Popping off ‘should’ be controllable by taking more air out, thus increasing the vac. If not you may be way over libbed or mis fit due to the #’s you provided. If your using it kicked back in a recliner, put a pillow over the capped end so it has to lift it, heavy enough would be similar to held in place but still with a little stroking.
Put he on/off switch outta reach so you can’t quit and force milk yourself.
Have had mine about 4 years and still enjoy it just as much 4-5 time/week. Don’t be afraid to experiment.