I think you may be mistaken about the reservoir cap.
Well, found out my reservoir cap is not sealing, so no riding today. That will cause a problem to not let them bleed all the way out because she can't build up pressure. There's another thing to check Tbump.
I'm not an expert on brakes by any means but I really don't believe the reservoir cap has anything to do with building pressure or brake bleeding. The pressure is generated in the spool valve in the master cylinder bore. The reservoir cap is there to keep fluid in and dirt and other contaminants out.
If you are having bubbles coming out through the line you have connected to the bleed valve while you are using a Mity-Vac or similar vacuum bleeder, you don't necessarily have air in the system. More than likely you have air passing by the threads of the bleed valve. Put some grease around the threads where the bleed screw goes into the caliper. If that doesn't work, remove the bleed screw and wrap some Teflon tape around it and reinstall and try again. That should pretty much guarantee that you are not passing air past the bleed screw threads.
If you are having bubbles coming out through the line that is connected to the bleed valve while using brake pedal pressure to bleed the brakes then yes, you do have air in the line. One thing that I have heard is that there is a way to get the bubble out. With the system closed for normal operation, place a weight on the brake peddle (or strap the brake handle to the handlebar) so that the brakes are applied. Let it set over night and then remove the weight or strap. I can' not verify that this actually works but I have heard of some that used this method and were pleased with the results.