While driving my truck last week, the brake pedal went to the floor when I came up on to a stop sign. Luckly, it was very early in the morning and no one was around. My first thought was the master cylinder. So I picked up one from NAPA and did the bench bleeding. Installed the master cyclinder and bled all the brakes. Pedal is high and hard when the truck is not running but as soon as you start the engine, the pedal goes to the floor. I adjusted the rear brakes and looked for signs of brake fluid leaking but could not find anything. I have done brakes for many years and never had this problem.

What else can I look for that would make the pedal go to the floor?

 

Have you had the rear drums off for a closer look at the wheel cylinders? You may have a leak there but have enough "crud" in there soaking up the fluid. If all that's good and no rubber hoses are leaking, then it is possible that the new master cylinder you got is no good. Unfortunately it happens...

 

Manual or power brakes?
Trying to figure out what the engine has to do with it.

 

yep, pulled the drums and everything is dry as a bone. Would the proportioning valve have anything to do with the pedal being low???

 

Power brakes. When you bleed the brakes with the engine off, I get a high pedal and very firm. Crank up the engine and the pedal goes to the floor.

 

Not to my knowledge... Unless it was leaking...

 

So it sounds like your booster is working and overpowering a problem.
Because the booster is a mechanical link to the mc piston that is helped via vacuum. So, it shouldn't go to the floor if everything is right.
If the booster were no good it still wouldn't go to the floor.
The mc would be suspect but it's new. However new ones have been known to be bad.
There is a piece of the puzzle missing here.
I'm afraid you're in the eliminate it mode.

 

Ok, I replaced the new master cylinder with another new (rebuilt) unit. I bench bled the MC before installing in the truck and got about an inch of travel and very firm. Installed it on the truck and fired up the engine and pedal is very soft and almost goes to the floor. When I pump the brakes with the engine running, I get what sounds like air coming from the brake pedal in the cab. Is this normal? This brake issue is kicking my butt!!!!

 

How's the brake booster? Forgot if you mentioned it already but to me it sounds like its leaking...I had this issue with my truck this summer. Pedal was soft and squishy with the engine running, firm with it off. Got the correct booster (there are different units for different applications) and boom... I had brakes

 

what half of your reservoir runs low after applying the brakes this would tell you what axle to look at

 

Brake booster. It has a hole in the diaphragm. Had the exact same problem on my old truck. It doesn't actually affect your braking since the hydraulic portion is still fine, but it is unnerving because you have to get past the "no brakes" portion of the pedal travel until you hit what is essentially manual brakes.

 

One thing I did last night was after I shut the engine down, I wanted to check the check valve on the brake booster. As soon as I pulled the check valve out of the booster, I got a big rush of air sound. I would assume that the booster was holding pressure. Is there a way of testing the booster?

 

So it sounds like the booster is holding air.
The air sound when you push the pedal down is not normal but explainable.
When you push down the pedal you open a seal that lets atmosphere into the back side of the booster helping create the push toward the mc. The seal is old so it's prolly making some noise.
You say the pedal is solid with no engine. But on engine start and vacuum introduced you have no pedal. There is a mechanical problem somewhere.
Try this. Unbolt the mc while keeping everything else hooked up and turn the end nut on the booster rod so that it actuates the mc sooner. Then see what happens. Take note of how many turns you go out (try one turn first).
This will take any slop out of the system. Maybe that's what you're dealing with.
Remember that if you extend the rod to much your brakes will be on. Not a bad thing if you're just testing, but don't go for a drive. If your brakes are actuated it's a good thing. I would say just measure the distances involved but you can't be to sure what they are when you rebolt. If it does actuate your brakes then you know the mechanical link between the booster and mc is ok. Just back off the rod a bit (you'll know how much because you're keeping track off how much you extended it).
Again, we are assuming the rest of the system is right.
I just went through the same kind of deal on a newly rebuilt front axle and found that I had installed the calipers on the wrong side. It worked at first then drove me nuts when I replaced a line. The point is you can think all is right but the system doesn't work. It seems simple but everything has to be perfect.
I understand it can get really disheartening but hang with it. You'll find it.

 

CollinsCD, did you ever find the issue here? I'm currently dealing with the exact same issue. Even replaced a rear wheel cylinder.

 

I had the same issue. It was a hole in the brake booster, also gave me a slight vacuum leak so it was impossible to tune.