The cap/bail/bellows/gasket depends on low pressure on both sides of the gasket to maintain a seal. If pressure builds inside the reservoir, the gasket will not hold back fluid. Excessive back pressure is caused by air in the system or a defective seal at the working end of the MC piston. The first opportunity for pressure in the wrong place is when you depress the pedal. Protect all the painted surfaces you want to keep that way. Remove the cap and watch the surface of the fluid in the reservoir while someone presses the brake pedal for you. If there is a geyser during brake application stroke, the seal on the piston is bad. Continue watching the fluid surface as the brake pedal is released. Does it geyser now? If so, there is an air bubble trapped in the system. You could have one or both problems. If the rubber seal is bad, the person pressing the brake will report the pedal is sinking and you may see a continuous welling-up of fluid while the pedal is moving. If brake pressure holds at full stroke, but there was a geyser during the stroke, there is a scar in the bore; once the rubber seal travels past the scar, it seals the bore. Just because the MC was brandy-new-rebuilt doesn't mean it is good.