In a correctly vented system the pressure in the MC is 14.7 or whatever the barometric pressure is. The fluid in the lines is horizontal and thus not adding/removing any 'head' in the reservoir.
Sorry, no. The atmospheric pressure is 14.7 psi, but it presses equally on everything in the atmosphere. It presses on the inside of a vented master cylinder with 14.7 psi, and it presses on the outside of the master cylinder with 14.7 psi. The pressure differential across the pistons, etc. is ZERO due to the atmospheric pressure. Look up PSIA, and PSIG on google.
I *really* don't think the that the spring in the vent is that fast acting that it will slam shut, and stay shut in the HOURS it takes for a parked MC to achieve these mythical pressure differentials.
Have you ever taken one apart? It is simply a stiff rubber disk with a very light weight coil spring on the atmosphere side of it. It will move very quickly. From my testing, it takes about 1/2 psi pressure differential to open one. One that isn't stuck, that is.
I mean yes, in mythbusters terms this is a PLAUSIBLE scenario.. but in real life the vent is NOT that that sensitive, and the pressures are not changing that rapidly by air temp/pressure alone.
That opinion is based on what testing? I've checked the bobble heads on my truck, and about 1/2 psi is all it takes to make one vent. And they do not leak back.
I mean the vent is designed to close when water hits it. Not at single mililbar per hour pressure differentials... its not that kind of check valve.
We aren't talking about barometric pressure changes, we are talking about pressure changes in a sealed vessel due to drastic temperature changes. You can crush those cheap 1 gallon rectangular metal solvent cans by sealing them when they are out in the sun, and then letting them cool over night.
Lets go line by line from the OP:
If the truck is used regularly, the brakes will stay pumped up, and the DOT5 will keep the piston cup expanded just from regular use.
There is no pressure in the system when parked. Its essentially just liquid in the tubes. If the MC piston is not applying pressure, the system is at basically atmospheric pressure with no force being applied to the brakes.
That liquid has a certain amount of head due to the master cylinder (and its reservoir) being above the wheel cylinders. It isn't much head pressure, but it is about 1/2 to 1 psi.
When air gets hot, it expands, and so it happens in the reservoir of the deuce master cylinder. The pressure caused by this expansion makes the one-way air vent burp the air out. But when it gets cold again, the air contracts, and sucks on the one-way air vent. Because the vent is one-way, the suction cannot be relieved through the vent, so it is reflected through the DOT5, and draws in the wheel cylinders instead.
Where is this mythical heat coming from on a PARKED vehicle?
The Sun, and the air temperature. Does your car get hot when it is parked in the Sun? Mine sure does. Does it get cool at night? Again mine does. When the Sun is boring down on the trucks, they get toasty hot. When night falls, they get sometimes down right cold. If the truck sits for a season, in the summer, during the day, the air bleeds out past the bobble head vent. At night, and in the winter, it gets cold, and the air space in the master cylinder contracts sucking on the hydraulic system.
-Chuck
) so we had turn in all our old trucks...we had like 15 M35's and 30 or so 939 5-tons...DRMO will not take a vehicle for turn in if it leaks anything so what do active duty wrench monkeys do? They drain the oil from everything, wipe it off and drive it in....Some of our deuce's had no brakes and the drivers would jam reverse to stop or use the park brake. Some active duty drivers and mechanics abuse the heck out of military equipment because they aint paying for it...