My truck drains the battery when sitting. There is a constant .8A draw. (It's a '77, no electronic brains or acessories or stuck trunk lights, etc.)
Trying to isolate circuit by pulling fuses until it stops. Nada... nothing fused is the source. Finally found it drops when I remove the main alternator connector.

Would it be likely that something in the alternator itself is bad and drawing current, or something upstream?

Hey Troy, I seem to remember you talking about alternator diodes relating to parasitic drains? Been to long ago to recall.

Any ideas come to mind for what to check next?

Yes, when a diode dies, the alternator may still charge a battery or keep the car running, but will reverse the flow when engine is off and drain the battery slowly.

Bad alternator. 0.3A is about the maximum allowable drain if you have EFI, Clock, CD player, etc...

Or carbon dust build up from the brushes mixed with oil vapor. Remove the brushes and wash the area as much as possible with electrical cleaner, blow out with compressed air and check again. If still draining, try washing around the diode pack and blow out that area also.
Tom

if you have a fancy dancy multi meter you can do a ripple test to prove 1 or more failed diods.

http://www.labscopes.com/index_files/Page441.htm

might sound silly !but make sure your brake light switch is working proper ...good luck

I'm curious about that comment... What do you mean?