I was reading that I have to keep air out of the system when working on brake components. How do I do this when replacing brake lines and other brake components? Do I do a complete drain of the system? If I do a complete drain, doesn’t that introduce air into the system? I’m also replacing the brake reducing valve so this question applies to replacing anything in the braking system.

Thanks in advance,
Scott
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Scott K - 1994 850 Turbo

Obviously, you cannot keep air out of the lines when you are working on them. Just make sure there is no air in the lines before you drive into a telephone pole.
This is a good time to replace ALL of the brake fluid. Do NOT try the brake pedal when the master cylinder is empty, you will break it.

Klaus
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(1975 164, 1995 854T, 1998 V70R)

you need to remove air from the system when you complete your work. exposing the system to air happens whenever you do work on it. I suppose not leaving it exposed for months would be in order to prevent oxidation and rust, but for a few days or weeks won't do any harm. Just flush/bleed the whole system when you complete the work.

Yes, you must bleed the brake system any time after you open a brake line. IPD and other parts suppliers sell a brake bleeder (Motive Products Power Bleeder)that enables you to bleed your brakes by yourself. It has a cap that fits onto the master cylinder. Once the bleeder is pressurized, the pressurized air and fresh brake fluid forces the old brake fluid and air bubbles through the brake bleed fittings when you open them. No need to touch the brake pedal. It works pretty well.

Autozone also sells (for cheap) a one person bleeding kit. It consists of a clear plastic hose and a platic bottle like container. You loosen the brake bleed valve on/near the caliper and connect the bottle to it with the line and then get in your car and gently press on the brake pedal. You have to do this with all 4 brakes. It's a good time to flush your brakes by adding brand new fluid. Instructions are included with the kit. It's not as easy as the IPD kit but much easier and works fine.
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1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon