Already completed. Mine were also plugged from a mouse nest which has now been cleared. No more leaks on the passenger side, just this lingering leak on the driver side.

 

^^^ This was a good lead. I decided to pull the last of the firewall insulation pad away so that I could diagnose this better. Then I started dumping water down through the cowl. A small amount of water came from around the steering column, but most of it seem to be coming from directly below where the brake booster attaches to the firewall. I'm thinking the booster needs to come off to get to this, yes?

 

Years ago I sealed the cowl with a good sealer and bed liner paint. So I do not have any cowl leaks from that.

My leak is due to the bad cowl to hood gasket that do not direct water away from the steering column/ booster area. Water runs off the hood along the firewall to those points. I keep forgetting to get that garage door gasket when I'm at Home depot.....

Yes there is a gasket between the brake booster and firewall, need to remove it to access that gasket.

If your cowl does leak, then better fix it before rust kills that cowl seam. That will be a pain to fix for sure!

 

If I remember correctly above the booster hole the fire wall is a double wall. The water leaks between these 2 sheets of metal and drips out at the top of the hole. Check and make sure water isn't backing up from drivers side cowl drain. If it is, it could puddle in the cowl and leak through the seam. If it's not puddling there you may need to remove the chalking and re-chalk where the cowl is spot welded to the firewall. Mine dripped and ran down the engine side of the firewall from the cowl drain being filled with dirt. I also had water coming out of the rocker panel and b piller where meets the floor. Cleaned out the block and stopped the leaks.

 

We had a leak in the same spot on our 93 for while and it actually wasn't from the wiper cowl on ours.

In the first picture there is a rubber flap behind the pedal that is covering the firewall. Behind that there was a hole just a few inches away from where the steering shaft comes through, and it looked like there may have been a rubber grommet in there at one time. We filled the hole with silicone about 4 months ago and it's been leak free since then. (We only used silicone because we didn't have a grommet on hand, if it starts to leak again I'll get a grommet to put in it instead.)

 

I'm digging back into this today. The rubber flap/carpet you mention above has been completely removed since the first pics were taken. Which direction from the steering column is this hole you speak of? I'm not seeing it on my '91, but the steering column on the '93 is entirely different, so we may very well not have the same leak point.

 

So this ^^^ seems like what is very likely going on in my case. Replacing the hood/cowl seal seems like something of a band-aid fix, but it's certainly quicker and should be done regardless. The "proper" fix would be removing the steering column and re-sealing at the firewall. That would also be a good time to check and see if the clip has gone missing as discussed in this thread...
https://www.ford-trucks.com/forums/1...sive-play.html

Stolen from the thread referenced above, here's a pic of the MD #03723 garage door seal being used as a replacement cowl seal. I just ordered the older version, MD #03004, which is vinyl.
https://www.ford-trucks.com/forums/a...1&d=1360340519

 

It's possible that the hole would not be there in a 91 as I don't have much experience with the cabs before 92. It's been awhile since I've been in there, but IIRC it was just below and to the left of the steering shaft, but I do know that I had to lift that rubber flap pretty high to be able to see it.

 

My leak comes from up high, so it can't be what you're describing.

One other possibility I discovered today is the seam in the firewall that's above the brake booster. But the steering column seal seems like the most likely culprit.

 

I pulled the steering column over the weekend. Short story, that's not where the leak is. The oddly-shaped rubber seal between the column and firewall was in good shape, in place properly, and all the bolts were snug. I put a dab of sealant on each bolt when reassembling, but I don't think that was really needed.

With the lower dash trim pieces removed, I can see better up under there, and it seems more likely that the leak is from where the brake booster is fastened to the firewall. A smaller amount of water does come down to the right of the steering column. It's possible that a bit of water is getting in from the clutch slave cylinder mounting, but I'm leaning towards a "no" on that.

The only other possibility is that it's something I missed when I had the cowl off, but I thought I was rather thorough in re-sealing everything under that that last year.

 

I did go ahead and install this new seal yesterday. I cut a piece 46" long and cut out a "1/4 moon" chunk where it goes over the wiper motor, just like the factory seal. It works better than the tired, cracked old seal, but would probably work better with a little sealant applied. I may go back and re-do it at some point.

I haven't even tested this yet to see if it helps with the water intrusion. Regardless, I want to fix the actual leak, but I'm realizing I may not get to that until next Spring if it's going to involve removing the brake booster. So hopefully this "band-aid fix" helps for now.

 

It's rained twice since installing a new cowl seal and no water intrusion into the cabin. I'll still want to find the leak (suspect it's the brake booster), but for now this is resolving the issue by not letting the water find the leak!

 

Yesterday I was using some Loctite PL Polyurethane Roofing Sealant (BTW, great stuff), and so I decided it was time to get after my firewall leaks. I had previously determined that both the clutch master cylinder and brake booster were leaking where they fasten to the firewall, so both got unbolted. The Clutch MC was easy enough, but for the booster, I found this crumbled plastic thing between the firewall and booster (the primary reason for my leak, I now suspect).

Wanting to get the job done, I just yanked it out of there and fastened the booster back up after smearing some sealant to the back of the booster. Do we think this will be okay? I put more sealant on the top where it seems there is only a single layer of firewall - it doubles up at the top bolt holes and on down from there.

 

It's been over two years since I've updated this thread. In a perfect world that would have been because I haven't had any water leaks. This is 2020. This is not a perfect world. I continue to have water leaks. The most notable place *seems* to be from around the steering column, but they may be a misleading hint; it may just be that the steering column is lower than some of the other thru-locations in the firewall such as the brake booster and clutch MC.

BUT, with a new bit of evidence, I'm inclined to think the leak is from a point higher up. Yesterday after the rain stopped, sure enough there was a puddle in the driver-side footwell. Aside from the usual water traces on the firewall, this time I noticed a drop of water coming off the clutch pedal. Keep in mind, I've re-sealed the clutch MC where it seals to the firewall, so I suspect it's coming from directly above, not through the firewall. If we assume this is correct, the leak would have to be from the cowl area or windshield, yes?

 

My 87 currently has a leak through the windshield, that ends up dripping down where you're describing, so yeah, it's certainly possible.