When I press the brakes I hear air being expelled under the dash. The brake pedal fades and goes to the floor.

check the brake booster a lot of times they will leak on the inside of the firewall and run down under the carpet inseide look under the dash where the brake pedal rod goes thru the fire wall and see if there is and brake fluid coming thru
to bleed brakes by myself i use a bottle of brake fluid and get a hose the size of the bleeder on the brakes and put the hose over it and put the other end in the brake fluid open the bleader a little and pump the brakes air goes out and it will pull the fluid back in

Seems like a vacuum leak. Either the valve, hose to the manifold or the booster itself is leaking air.

if a master cylinder is bad and you are sure you have all the air out and no leaks.pedal will go to floor , running or not. if booster is bad it will cause a hard pedal. but if master cylinder is bad it will turn on a brake light on dash if it works. make sure the wheel cylinders are not leaking. I beleive that the master cylinder is bad, or you have a leak somewhere. If the master cylinder is functioning properly, the brake pedal will depress to a certain depth and no further.

Chances are you have a small pin sized hole in your brake lines were your fluid id slowly leaking loosing brake pressure. I would fill the master cyl. full then have a friend look under your car while u step on the peddle to see if u can see any brake fluid leaking anywere. If there is none i would try bleeding the brakes. There could be air in your brake lines also causing this to happen.

Get someone to help you, have them press down on the brake pedal while you are outside the car. Go to the back of the car and look under, look to both left and right as the helper presses the pedal. Do this both front and rear left and right. If you see brake fluid on your tyres either your seals are leaking or your wheel cylinders are leaking.

If no fluid it is possible that your brake pads have worn beyond their limit and are almost contacting metal to metal.
Failing this, if your pedal goes straight to the floor you have lost pedal pressure. This sometimes indicates worn seals in the master cycliner which when you look under the bonnet you will see your fluid reservoir and a big round thing which is the vacuum booster. Attached to both of these is a metalic cylindrical item that is your master cylinder. It all attaches to the firewall.

This needs to be correctly diagnosed by a mechanic though.

You need to crawl under the truck and inspect each brake line for leaks. You also need to check wheel cylinders and calipers. The chances are pretty good that you will find a small leak in a line or a wheel cylinder leaking, pay close attention to the rear of the truck because that is normally where these problems occur. Hope this helps.

A vacuum leak. Look & listen for hissing noise where rubber or nylon lines come through firewall under dash near brake pedal. Also look under hood at large hose going to back of brake master cylinder, power brake booster.

IF YOUR HEARING IT IN THE CAB YOU MAY HAVE A LEAK AT THE BOOSTER WHERE THE PEDAL ENTERS IT GET UNDER THE DASH AND PRESS THE PEDAL BY HAND AND SEE IF YOU CAN HEAR IT COMING FROM WHERE THE PEDAL ROD GOES INTO THE FIRE WALL

Sounds like air is still in the brake lines, the air sound that you hear is the power assist, but the brake pedal needs to be "high and hard".

if the pedal goes to the floor, that is not good. Sometimes, when pumping brakes for bleeding purposes, the master cylinder gets damaged, as the piston goes past it's normal "travel zone" and gets damged on deposits.....

Now, with that being said, if everything was perfect, brakes are typically not as firm after replacement, due to differences in new pads/shoes and old rotors and drums. Pedal to the floor, however, is never okay.

Doc