VERY hard brake pedal while running
 

06 lt
Came out this morning and my brake pedal is such that I wouldn't need to go to leg day at the gym. I've drove cars without power brakes, and those were cake compared to this. It was fine yesterday. Overnight it dropped to -15f, but it was colder earlier this week. The brakes "work", but I have to push the hell out of the pedal. They do seem to release fine. I'm thinking the almost week of double digits below 0, have caused something to freeze up in the master, since it is affecting all 4. There is a few years on the fluid, and it could use a swap sometime. It's supposed to warm up on saturday, into low 30s. That should allow things to thaw, if that's the problem.
There is a small change it's abs related, I suppose. My left front hub has a finicky slip ring sensor, and when it gets cold, it likes to trigger the abs and traction lights, but it's been like that for weeks. So, I don't think that is the problem.
Just wondering if anyone else has had an issue like this, and what their problem was.

Has the brake fluid ever been flushed and refilled?

It's been some time for the whole system. Which is why I am guessing it's frozen, just enough water that it's mostly clogged with ice.

Water vapor in power booster?
You can't replace the sensor only. More likely there is moisture in the connector. Maybe, replace the front hubs.

Could be vapor in booster, it has the signs of booster failure. I don't hear the sounds of a vacuum leak, Pedal height is normal, but it's just hard as hell to push. Aside from hard to push, braking action works as normal. Minus abs, which I'm not worried about. The left hub is intermittently a problem, but is very unlikely to be the issue here.
So, I moved it into my garage, maybe it'll warm up enough to see if anything thaws. Otherwise, I'm probably looking at a new booster.
However, rockauto does sell the vacuum check valve, since since lack of vacuum is usually what results in a hard pedal, that is the easiest to avenue. So, I'll probably replace that, and see what happens. I'll have to look for signs of a fluid leak into the booster, since that can cause diaphragm failure in the booster.

Check valves are awfully easy to test before replacing. So is vacuum in the hose.

Careful with the vacuum hose, the plastic barb on the intake manifold cracks and breaks off easily

Hose is a bit of a pain to get off. Kind of funny how easy a check valve comes out. Hose looked good. Pushed around the scuff cover and looked for cracks. The check valve appears to work. I could only blow 1 way through it.Stuck a thin hose into to booster, and didn't find any fluid. Master looked ok. No fluid leaks. I did blow some water out of the vacuum line. And when I looked through the butterfly(which was kind of dirty) saw some moisture. But I put it all together and now brakes work. My guess, the check valve is an amazingly simple thing. I think it might have froze open. Possibly a ice block in the vacuum line, I guess I probably won't ever really know, since there was nothing obvious.
Maybe it was just feeling neglected and wanted me to work on it, just because!

It’s fixed, enjoy the ride!