[SCOTI]

I would check for vac leaks. A stock 283 should have more vac @ idle I would think. Check everything again (check valve for the unit installed correctly etc....). But even @ 15hg, it should work @ idle. Low vac probs will be much more noteable when you stand on the brakes after hard, WOT driving situations (not just cruising) or under repeat use of the brakes w/o time between.

If everything double checks from the engine side, I wouldn't rule out the vac booster being faulty either. I've had one that passed the same 'testing' routine but would be hard as a brick when in use (because the pressure applied was different is the only thing I could guess).

I don't know what the internal failure was but the problem went away when I swapped the vac bosster for another. Possibly hit your local auto parts store for another to 'gently' try & see if the issue persists. Be cautious so you can take it back the same day & you usually won't get grief if you have a good poker face (I didn't need this after all....).