Where can you find a vacuum diagram for a 1981 Chevy 305 truck?

I need a VACUUM diagram for a 1981 Silverado utility (aka pickup) truck.

I have tried ALL the solutions suggested here over the past 3-4 years with NO luck at all. I can find the 1980 and the 1982 vacuum diagrams, but the 1981 is significantly different and I can not find it anywhere, including through two Chevy dealerships. I have my vacuum systems working fine. I have traced out the existing 5 lines (one, by the right door, branches to 2 components) and made a drawing of the vacuum system. BUT I wish to install the vacuum operated valve that controls the hot water to the Heater. The last possible outlet on the silicone manifold on the rear of the A/C- Heater control Valve in the Dash has a factory blank for the 6th possible outlet (no hole through the factory molded part) and punching / cutting it through to TEST whether that is it or not could ruin this whole (very small) manifold. I have found two drawings, supposedly for the 1981 305 (my original engine), but it had a 1978-79 engine when I bought it). Neither drawing comes close to my engine cmpartment and the factory components therein. = Murphy's Law !

New info (?) as of 13 May 2011 - I have been told the '81 Silverado did NOT have a vacuum operated hot water control valve. A flapper in the under-dash ducting allowed or blocked the hot water heated air to flow (pretty undependable if true). Anyone out there know which is the case?

I have tried the Library, dealerships, repair shops, on-line, etc. and the manuals, et al, do not cover the vacuum system for my year. NO SOURCE anywhere shows this one year, 1981, which is different from the '80 & '82 or any others I did find.