It seems that all other connections are ok, but... before distributor there is a valve/connector (or whatever) with outlet -dist- and two inlets -carb- and -delay-. Carb is connected to carburator and delay is plugged by sheet metal screw, so, is that delay valve missing. One of troubles is that transmission can not decide should it be 2nd or 3th gear.

If you are talking about a truck that has a THD 350, that to me, sound like a leak in the center seal in the transmission. Not a vacuum issue. In other words your transmission is going bad. Do you have a small delay in reverse before the truck starts to move? When at low speeds does the transmission downshift on its own? Or even have you noticed when going approx 30 +- 5 or 10 MPH and hit a fair bump does the tranny down shift?

The other thing to check is the Modulator valve on the transmission. I could be wrong. It is really hard to say without driving it.

Thank you Pete...
my first was '38 (Ford, sorry to say:), but now this '81 El Camino 229 is my first automatic. Prolly that explains 2nd part of my message.