Slots in Firewall for Master Cylinder

[Silas]

Hello All,
The firewall on my 68 D100 has four slots-not holes-for the brake master cylinder. The slots look original and not PO-made. The four bolts in the master cylinder (from a 72 D100) fit through the inner portion of the slots but the master cylinder leaves uncovered the outer portion of the slots, therefore allowing water to enter into the cab. A gasket between the master cylinder and firewall won't entirely cover the slots either since the mounting flange on the master cylinder doesn't entirely cover the slots. Are these slots common? I was going to seal the slots with caulk but thought there must be a better way and that someone must have experienced this before. I would appreciate any helpful input. I haven't been able to find anything through forum or google searches. Thanks for your help.

[Silas]

Here are two photos illustrating my issue.

The attachment DSC02850.jpg is no longer available

[Silas]

[PwrWgnDrvr]

How about a large neoprene flat washer under a large steel flat washer on the inside.

[Silas]

I considered that but the pedal mounting box on the inside comes off the firewall right next to these slots so there isn't room for a large washer on the inside. And if I make a covering plate for the outside to fit under the master cylinder I'm afraid that will affect brake pedal pushrod length in an adverse manner. Was the original master cylinder flange bigger than the one I have now or was there a covering plate or was some other method used to seal these slots from the factory?

[Jims68]

The holes are original. I'll have to check to see what I did about the "slotted" area. I just can't remember now.

[Wildergarten]

Hello All,
The slots look original and not PO-made. The four bolts in the master cylinder (from a 72 D100) fit through the inner portion of the slots but the master cylinder leaves uncovered the outer portion of the slots, therefore allowing water to enter into the cab. A gasket between the master cylinder and firewall won't entirely cover the slots either since the mounting flange on the master cylinder doesn't entirely cover the slots. Are these slots common?

I've seen a factory installation of a flat piece of sheetmetal there with a white polyurethane foam gasket. Not pretty, but it works. If you're going to fabricate one, I'd go with stainless (probably 16ga). Could there have been a vacuum booster that would cover it there at one time?

[Jims68]

Could there have been a vacuum booster that would cover it there at one time?

Mine didn't have a vacuum booster. Looked like original MC when I removed it too. However, maybe that is what they are slotted for?

[my5thmopar]

64 has them. Lift up master a little.

[Silas]

Well, it is nice to hear the consensus is the holes are original. However, the master cylinder on my truck wasn't original when I bought it so I can't see what the factory did about sealing the slots. Thanks for all of your observations and input. I'm eager to hear what any of you may have done to seal those slots. I'm putting the truck back together after taking it down to the frame for a rebuild so I would like it to look reasonably good. Learning lots through the process and from all you fine folks in this forum.

[PwrWgnDrvr]

All the cabs and trucks in my yard with a hydraulic clutch are slotted, as are all W300 and larger. 69-71 100/200 trucks are holes only, no slots. None have gaskets or plates to seal the slots. Most are covered by the m/c.

[Silas]

PwrWgnDrvr,
Thanks so much for the info. If I understand you correctly, it appears the factory used a brake master cylinder with a large enough mounting flange to hide (cover) the slots. But with no gasket, rain water surely would have leaked into the cab unless the factory caulked or otherwise sealed the holes. Jeez, that is incredible. Thanks again for your research and taking the time to respond.

[Silas]

Wildergarten,
I'll probably follow your suggestion of fabricating a sheet metal cover. I'm pretty sure there wasn't a vacuum booster on my truck. Since my truck is an automatic, there's a factory block off plate on the firewall where the hydraulic clutch would normally be. Guess I'll copy that so it looks somewhat uniform. Cheers

[Wildergarten]

Guess I'll copy that so it looks somewhat uniform. Cheers

Found this piece of related hardware. Looks like there was a gasket but it was mounted from the inside.

[Silas]

Hey thanks Wildergarten. Now that makes sense. Hmm, I've never seen anything like that before on either of my trucks. I wonder if POs removed them when changing the master cylinders or that is a homemade/special piece from your truck. I'll look into going that route since it more maintains the original look from the outside and should still seal well. Cheers

[Wildergarten]

I wonder if POs removed them when changing the master cylinders or that is a homemade/special piece from your truck.

I suspect it was manufactured because it is square enough to have been jigged and then spot welded.

[hmaynord]

pretty sure came from the factory this way