If the float in the carbs, any 1 of the carbs or all, can't seal off the fuel inlet needle valve due to maladjustment of the arm to which it's mounted, or binding, or ceases to float (waterlogged....is the term but for this it's fuellogged) you can't expect the valve to seal. Also, if the valve is defective, primary reason, a ring around the conical sealing part, you will have the problem mentioned. The bulb should be able to get good and hard with moderate hand pressure with the engine stopped.....assuming the check valve(s) in the bulb are working properly.
