We just got done with a plenum pull to locate a secondary vacuum system leak on George's (LT5-1935) recently purchased, 1990 ZR-1. Everything under the plenum was in excellent condition. All the tubing and fittings were tight. The leak was from the Secondary Actuator Valve. We repalced it with a new one and he was on the road in under an hour and a half.
He returned from the test drive with a big grin.
It was the first time he had really experienced the LT5 in full power mode.
BTW, the 1990 plenum pull is easier than the 1993, 1994 plenums I have done. There are less vacuum connections and electrical connections under the plenum.
Jim