Lose Power Brakes when Below Freezing

[Colin]

04 4.0L 370k kms.
This issue started last winter, but I of course forgot about it until it started getting cold again, and it seems to happen more frequently/easily.

When the truck is first started in below freezing temps it acts like there is no brake assistance. The pedal drops at least halfway and then they start to work. After a few mins the brakes feel normal again. Takes longer to come around if its colder.

I'm leaning towards the booster, but maybe its the check valve? Or something else that I'm missing?

The master was replaced a couple of years ago when I add cruise so I don't think that's the issue, and I'm not losing any fluid.

Any input is appreciated

 

[Josh B]

I think you're right about the water possibilities

 

[Colin]

Sorry what water possibilities?
The issue is temperature related

 

[Josh B]

Simple, water freezes when it's below freezing. I'm simply supposing water has gotten in it somehow

 

[Colin]

Ok, now I get you.
That's possible I guess. Once I get the booster off I'll give it a shake and see if I hear any sloshing.
I'm hoping I can remove it without disconnecting the brake lines, but we'll see.

 

[RonD]

Power brake booster should hold vacuum for weeks if not months and months
When you get in, after truck has been sitting, at ANY temp, press the brake pedal, should have power assist, if not there is a leak
Also after driving and you have power assist, after shutting off the engine press brake pedal down 3 times, on the 4th press you should start to lose power assist
If it doesn't last 3 pedal presses you have a leak
The reason the booster is so large is to hold enough vacuum in reserve so if the engine should stall while driving, the driver will still have power assist to pull over and stop safely

Could be the check valve, but they do not tend to fail, but not expensive to replace, $6-$7

Heat expands things
Cold shrinks things
That applies to seals and gaskets, so you could have a booster seal that leaks more when cold

The water inside is an interesting direction, but temp would have to be "just right", cold enough to freeze the water but not too cold because it would have to unfreeze fairly quickly, for booster to work again

Long shot
See if you can lift the brake pedal up more with your foot
If you can usually the brake lights would stay on, which is why its a long shot
But in the cab end of the booster is its Atmosphere valve, when you press down the brake pedal the push rod pushes in the Atmosphere valve, opening it up, this allows in the 15psi outside air pressure to Assist the driver in pushing down the brake pedal along with the -9psi vacuum inside booster
If brake pedal doesn't come all the way back up the Atmosphere valve is left partially open..............a leak

 

[Colin]

Thanks for the tips RonD.
We'll see how the truck behaves when I drive home this morning

 

[Colin]

Ran out of time yesterday so I threw in the new check valve to see if that was the issue. It was not. Guess I'm changing the booster. This would be alot easier if the truck didn't have abs.