I just replaced my brake booster but now I notice that when I keep pressure on the brake pedal for an extended period of time the pedal seems to slowly go down toward the floor. Not all the way to the floor. The truck stops fine but the pedal just seems to get soft after the initial stop.
3 questions;
This doesn't seem normal but is it in fact nomal?
Could dirty brake fluid cause this?
Is there any way to test the master cylinder instead of just replacing it?

 

If fluid isn't leaking out somewhere you have classic symptoms of an internal master cylinder leak. Dirty fluid could cause it but a more common cause is pushing the pedal all the way to the floor when bleeding brakes. What happens is the part of the master that never gets used gets rusty. When you push the pedal all the way to the floor the rust tears the master cylinder seals.

 

Hmm may need a master cylinder.
I noticed something else strange after changing the booster.
It seems that the pedal wants to fall away (about 1") from the stop where the brake light switch is while I'm driving. I can shut down the engine and lightly pump the brakes a couple of times and it stays up. Is there a way to adjust the pedal play other than adjusting the brake light switch it's self?

 

Well I've replaced the master cylinder and the pedal is still soft.
Also it seems to have too much slack in the pedal. It tends to fall away from the brake light switch causing the light to stay on. when I kill the engine and pump the brake bleeding off the vacumm the pedal stays up against the switch.
I bench bled the master cylinder before installation then I used a vacumm pump to pull all of the old fluid and any air from the lines. The front brakes had a little bubbles but cleared up quickly.
Now the back brakes never quit bubbling. I pulled more than a pint of clean fluid through each rear brake line and the fluid was full of bubbles.
Am I doing something wrong here and why is my pedal so loose?