1985 745t

Took my brick in for some service at with my indy mech. earlier this week. The rear calipers were sticking pretty badly and the brakes needed some major attention (I don't do brake work myself). Turned out the rear calipers and rotors were bad, so I had them replaced and everything seemed fine. This was done Monday.

However, tonight I popped the hood to fill up the windshield fluid and found teh brake fluid expansion tank caop sitting on the cowling over the aux fan. Seems my indy guy forgot to put it back on when he was finished. When I looked at the fluid it was down at the minimum mark.

What has me concerned here is the fact that:

1. It has been wet/cold/raining off and on the last few days, with major ambient moisture in the air

2. My hood insulation is falling down around the area where the brake fluid tank is.

My question is simple; how concerned do I need to be about the ambient moisture trashing the fluid and trashing my old and new rotors? I just spent $500 on my brakes, so I'm obviously doubly concerned here. My mechanic is good and honest and I have no doubt he'll do me right, but I want to know how hard to push on this if he kind of waffles. Will ambient moisture frag brake fluid in this amount of time? Do I tell him he needs to flush the brake fluid now, period? Does he need to do anything else as well on the caliper side?

Any help or insight is appreciated. As I said I don't do brakes, I just know the things to be concerned about.

Thanks,
Rick