How to clean main vent?

I have reason to believe that the vent to my main sewer drain may be obstructed. The vent is about a 1.5" metal pipe that comes off the drain, thru the outside wall and up along the house. It is about 20' long. I don't have a snake but I do have about 30' of 1/4" cable that I could run down the pipe to see if it hits anything. Or would it be a good idea to put a garden hose in the pipe and see if I can flush any obstruction down? Is there a good test to confirm that it is blocked? Thank you.

I doubt it is obstructed!

The "clogged vent" is the most blamed part of the plumbing system....

Yet it is the least problematic part of the plumbing system...
In my entire career as a plumber I can count the number of clogged vents I have found on a single hand....

What are the symptoms of your plumbing problem?

"You have reason to believe the main vent may be obstructed". What do you base this assumption on? Why not give us the symptoms and perhaps we can offer some suggestions.

OK, a little background first. House was built in 1942, I have been here for 20 yrs with no issues. I doubt its vented properly to begin with but haven't had an issue until now. Upstairs toilet drain makes 90 degree turn then runs along floor to outside wall, sink and tub drains connect to toilet drain before it makes another 90 degree turn to run down wall to 1st floor. Right where the upstairs drain turns to go downstairs there is a vent pipe connected that goes outside and up the wall. This is the part that I believe is incorrect, the vent is between the upstairs and downstairs plumbing and I don't see any vents above the upstairs plumbing. Starting a few days ago when I flush the upstairs toilet I hear a gurgling sound from the upstairs shower drain. Last night I noticed the downstairs toilet water level in the bowl is low and sewer smell from the downstairs shower, seems like a vaccuum pulling water from the traps and out to the street. Water level in the upstairs is normal. When I flushed the upstairs I noticed the downstairs toilet water level going up and down and the laundry tub trap making sounds. Now this is the weird part but I'll try to describe, maybe take pics when I get home. Years ago I noticed a plastic hose about 1/2" OD connected to the top of upstairs toilet drain where it turns to run along the floor, between upstairs toilet and outside vent. I followed the hose about 30' across the 1st floor ceiling (easy to trace, removable ceiling panels) where it just ended, not connected to anything. I had no idea what it was for but thought that it would be a good idea to seal up the end with tape so no gasses would come out of it, this was about 15 years ago. Last night I removed the tape and when flushing the upstairs I no longer get sounds from the shower, basement toilet water level and shower drain are normal now. Seems like I was adding a vent by removing the tape from the hose, thats why I think the main vent may have something in it like bugs, small animal, rust flakes?. I hope the main drain isn't blocked because that is over my head and would need to call a professional. Let me know what you think. Thanks

From the way you describe your sewage system I think you have a mess that was installed by someone that didn't have a clue what he was doing, thats way too much for someone on the net to figure out, I think you need a plumber on site, sorry

If you run water down the vent and it is plugged it will fill up with water and flow over. If there is any other drain tied into this vent it could back up into that sink . If you try a water hose have someone on the inside of the house checking all the sinks/tub/toilets to make sure they are not filling with water. If it takes water from the hose down the vent without backing up or running over the vent is clear.
If there is a section of horizontal vent pipe going out through the wall before it goes vertical it is very posible waste has gotten backed up in there and blocked the vent.