I've got to check my 150 for its annual shortly so will look more closely at the priming mechanism then. However I only ever give it one pump on the primer and about 1/2" of throttle and it starts first time, every time, with no fuel leakage.
The Cub I fly mind you, same engine (RR 0-200), is generally started without primer and a pump on the throttle (also no fuel leakage). It's the owners preference and I guess he should know given his experience.