I need some help with this one, folks. I have a brake problem.

06 extended cab, 4x4, 4.0, automatic. 79,000 miles.

The problem: The brake pedal will intermittently go to the floor with very little weak braking pressure at the bottom of the pedal travel. It started happening yesterday morning, it was very cold (4 deg). It did it a few times and then it seemed to be fine the rest of the day. It did it a couple times this morning too (it was 14 deg). I really have no idea if the temp has anything to do with it, it just started when the cold air moved in.

I checked the brake lines front to back and there are no leaks. There is no leak at the back of the master cylinder and the fluid level is fine. There are no "ABS" or "Brake" lights lit on the dash, and no ABS codes stored in the computer.

The master cylinder seems to pass the old test of slow, easy pedal pressure building pressure at a reasonable pedal height.

Anybody have any ideas about solutions, or further diagnostics I should do? I just don't want to start firing the parts cannon. Thanks for your assistance!

 

Probably master cylinder is wearing out.
The internal seals are not doing their job anymore, and the fluid simply goes by them instead off being fed out the brake system.
With this situation the pedal would go to the floor.
Pumping helps because the friction warms things up a bit.
It's basically an internal leak.

It happens more when it's cold because the seals are stiff and can't make good contact with the bore.