Master cylinder reservoir fills up

[87924gt]

I just bought an 86 Carrera and the MC reservoir is filling 1/2 way to the rim every 10 days. I checked the archives and I think, (that's gotten me in trouble before) that the MC is the culprit. The car (Betty) has 140K on her and I have most every record on her back 16+ years. She's been to over a dozen DE's and was rightously maintained by the previous owner. 1st question is, is it the MC and 2nd, how long should one last?

[evill]

I have seen this a few times, when they wear, pressure bleeds past the piston and up the reservoir feed lines. This is what pushes the fluid out.

Replace the master.

As far as how long they last, it depends on things like how often you change your fluid, amount of use, etc..
I don't think there is a definitive answer.

Ed

[Bill Verburg]

the m/c level will go up and down w/ piston movement and temp changes. When it rises does it go down of it's own volition? or do you remove fluid manually?

a bit more info might help

Basically they will last for a very long time(for ever almost) as long as the fluid is changed regularly. I like to do mine at least once a year and more often if tracked.

[87924gt]

Ok, this AM got in the car and the dash board light was on telling me that the MC is low. I park on an incline (front higher) and I thought with the fluid that's been lost it makes sence that the fluid would be low. The temp here in Orlando has gotten much hotter the past several weeks and the fluid does go down on its own. Yesterday it was at the botton of the reservoir.
One more thing, I replaced the clutch cable yesterday and now the throttle will not return to the closed position. I checked the linkage behind the pedal and it doesn't return. It sticks about 1/2" before it should return to idle. Baseball season just started and I've already got 2 strikes on me. HELP!!!!

[Quicksilver]

You have a number of problems so lets look at them one at a time. The brakes are more important as they are the "Anti-dying" pedal so worry about them first.

Brake fluid doesn't expand and contract with temperature change. If it did it would be useless as a hydraulic fluid in a system with the temperature requirements of your brakes.

There are only to three reasons that the brake fluid should rise in the reservoir.

1 - You have changed the brake pads and retracting the caliper pistons pushes fluid back into the reservoir. I doubt this is it because it would only happen that once.

2 - You have a lot of air in the lines and you have to pump the brakes up to get a pedal. If you checked the level when the brakes were pumped up it would appear low and when the pressurized air pushed the fluid back it would go back up. I would hope you would notice the bad pedal and bleed it and if it comes back then replace parts.

3 - If you expose brake fluid to the air it will absorb an amazing quantity of water and will fill up and even overflow. If there is a large hole or leak in the top of the reservoir then it could be pulling a lot of moisture from the air.

More details on what you are seeing and feeling in the brakes would help. I'm betting on #2 unless you have no lid on the reservoir.

---

Fun experiment: Fill a shot glass 1/4" from the top with brake fluid and see how long it takes it to overflow from moisture in the air!

[87924gt]

Looks like I'm bleeding later today.
Thanks

[Bill Verburg]

Brake fluid doesn't expand and contract with temperature change

Oh really? Guess the guys at Dow got it wrong somehow

Dow Propylene Glycol for the production of polyglycols for use in hydraulic and brake systems (brake fluid to the uneducated)

Perhaps you have confused the incompressibility characteristic of brake fluid w/ the normal thermal related expansion and contraction of all materials.

further proof can be found at any race track where overheated fluid is being spit out of the brake fluid overflow tube

[masraum]

There goes that Verburg guy again. Spoutin' off like he knows somethin' about brakes er somethin'.

What we want is some sort of proof, do you have documentation or numbers??

[Bill Verburg]

Did you bother to follow the link?

specific gravity changes w/ etmperature for only 1 reason

[sammyg2]

"WET" brake fluid contains about 3% water by volume. I did the shotglass trick (but i used the cap off an old brake fluid container).
I filled it almost to the top and am checking it every hour or so. I haven't noticed much movement yet. How long should it take?

[Quicksilver]

Originally posted by sammyg2
"WET" brake fluid contains about 3% water by volume. I did the shotglass trick (but i used the cap off an old brake fluid container).
I filled it almost to the top and am checking it every hour or so. I haven't noticed much movement yet. How long should it take?

We don't have much real high humidity today but it will overflow. Give it a few days. 87924gt is in Orlando (drip, drip, drip) and he said every ten days.

The 3% is a figure that the auto industry came up with as a standard based on, I believe, one year's average absorption in an ill maintained brake system. Brake fluid will absorb a lot more then 3%. I would suspect it should absorb the range of 33 or 50%. I'm not a chemist but I play one this board as Bill will shortly point out.

---
Bill, as far as expansion; yes, all matter expands when heated except real odd cases when certain molecules shrink when they go through a state change. The notable example is water when it freezes, but even water always expands when heated or contracts when cooled when you disregarding the point of freezing.

BUT! Did you look at how small the rate of thermal expansion is for brake fluid? Very small. There are lots of uncompressible fluids out there that won't boil until much higher temperatures but they use Ethel Glycols or Ester Glycols because they have an extremely low rate of expansion. In practice we can ignore it unless we boil the fluid.

As far as expanding so much they spit out of race cars... It will happen if the fluid is heated outside of its operating temperature (boiling). It is somewhat common to see it when entering the pits as you have a maximum braking event that isn't followed by the normal cooling event. The heat soak will fry the fluid, but you know this.

I'm not sure what your point is. I would expect that you agree with my three reasons for expansion. You may not agree with my disregarding a 4th negligible form of expansion but I bet you don't think that is the cause of his problem anyway.

Does he need to bleed the brakes and then possibly change the master and/or the slave cylinder if the problem continues? I can't imagine you have any answer except "Yes".

[masraum]

Originally posted by Bill Verburg
Did you bother to follow the link?

specific gravity changes w/ etmperature for only 1 reason

Not sure if this was for me, but if it was then my sarcasm and tone didn't come through in my post. I was just kidding Bill. You're the local brake expert, I was just making a funny.

Sorry