Under Dash water leak

I read once a while back on some guys dart restoration website about a "common leak" found on the firewall of darts and other a bodies.

of course I cannot find that site anymore, and I think he mentioned it being at a seam of the firewall / cowl.

well.

My car has a under dash leak on the driver side. When I had the dash tore apart I inspected the wiper pivots they show no sign of being the source.

one of the bolts for the voltage regulator was missing and I could not get another to thread back in, bolts of the corrects size would not stay tight and could not fit a bigger one. so I smeared a dab of silicone over the hole for a quick leak preventative. it has not rained since and I have not had time to venture to the carwash to see how that did for the leak. there is a bit of surface rust inside on the firewall around and below where the master cylinder is. could that be the source? would it be worth my time to remove the master cylinder clean things up and reinstall the master cylinder with some sillycone?

or is there an actual mystical common leakage that others have suffered and conquered? having the leak prevents me wanting to put new carpet in.