Try itIf you plug the return line you won't be able to push the primer bulb. If you push hard enough you may be able to blow through the plug in the return line.
The return line is not essential to engine operation. In fact, if you don't have a return line you can PLUG the nipple on the return and operate the motor that way without the primer.
The sole purpose of the primer is to flood the carb and fill the lines with fuel for easy starting. Until the engine is TURNING over, the carb float will not operate or open to allow any fuel in the motor intake.
If you have leaks here and there, then you have some mechanical issue with the carb, etc.
MA 1005 Hanson 280, 4196 flts
Spectra 27 3DMax, 3272 flts
Whiplash V1-2 Hanson 300, 1540 flts
Whiplash V2 Hanson 300, 380 flts