Rainwater Leak into 1978 Shortbed Cab
 

I’m about to lose it over this one! My 1978 C10 has leaked rainwater, on and off, for the year that I have owned it. When it rains, water drips into the cab in two places. The predominate place is onto the driver’s-side kick panel. However a leak of lesser magnitude seems to come from the circular air vent (maybe 2.5 in. O.D.) located over the brake pedal, in the duct which feeds the far-left dash air outlet. No water is evident on the passenger’s side.

When I first realized that I had this issue, I discovered that the lower windshield retention flange was badly rusted. I had a body shop cut out the rust and weld in new metal—installed new glass (old was cracked), and also a new gasket. Things were better for a while, but have returned to what I had originally.

A few days ago, I taped poly sheet over the windshield, leaving access to the cowl cavity open (bottom of plastic taped just below wiper shafts). Plastic was wrapped around the A-pillars and was taped inside the door cavities. It draped over the roof and continued down below the rear window. A heavy spray from a hose, directed at the windshield, and including the roof, produced no water in the cab after an hour.

I immediately removed the plastic and restarted the water. After 20 min. water was beginning to show up on the driver’s side.

Yesterday, I pulled off the rain gutters, sealed screw holes and reinstalled. Also brushed seam sealer on a suspect seam on the inside of the A-pillar.
An hour of water spray today produced considerable water in the floor on the driver’s side and a significant amount was dripping from the previously-mentioned air vent over the brake pedal. I can’t be sure, at this point, that the water is coming from inside the duct, as opposed to running down the backside of the duct and dripping off at that point.

I apologize for this lengthy post but I think I am suffering from info overload and perhaps can’t see the forest for the trees. Please offer any suggestions that you might have.

Clockdoc

Re: Rainwater Leak into 1978 Shortbed Cab
 

Try a product like this around the windsheild seal: https://www.permatex.com/products/ad...-glass-sealer/

Re: Rainwater Leak into 1978 Shortbed Cab
 

The lower corners of the windshield often leak on these trucks. Water will sometimes wick between the glass & seal, but more common...between the seal & the body. The corners puddle up & run over the flange. Originally there is a small ditch/divot/gutter...what ever you want to call it, in the metal from the flange outward to aid in the water getting out & down into the cowl instead of pooling up & going up & over. These may have been eliminated when the patchwork was done? Even with them still in place many still leak.

The fellow that does my glasswork uses a bit of sealant between the glass & seal & also the seal & body to prevent the leaks in the corners. Hope this helps, Lorne

Re: Rainwater Leak into 1978 Shortbed Cab
 

Thanks for the comments. I think I am getting some through the lower corner on the drivers side. I am planning a few more tests today. The biggest puzzle to me is how water could get into the ductwork. The firewall appears to be sound and does not seem to be getting any water on it during my tests.

This one is aggravating

Re: Rainwater Leak into 1978 Shortbed Cab
 

Thanks ACS. I was not familiar with this product. Have you used it and did it work as claimed?

Clockdoc

Re: Rainwater Leak into 1978 Shortbed Cab
 

I know this is an old thread, but wondering if anybody had an update. I have recently experienced the same thing. I suspect it is the windshield gasket along the top of the cap corner on the driver side, and it is running down the A pillar and into the cab along the top of the dash.

I plan to soak water into the cowl area to rule that seam along the firewall out. if that doesn't produce water, I plan to seal along the top of the windshield and then soak that area after a 48hr cure to see if I have resolved the issue.

Re: Rainwater Leak into 1978 Shortbed Cab
 

I am currently restoring 75 Silverado. Water seeped between the glass seal and the corner. Rusted out front cowl corner where it meets the floor pan. Area around the heater core Hole in the windshield frame. Only way I knew it was actively leaking was dash pad was torn by the glove box and it would leak water

Re: Rainwater Leak into 1978 Shortbed Cab
 

to post my resolution, I cleaned the cowl drain outlets at each fender, flushed water, and repeated until nothing else washed down. I couldn't believe the amount of stuff that came out after 30 years. I also sealed the top of the windshield gasket to body gap. everything has been fry through several rainstorms. I suspect the biggest issue was the stuff in the cowl drains, but I can't be certain. hopefully this helps somebody else.

to access the cowl drain outlet locations, a plastic trim piece has to be removed at the base of each fender behind the wheel wells.