Nichole340,
I looked for a schematic of that model Mercury, but could not find any. Even my Mercury Outboard Repair Manual details how to drain and fill but the individual drawings are rather simple.
What I do know, is that the Vent your describing, the Lower Unit Top Vent, is a straight line to under the Water Pump Lower Housing, showing in your photo.
If you remove that piece you will see the line and probably a blockage.
You said you were replacing gaskets, you'll need a new gasket for that Lower housing also.
Could be....A small amount of metal filings, and that's acceptable, especially in a 19 year old motor. Over time they fuse together and caused the blockage. Or simply crusted over gunk....that's what I'd prefer.
You can start by trying to hand twist a 1/16 inch drill bit into the vent, using a Tap and Die T Handle, to loosen the blockage. It may work....if not, a drill on very, very, very low speed.
Your only looking to go about 1 1/4 inches in to the Drive Shaft!
Then, when through, go to a 5/32 bit but NO larger. That will open the vent to the Drive shaft.
Test it with compressed air, a tire inflator would be enough air, once free.
When done.....Remember to flush out the housing and gears with a cleaning solvent, WD40 etc, to remove any contaminants, fill with fresh oil and you should be good to go.
Your doing things right after all these years. It's a quality motor and people look for them.
Hope this helps