Based on researching the Board, it appears that I have a vacuum leak associated with the brake booster thingy.

Here are the symptoms. During normal idle, my RPMs are unusually high...sometimes up to 1500. But when braking, they drop to about 1000. This is when the engine is warmed up.

When the engine is cold and the brakes are applied, the RPMs drop to 800 which causes the computer to compensate and revs the engine a bit. My vacuum (fuel economy) gauge also drops with the brake applied.

Should I replace the booster, or try to find the source of the leak and patch it up somehow? Could it simply be a bad check valve? I haven't yet had a chance to listen for a hissing sound under the hood.

Speaking of which, I bought this climate control check valve a while back but am not sure where it's supposed to go:

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Mike F - 1984 244 DL - 317,000 miles
Original engine, transmission, drive train, starter
Undergoing reconstructive surgery with POR-15

1972 142 S - The Yellow Brick - 135,000 miles
All stock except for Weber Carb. B20B engine. M40