2008 Sierra brake issues

[Lateapex911]

I did a search, but haven't found the right thread. If I missed, I'be grateful for a link.

 

Basic problem. One morning, the truck has very limited braking. Was 100% normal the previous night.

Essentially, it slows but stopping power is greatly reduced. Pedal travel is soft and long. With the engine off, one push uses up the booster vacuum, and then efort goes up, but it's still soft and long.

 

Previous ambient temps were in the 30s and up, but the night it happened it went down to 3.

 

Inspection showed no leaks along the lines, the rubber hoses or at the calipers. Hmmm.

 

Friends agreed and suggested that Master cylinders fail in sudden cold snaps, (something to do with seals breaking internally and the cold pushes them over the edge?)

 

Truck is an 08, with 80,000. AWD, 6.0.

 

So, it sure felt like a MC problem, so I replaced it. The problem remains, and may be slightly worse. That could be due to air in the lines, but I was very careful to swap quickly with a fully bled MC.

 

It sure doesn't feel like the problem is solved. (Once I can get assistance, I will bleed the brakes!)

 

Ideas?? Known failures?

 

Barring any other direction I'm probably going to try and isolate the various parts of the system, using plugs, so I can eliminate components.

 

Does anyone know the sizes of the ports on the ABS and MC? I think the GMT 8000 are 1/4 pipe, 7/16"-24.