Fairly new to me truck I noticed a couple days ago driving in heavy rain dripping water onto my feet. I've had several of these trucks and never had a leaker before, didn't think they were all that susceptible to cowl rot. Is this likely to be the cowl rotted out? Could plugged cowl drains on each side cause water intrusion? Rusted channel around windshield? Seemed to dump a bit inside when going around a highway offramp long right turn, so thinking pooling in cowl and maybe a hole in the right side. Fortunately our cowl panels are quick and easy to remove. Also heater box has a small piece cracked and missing a bit around the evap core. Doubt it's coming from there though as it's a fairly sheltered location.
I'll just have to pull the cowl and windshield trim and look, but curious if anyone else has had water leaking under the dash and found a particular problem area. Unfortunately it's too cold to do a hose test until we get a warm snap.
The best way to test this is with a water hose. Run water down inside the cowl only and see if it leaks. If it doesn't after running it a little while, move up to the base of the windshield. Move back and forth on it, then move up each side, and then move along the top.
I had a floor leak on the little ranger pickup someone gave me, and I did a lot of messing around with the door seals and other stuff till I finally got the water hose out. After pulling the carpet back and drying the area out, I ran the water hose here and there and finally found the seam at the floor/kickpanel junction was leaking when I put water in the cowl. After tearing different pieces apart I found I could not access this area, so I dried it off good and put some roof sealer on the seam, not expecting much success from that, but believe or not it stopped the leak.
Try gently, but firmly, pressing against the windshield from the inside and then the outside. On my son's 1984 F150, which was leaking as you have described, the windshield would actually move when doing the above. The seal had let go. This can become dangerous... imagine if the windshield lifted at speed!
Definitely windshield seal is leaking. Pulled the outside trim and could see a couple gaps between the windshield and the seal/goop. Whatever goop was used originally or to try to reseal later is dry and deforms/crumbles pretty easily. I used RTV on it this morning pushing it in as best I could and after rain tonight it's dry so far and no more drips below dash. Wasn't a heavy rain so time will tell if that's the only source. At some point I'll just have a glass shop pull the windshield and reseal it, I can't imagine that would be terribly expensive.
I had a similar situation with El Guapo, but in my case, it was coming through cracked caulking in the cowl. Cause? Cracks in the firewall behind the clutch master cylinder. If you have an automatic, you shouldn't have that problem. It sounds like that may not be the source of your leaks, but I wanted to throw it out there for others who might read the thread and have manual transmission with hydraulic clutches. Good luck with the fix on yours.