hard, or otherwise rendered useless. It's possible that the lower chamber diaphragm in the carburetor is bad--hardened or cracked. The primer is supposed to pull fuel from the filter in the tank, into and through the carburetor, then out to the primer bulb, then back into the tank through another line. Go to: http://www.drystacked.com for a 12 page article on Walbro brand carburetors--very helpful. Hope this helps!
A new carburetor would be simplest solution. You could try disassembling the carb, cleaning and replacing diaphrams, but just replacing the carb would be my choice. What is the model # on the trimmer and I can tell you the part # of the carb you need.