Ours was rotted so bad that I could stick a finger through the bottom of the cowl all around the right side vent.  Some PO had tried to fix it with about five pounds of MS's favorite goop butt (TS&T) of course, that just made a bigger mess.

It's getting a new inner and outer as part of it's body rehab.  Looks like it's gonna be very nice.  I removed the old cowl which took a couple of hours but was nowhere near as bad as I had been led to believe.  However, due to my own problems I'm having the body guy do the metal work on the car and he'll re-install the new cowl.

In my opinion DO NOT try to do a quick and dirty fix.  That's just wasting time and $$ and won't work.  It really isn't that bad to fix correctly, and a number of guys on here have done it.

BB
 

Water should not enter regardless of which position your vent levers are in.
 Is your leak on the right hand side? Heater core?

Left hand side

Derek wrote:

Here's what the inside of a new cowl looks like.And here's what an original one looks like.

Derek, is that "strip chalk" that you used  to seal between the inner and outer?  How did  it work after welding?   Looks like you welded the innner down first, then welded the outer to it.  How'd that work?

 

Process of elimination:
Get everything that was wet as dried out as possible.
Place masking tape over the cowl vent slots to block them from water entering the cowl
Spray garden hose water heavily all around the windshield gasket, starting at the bottom and working upward.  If you start at the top, you won't know if a gasket leak is at top or at bottom.  Check bottom of WS first, then move upward.

If you get water inside the car, the windshield gasket is the problem.

If no water gets in the car, remove the tape and pour water into the cowl vents. If you spray with a garden hose, there is a chance you can spray higher than the dams around the vents.  If you get water this time, you know what is leaking.  You can take a coathanger wire and poke around the dams to see if you find any holes, reaching through the cowl vent slots.

If you find rust holes, there is only one way to do it right.  Replace everything and be sure to seam-seal and paint afterwards.

A temporary fix for MINOR rust at the bottom of the dams is to insert some of those repair kits that look like a top hat, yes... and they use RTV to help seal.  Those are usually good enough to get you through the winter until you can fix it right, but if the rust has invaded the horizontal metal around the dams, even those won't do any good.

Well I use to not have good lunch well that changed. I dumped some water down the cowling to test it and see if it was bad. I was suprised to see none come pouring into the car. It turns out it was my windsheild seal which id rather deal with but the funny/crappy thing is the windsheild and seal were just replaced not even a year ago

I know I'm excited usually that kinda good stuff doesn't happen. Oh well hopefully I can find my receipt for it.