Cold Power Brake Problems

[FoMoCo]

I hope you guys can help. I have a 67 with the large diameter power brake setup from a mid 70's f-100. The rest of the system has been completely replaced with new but is still 4-wheel drum. The problem I'm having is during cold weather. When I start the truck in 20 or below weather the brakes will hiss when depressed not when the truck is moving but sitting stopped. When it hisses it will stall the engine. It also takes about 1 minute for the brakes to warm up or they feel like "hard-pedal". My 300 engine has an agressive cam which made me think it could be a vacuum problem. Brakes shouldn't have to warm-up? Bad booster? Thanks for your help.

Nick

[bluef250]

Vacuum problem yes. Try the check valve to see if it is stuck or has some moisture in it. Sounds like some water in system freezing things up.

[Sharkdance]

IMO, definetly a vacume problem.

A radical cam can cause low vacume but I would think that if that were the problem then it would be consistent.

20 below is a pretty hostile climate. Rubber, plastic, steel and iron tend to contract in temperatures below freezing. Then expand when they warm back up.
Your description of symptoms makes me think of something that is loose or not functioning correctly untill it heats up to normal temps. Then it expands or begins operating to make your brakes work correctly.
No brakes do not have to warm up to operate. At least the hydraulic portion of them dont. Brake fluid doesnt freeze in 20 below so everything from the master cylinder to the wheel cylinders will function normaly in those temps. But the vacume assist portion of the brakes is a bit different.
It consist of a diaphram vacume booster and a check valve to make sure that its vacume source only flows one way. Not to mention it needs a vacume source that is sufficient to operate the vacume booster.
Rock hard brakes is a sure sighn that a booster is either not functioning or has lost its vacume source.
Intermittent booster problems are almost always vacume related because the mechanics of a booster dont change otherwise. Meaning, if a booster works, then it is not likely that the booster is bad. All it does is apply vacume assited pressure to a piston that is actuated by your foot pressing on the pedal.
I guess it is possible that the diaphram in the booster is checked (cracked) and with heat the crack closes up enough to seal. But that seems unlikely to me. The same goes for the seal around the band that holds the boosters two halves of the can together. Possible but not likely.

I would suggest you replace the check valve in the booster since its not only easy to change but very in-expensive. Maybe somehow the cold is effecting it causing it to lose your vacume during extreme weather.
The hissing sound you describe sounds alot like what a bad check valve would cause.
Also check all of the hoses between there and the engine. Maybe one of them has gotten soft and only seals on its fitting during operating temperature.

If your brakes work normaly except when its that cold then your brakes are fine. Im thinking its a vacume loss situation instead.

Hope this helps.

[fordman]

start the engine and spray some starting fuild around all your vacuum hoses and the booster area . if the engine revs up then you have a vacuum leak in the area you were spraying.

[FoMoCo]

Thanks guys. I'll get a new check vavle. It has a spare from my lightning that I used during the build up , then never repleced.

NIck

[68F250]

When I start the truck in 20 or below weather the brakes will hiss when depressed not when the truck is moving but sitting stopped. When it hisses it will stall the engine.

I've had that happen before, the problem is definitely in the booster itself. It's leaking out (leaking in) the side where the rod goes in from the brake pedal, that's where the hissing is coming from. I kinda doubt it's the check valve or an engine vacuum problem.

How does vacuum leak anyway?

[Sharkdance]

68F250 Wrote:
"It's leaking out (leaking in) the side where the rod goes in from the brake pedal, that's where the hissing is coming from."

Interesting. I hadnt thought of that.

68F250 Wrote:
"How does vacuum leak anyway?"

Heh, yea I guess its sucks doesnt it?