Power Brakes (1 Viewer)

[Brian Macgyver]

I have an odd deal to report/ask about.
When I start up my 80 at below 30 degrees outside temp, the power brakes do not work. The brakes are manual, and barely enough to hold the truck in drive. I have to press the pedal hard.
Within 1-2 minutes, the power brake is warm and works normally. It stays that way all day. It is reliable when warm. It is as if the PB diaphram is too cold to function.
It is not a huge issue as the power brake comes on line within 3 minutes. But the lack of real brakes in the first 3 minutes is unnerving. The truck tends to wander when it is cold. I can hold the brake, and the truck still drives away.
This has mostly happened when our overnight temps have hovered below 30 degrees. That has happened most nights lately. Once the truck or outside temp warms up, the issue is gone. The truck brakes normally.

Does anyone have an answer?

 

[96r50]

I have a very similar problem. From what I have read on here there is a check valve in the brake booster system that maintains vacuum in the booster. My layman's understanding is that this check valve can freeze open and not maintain vacuum in the booster to provide the vacuum assist for the brakes.
I don't understand how the check valve manages to unfreeze so quickly after startup though.