Can,t prime fuel with primer ball on my weed eater XT 400, lots of fuel and lines look good

It sounds like the bulb is not sealed properly. That may also explain why it shut off after 20 secs. (sucking air and not getting enough gas). Fuel line or carb may be cracked or clogged. Or you may have accidentally reversed the fuel lines. Since it worked fine before you removed it, the problem most likely lies with the bulb, its gasket, its fuel line connections or the one-way valve beneath the bulb.
Good luck

check the position of the fuel lines to the primer or if part of the carby from the tank to the carby
the way it works is that the primer bulb sucks fuel through the carby to prime it so the tank line goes to the carby and the line from the primer back to the tank
could be flooded or the metering screws have to be adjusted

Replace the primer bulb.

check the primer bulb to be sure it is not cracked. check the fuel lines also. they crack and wont pull fuel.

Lots of help videos on youtube.
ryobi weed wacker wont prime YouTube

If it's the one that comes 'out' of the primer bulb it is the return line that goes in the fuel tank.

Hello,
Could there be the possibility that the lines may of been put on the wrong way i would try swap them around first, also the primer bulbs do not like air make sure all the fuel lines are all on tight and that there is heaps of fuel in the tank

Make sure lines are connected correctly. On the back of primer it is stamped In & out

change the fuel lines over on the carby
you are trying to feed fuel through the primer line when the engine is running and that is why it is cutting out

Prime and watch the fuel move up to the carb. This is the fuel line and it should be long. Make sure it goes to the bottom of the fuel tank.