Hello Forum, I have a vacuum problem I think, and would like to run it by you all for any input.

I have an 86 stock corvette that had to have a lot of work done to it. I finally got everything working in it, and its been a pleasure to drive
up until just recently. The HVAC system would always blow cold air on auto (BTW I have the electronic version for controls) no matter what the driving
condition was like, uphill, under heavy acceleration yada yada yada. Now it has begun to blow warm to hot air under moderate acceleration or if I'm climbing a hill. It has never done this before so I dont know what has changed.

So, this is what I have done so far. I looked for a vacuum leak in the tube that comes off the 3 way valve. I put a vacuum hand pump on that tube and it does not hold any vacuum.
I dont know if its supposed to. I found the 3 way valve to be broken (34 years old) and installed a new one. This helped a bit but it will still blow warm air from time to time even under flat normal cruise.
I also installed another check valve in the line that feeds the the vacuum ball according to a diagram I found on the internet for 86 vacuum routing.
I guess I just want to know why it has started to do this when it never did it before. As always any help is greatly appreciated.

Tony