Brake flush -- thoughts on doing a 3/4 job

Hi --

I had a recent oil change and it looks like the shop broke the lid on the master cylinder reservoir and didn't tell me, and so I discovered that I have been driving around for at least three weeks with a broken lid, letting moisture into the system.

I know I need to flush the system -- aggravating because I did a flush a few months ago, at which point I broke off the head of one of the new speed bleeders I installed just as I was trying to tighten to final torque as a last step.

So I'm wondering if I can get away with flushing now through only the other 3 lines? I'm thinking that maybe the amount of fluid in the 4th line will be small enough that the amount of moisture left won't be so high that it does damage?

I don't want to spend the time at the moment to deal with getting the screw out -- and/or replacing another caliper. But I also don't want to rust out my brake system.

Thanks for your thoughts!

Monica