89 Aerostar - Brakes

Hi,
I have a slight fade in my Brake pedal. I noticed the other day, that the fluid was REALLY low, so I got some DOT 3 and topped it off...
Drove it for 100 miles, start and stopping... pumping brakes, etc...
checked the fluid level, still full...
I noticed before the fading petal, that the rear drums would tend to lock up at low speed, just from applying light pedal... like at 15mph, you could hear the rear wheels skid. - This used to only happen when it was wet, and then when it dried up, the rears were fine....
The brakes stop the car on a dime when need be, but I hate that "slightly sinking feeling" at a light.
The question is, will this be cured with simply bleeding the system? and the second part is, how did air get in there, since the system hasn't been opened in some time (like 4 years) Could air have been brought in from the level being too low? How serious is this, and can it wait til spring?
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Could be moisture in the lines instead of air. Brake fluid attracts moisture, which is why you should toss any bottles of left over fluid after a year. Hygroscopic I think is the word. You could take it down to a shop and have the system power bled. The night before spray the bleeder valves with some penetrating oil, like Liquid Wrench so they don't get accidentally broken if they haven't been opened in few years. Matt
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yeah, I was thinking of doing that... I guess the moisture will not "go away" come summer... or will it? I wonder why it was so low to begin with, and there is no leak? does it evaporate once it takes on moisture or something?
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It's probably magic fluid
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snipped-for-privacy@optonline.net Could be moisture in the lines instead of air. Brake fluid attracts moisture, which is why you should toss any bottles of left over fluid after a year. Hygroscopic I think is the word. You could take it down to a shop and have the system power bled. The night before spray the bleeder valves with some penetrating oil, like Liquid Wrench so they don't get accidentally broken if they haven't been opened in few years. Matt ===========
hi matt - having taught hydraulics for many a year - may i offer some suggestions.
having any hydraulic fluid around is not going to cause it to attract moisture if it in a tightly sealed container. if that were the case, they would have to throw away all the hydraulic fluid (brake or otherwise) off the shelf from the stores if it wasn't sold within a year.
the problem of moisture in the brake line comes from moisture of the air and pressure on the fluid. when pressure is applied to fluid, it can hold more air - example - open a bottle of pop. it was under pressure and when the fluid is exposed to atmospheric pressure, it releases the air bubbles. those air bubbles have moisture in them, and that's were the problem starts.
as the fluid is squeezed in a brake line, the air is absorbed into the fluid because the auto braking system in not a sealed system but exposed to the outside air. over the period of a year, it has squeezed the fluid and released the pressure causing the trapped gas to be released, but it will leave the moisture behind. this principle has to do with pressure and moisture properties. if the fluid can be warmed up and exposed to the outside air, then you don't have nearly the problems you have as you do with brakes.
case in point - how often do you change power steering fluid, transmission fluid, or diff. fluid? these are oils too, but moisture is nowhere near the problem as it is in the brakes
on big machines using hydraulic oil such as plastic molding machines, the maintenance man will remove the plug in the bottom of the reservoir tank and drain the water buildup that has accumulated over the week or month, but they don't replace the fluid at that time.
probably the biggest reason i can think of bleeding the brakes is getting the moisture out is because of the heat produced by the pads of today. the temps really climb when brakes are heavily used. and since water in the brake fluid evaporates at 212 verses the temp of DOT 3 or DOT 5. it will vaporize and cause the brake pedal to get spongy or can collapse.
this may explain why someone could be going down a mountain side and using the brakes heavily. 1/2 down they go to the floor and somewhere from there to the bottom they wreck. much later, the brake fluid has cooled down, the water vapor is gone and is a liquid again....the person tells their story about the brakes and the officer pushes on the brake pedal and it behaves like it did at the top of the hill, not at the bottom where the wreck took place. now, try to explain that to the insurance company........
hope this info helps.
~ curtis
knowledge is power - growing old is mandatory - growing wise is optional "Many more men die with prostate cancer than of it. Growing old is invariably fatal. Prostate cancer is only sometimes so." http://community.webtv.net/PALMER_ENT/doc
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RobGute opined in

Pulll oyur rear drums and check the shoes and cylinders
--
- Yes, I'm a crusty old geezer curmudgeon.. deal with it! -

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