I've been diagnosing various problems that seem vacuum related with my 68 factory A/C system. Yesterday it acted really strange. The automatic trans kept slipping out of gear, forward or reverse. I could put it in neutral, rev it a bit, drop back into gear and it would usually catch. Then I also noticed the power brakes were super sensitive. I only had it running for a minute while I moved it from one garage bay to another. Sound like a vacuum issue? A few days earlier I noticed a back leak from the A/C reservoir canister (i.e it wouldn't hold it's vacuum when the engine is revved or after it is shut off) so I installed an inline check valve in the hose supplying vacuum to the canister. That seemed to fix the back leak. Could it be a culprit?