'83 - Brakes drag down idle??

Normally (well, not normally really as it's a malfunction) vac leaks from the brake booster hose or pancake cause the idle speed to go up when the brakes are applied as the leak is introducing air in a fashion that is akin to cracking the throttle open a small amount. (usually 200-300 rpm increase)

That aside, a horrific vac leak in the brake circut can cause things to drop right off too, as the bigger the leak, the less metering the AFM will do. (all the air comes in a different route than normal so the AFM starts to close up resulting in a really weak fuel mixture)

While you're looking over the hose and booster, check the one way valve at the plenum end of the vac hose onnaconna if it sticks open it'll also cause some goofyness in the way things work when on the brakes. (up to and including a pulsing brake pedal)

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Check the valve lash before getting all excited about shimming them til they are quiet. There is a certain amount of clatter involved with this type of engine even when they are well within spec.