Brake Fluid Rising - Why?

[vasy]

I've been keeping brake fluid level in the reservoir at 1mm below the full line since I replaced the left rear caliper and hose about 3 months ago. At that time, I flushed brake fluid at all 4 wheels and everything is fine after that (no spongy pedal). A few days ago I noticed that the fluid level in the reservoir went up to 1-2 mm above the full line. That's odd. I've never seen brake fluid level going up by itself (no work done to the system). The old caliper's piston was sticking causing very hot wheel. The new re-built one works fine, no more hot wheel. Why did my fluid level go up?

[1 Lucky Texan]

for one possibility, higher ambient temps in a 3 month period could expand the fluid. Also, brake fluid is hygroscopic, could be the volume increased from atmospheric moisture absorbed?

[Fairtax4me]

Brake fluid expands a little when it gets warm. Did you check the fluid level after driving?

[vasy]

for one possibility, higher ambient temps in a 3 month period could expand the fluid. Also, brake fluid is hygroscopic, could be the volume increased from atmospheric moisture absorbed?

Possible. But this much rise in fluid level in short period of time has not happened to any of my cars before. I checked my cars at least once a week. This Subaru, due to its old age, usually 2-3 times a week.

 

Brake fluid expands a little when it gets warm. Did you check the fluid level after driving?

I first checked after driving. But the level has remained the same overnight.

I also notieced the color of the fluid is a little darker than just a couple of weeks before.

[1 Lucky Texan]

well, the system holds a lot more fluid than the reservoir volume, yet, the reservoir surface is the only place the fluid is not 'restrained' so, it must reflect ALL changes in static volume.

 

Plus, there is no connection to any other fluid that can infiltrate the system.

 

What kind of fluid was installed? Something exotic? Maybe there's some internal swelling of brake hoses?????

 

If you also reported some kind of braking problem at a wheel after suspension work - OK, maybe a hose got twisted/collapsed, pushing a little fluid up, when a caliper was re-installed. But.....

Edited August 21, 2014 by 1 Lucky Texan

[Fairtax4me]

Could it be that the vehicle was leaning or pointed uphill during the initial fill and is the level now being checked on a flat surface or downhill?

[vasy]

I've been using Valvoline synthetic brake fluid for all of my cars in the past 5-6 years. Internal swelling of hoses is possible. 1 hose is new (OEM) and the other 3 are 18 years old from the factory. But there is no driving-related issues except the left front caliper might have a slight piston-sticking problem. I said "might" because that wheel is always about 10-20 degrees hotter than the right front. But when the left rear caliper (which I replaced 3 months ago) was sticking, it's much hotter (30-40 degrees) than other wheels, and the fluid level in the reservoir did not go up. I do plan to replace the left front caliper before winter this year.

 

I filled the fluid in my garage and always checked the fluid in the same garage. So it's not because the car is on an incline. Well, I'll see what happens after I replace the front caliper and put in new fluid.