The 78 B tunes up fine and I can even get the idle set to specs, but there
seems to be a buildup of vacuum at the distributor. After driving and
getting up to 4th, the idle goes up to 1200 from 900. Checked the timing and
found it advanced to about 18-20. Pull the hose at the distributor and I
hear a distinctive hiss and the idle drops back to spec. Isn't the vacuum
supposed to equalize at some point? The fact that it has to get to 4th to
trigger the high idle would seem to indicate that the transmission switch is
activating the solenoid properly. Why doesn't the vacuum advance release?