The pump does not really supply pressure to the brake valve. It only trickles oil in to keep reservoir full. The brakes are applied by manual pressure only from foot pressure. Yes you can bleed the brakes while the plug(s) are removed from the top. But, if your pistons are fully retracted, they must be stuck and need to be freed up. Sometimes there is paint or rust built up on outside diameter of the pistons that makes them stick when they go in farther than their normal travel. when you lift up on brake pedal, do they still stay in? Sometimes the pedals get tight on their pivot shaft. If the pistons are protruding from the valve housing, you might be able to wrap a heay rag around the piston to protect the outside diameter and then grip them with pliers or vise grips and twist them to free them up. Then polish the outside with fine sandpaper to clean any rust or paint. If there is not enough sticking out to grab, you will need to remove the large plugs on the rear of the housing to access the pistons from that end. Disconnect the small brake lines going to each wheel and then remove the plugs. Be careful because there will be alot of spring pressure pushing the plug out. There is a ball, cup, spring, and then the piston. Remove the plug, ball(about1/4" diameter) cup and spring. Then use a long punch from the back side and tap the piston towards the front untill it stops. It helps to tie the brake pedals back so the piston makes full travel. Then polish the piston so it slides freely. reassemble, fill the reservoir and bleed the brakes.