Feels like a misfire under load 🤷****♂️

[blue62]

So first:

It looks to me like the black and white check valve has been broken and an attempt to repair it has been made.

You can check it by removing it then blow into one side then suck on it.
It should pass air in one direction only.

If it has been repaired in should be replaced.

Second:
Your vacuum gauge should have come with a "Tee fitting"
Insert one leg of the "Tee fitting" where your finger is pointing in the second picture.
Then Hook the Vacuum gauge to one leg of the "Tee Fitting" via a section of hose.
With a short section of hose connect the black end of the check valve to the third leg of the "TEE fitting.
Leave the white end of the check valve connected where it is in the picture.
Connected this way your vacuum gauge is in direct line with the intake.

If you connect the gauge the way STL-986 describes then the gauge is connected after the check valve.
You do not want that. That is an improper way to connect the vacuum gauge for and intake vacuum test.

When you do the vacuum test make sure all your hoses in the picture are reconnected.
Have the car fully warmed up.
Connect the gauge then just let the engine idle.
Record the reading and the action of the needle if any.
Then just watch the gauge for a few minutes. See it the needle wanders or ticks or exhibits any action or movement.

Then disconnect the check valve from the TEE fitting and plug that leg of the TEE fitting off.
Now you just have the vacuum gauge connected to the intake.
Run the test again the same way. check for any changes in readings or needle action.

Let me know how you go.