79 F-150, power brakes, disc/drum. When the truck sits in really cold weather for a couple days, the braking often sucks, with a very hard pedal. After several applications over a few minutes, the braking improves to where it should be. My guess is that the complete brake system needs to be completely flushed, possibly due to moisture freezing in the lines.

Thoughts?

 

I'd definetly start with a flush.

 

Good chance it's your booster.

 

Might be as simple as a hard old PB booster vacuum check valve grommet softening up with some heat and sealing better.

 

Right, replace the brake fluid for GP's
But
The brake booster will be the cause of the hard pedal
Either a one way valve that does not work or the booster itself
There is a test in the book for brake boosters
You apply the pedal a few times with the engine off
That will depleat any vacuum in the booster
Then, while holding the brake pedal down firm, you start the engine
The pedal should drop an inch or two if the booster works when the engine starts
If it just sits there, or remains hard, replace the booster

 

I've never "flushed" the brake fluid in any vehicle that I have owned.
1989 Mustang LX 5.0HO 325,000 miles.
2011 Chevy 2500HD 257,000 miles.
2007 Lincoln Town Car, 175,000 miles.
1979 F-250 4wd, 250,000 miles.
Freezing cold snowy winters and blistering hot summers.

I'm sure the brake fluid was flushed when I did the brakes on my '76 F-250 4wd when I replaced the calipers, wheel cylinders, brake hoses and master cylinder.
It would have been kind of hard not to.

 

Cheap friction materials (pads - shoes) (also HP) will also cause this. Once the friction material has a few applications and warms they will begin to grab correctly.

The caliper pistons may also be frozen due to bad fluid.

 

Brake fluid is hydroscopic, it attracts water out of the air. I supposed if you get enough it could freeze in the line. Worst part of not flushing from time to time is a rusted up system from the inside out you can't even see.

 

If it was the booster, why would it only happen when it's VERY cold outside (10 degrees and colder) and the truck has been sitting for several days in the cold, and only for 2 or 3 of applications to make the problem go away?

 

Maybe water in the booster?

 

How long has it had this problem???
10° and sitting for several days?? Only 2-3 first time applying the brakes??
Maybe there's nothing wrong with it.

 

CLICK HERE - https://www.brakeandfrontend.com/vac...r-diagnostics/

DIAGNOSE BOOSTER Before First Start-up



If water is in the system, it will be absorbed into the fluid (as designed) - hygroscopic) and if extra saturated (beyond designed absorption rate) the water will separate and eventually settle/pool to the lowest point(s) in the system, calipers and wheel cylinders, and it will freeze. Friction heat from several brake application(s) will/may eventuality un-freeze the moisture.

Fluid should be flushed at the least every two years.

 

Now it just sounds almost normal, to hold a vacuum for several days requires NO vacuum leaks at the booster. That check valve and or the rubber it goes in could leak, and if only doing it cold, replace the round rubber seal.

Have you yet tested the booster as detailed by both KULTULZ and manicmechanic above? Until you do, we are all just guessing.