I drilled through my firewall to ground a switch for my airhorns and when I start my truck there's a loud air sound like its escaping from somewhere but when I step on the brakes really hard the sound goes away, my brakes still work. What did i screw up?

It's the hole just under the steering and above the ground:

 

Here's a vid:

http://s65.photobucket.com/user/atro...11dab.mp4.html

 

Just guessing here - but a hole in the brake booster?

If there is a leak in the diaphragm, I assume it would do that.

The brake booster diaphragm works on vacuum...

EDIT: Yep, those four larger bolts are for the brake booster. I'm guessing your drill went right into it.

 

Yup stupid rushed mistake.

I drove it around the block and the braking was fine but If i slammed all the way down on the brakes my rpms would drop and truck would almost stall

 

Would it be possible to patch the brake booster?
Maybe JB weld in the hole or weld on a small patch plate?

I'm not too sure about how thick or thin the material is making up the walls of the booster.

 

It sounds like you might have to replace the brake booster.

 

id replace it, and double check next time you run a ground and before you drill.

 

You could have avoided the hole to begin with. I just poked a little hole in the grommet under the engine fuse box and ran wires for my sub, back-up camera, and off road lights through it. Much easier.