240D vacuum readings/downshift

[240Joe]

I'm trying to figure out why my 1980 240D won't always downshift from 4 to 3 when you put it to the floor running at about 30 to 40 mph.

I put my vacuum guage on the line that runs to the transmission and this is what I saw.

At idle, I get 20 inches of vacuum. Just a little off idle, it immediately switches to 9 inches. From just off idle to full throttle, it goes from 9 inches all the way to 0.

It seems to vary nicely with throttle position, but what I'm not sure about is the immediate jump from 20 to 9 just a little off idle. I know that is when the vacuum switch on the valve cover switches to the vacuum control value on the injection pump.

So three questions...
1. Does 20 inches seem ok for my vacuum pump, and

2. Is the immediate switch from 20 to 9 inches when the switch kicks in the vacuum control valve on the injection pump normal?

3. and if this is all normal, what is next, change the modulator in the transmission, or adjust somehow??

Thanks

Joe