Hey guys, let my 4runner sit for a week over the Christmas holidays, fired it up no problem this morning in prep for work tomorrow.

Then I was checking my fluids and my brake master cylinder is over full, and looks like it over flowed a bit? Can water seep into the system when sitting?

I'll siphon some out in a bit and test drive it.

Haven't found anything searching yet, but will keep looking.

Thanks guys!

 

Most likely it was near overfilled before you parked it. If you didn't have the parking brake set all of the pistons would have retracted and raised the level in the tank. Could be an air pocket that expanded due to temp cycles.

What does the fluid look like? If it's milky or burnt just suck it out and replace it.

No you should not be getting water seeping in as a general rule unless you parked it in a lake with the hood submerged.

 

Thanks for the reply Co_94_PU,

That makes a lot of sense. It was just under the Max line before I parked it, and I do not have the E brake on as I need to rebuild my bell cranks first. I will double check how the fluid looks and proceed from there.

You are gentleman and a scholar!

Edit: If my fluid looks okay I should be able to pump the brakes with the truck running to get the pistons to reseat and return MC level to just below max, correct?

 

Yep sound planning, pump the brakes and maybe remove some fluid.

 

I've never heard of that before; how would it happen? The front brakes (disc brakes) don't retract on their own. The rears (drums) have springs that would pull the shoes open (without the parking brake applied), but if the brakes are even close to adjusted they should push a very tiny amount of brake fluid. Certainly not from "MAX" to overflowing!

But other than that, I can't explain what's happening to 5 fists.

 

I was wondering about that myself, wasn't sure if there was any way the fronts would retract on there own. It's definitely possible my rears are out of adjustment quite a bit, but then I'd think my braking would be a lot worse. Time to take a look at them I guess!

All that being said, I fired er up, pumped the brakes a few times and sure enough the level dropped back down to MAX. Took some fluid out and test drove, all good it seems. I will be keeping an eye on the level for sure though.

 

87 4runner from your signature?
Parked outside?

There are a couple of more possible causes for this, #1 you could have a blister on one of the flexible brake hoses that over time will tend to collapse back to normal and push fluid back to the reservoir. or #2 you could have water in the brake system that froze and expanded (this is unlikely).
Brake fluid collects water (by design) which is why you should flush it out now and then so that it won't get saturated and allow water to collect at the low points and rust out your cylinders/calipers from the inside out. Also why you need to keep the cap on your reservoir and bottles of brake fluid.
Are the hoses original? is the fluid original?
Check all of your brake hoses carefully for any bulging/cracking or leaking. Since you commented that the level went back to where you left it after some pumping I wouldn't worry too much if you don't find an issue with the hoses.

 

Thanks for the thoughts akwheeler.

I replaced both front flexible brakes lines about a month ago when I did my calipers and pads, but I haven't touched the rear one. I should replace soon.

I did a complete brake fluid flush in the summer after one of my rear lines rusted out and I had to redo some hardlines and bleed the whole system.

Edit: sorry, didn't answer your questions. Yes 87 4runner, and yes parked outside.