Here's a diagram that should help. Let me know if anything isn't answered.
If I remember your saw correctly, don't you have a barbed fitting and a smooth brass fitting on the carburetor? The barbed fitting is fuel, the smooth is return to the tank.
If yours is different, please let me know the type or serial number of the saw. The straight 2150 has been made in 7 different versions, plus the Predator, LE, and LX versions for another 11 variations.
The line from the fuel tank filter goes into the upper fitting of the carburetor. The line from the lower fitting of the carburetor continues on to the small nipple on the purge bulb. Then the return line goes from the large nipple on the purge bulb to the top of the fuel tank.
Kind of a confusing setup, but does this make sense?
Here's the diagram, but I don't know it will tell you much. I asked so I could verify which Walbro carb you have (WT-324). I'll link to a carburetor service manual as well and then try to explain it so it makes sense.
2150 Type 4
Walbo WT Manual
First, although many refer to this as a primer system, it's not. It's a purge system. This will not "prime" the carburetor by pushing fuel up into the throat, it simply "purges" the air from the fuel line so that a solid fuel supply is available for easy starting. If you notice, you also have a choke. That's what richens the mixture during startup, not the primer.
The purge bulb has two one-way check valves that make it work like a one-way pump. When you press the bulb, it draws fuel from the fuel filter in the tank up through the carburetor. You will see the bulb fill with fuel as this happens. It's a lot of line to fill up, so it takes about 4-6 pushes for all this to happen. When the lines get full of fuel, the excess gets returned via the large nipple of the purge bulb to the tank through the line coming in from the top.