brake fluid leaking from overflow/vent line

[fredmeister]

I was attending a DE at Mosport 2 weeks ago and noticed that my brake vent hose was leaking fluid onto the drivers side front control arm and the ground after a few sessions on the track. The vent hose runs down in front of the drivers side tire more or less next to the A/C condenser under the front spoiler. The car is a 79 911SC.
Can anyone explain why the vent hose would be draining fluid after a few very hard sessions?
The level in the reservoir dropped significantly to about an inch below the low level/add mark. I had never seen it drip before and was wondering if it was possibly caused by the high cornering g-loads getting fluid to slosh into the vent line and later having it stop after the level reached a signifcantly low level in the reservoir?
How do other passenger cars vent their systems as I don't see any external hoses and it appears to completely sealed systems?

[Wil Ferch]

Other cars vent directly into the engine compartment, where most other ( front engined) cars have this hardware.
Sure... a combination of high cornering loads and heating up ( expanding) the fluid can case you to push fluid through the overflow line.
---Wil Ferch

[BGCarrera32]

Sure... a combination of high cornering loads and heating up ( expanding) the fluid can case you to push fluid through the overflow line.

If you have never noticed this during normal driving the above explanation is correct HOWEVER if you can duplicate this with normal driving and say a dozen or so stops, you need a master cylinder. I had this same problem and it was extremely difficult to figure out... BUT there is seal at the back of the master cylinder where the piston shaft exits the casting into the brake booster donut and if the seal is failing it won't necessarily leak fluid there but will suck air in on the return stroke of the piston, shove that air into the reservoir and displace large quantities of fluid out the overflow tube. You will also notice a decrease in rear brake performance as the brake line for the rear is closest to this rear seal.

You probably don't have this problem, but thought I'd post anyway as I spent more than a few hours trying to figure this out.
I highly recommend you construct some sort of catch bottle for you overflowing fluid as well...it makes a mess underneath the car and others won't be crazy about driving through your brake fluid drips.

Good luck-
BG

[surflvr911sc]

When I did my first system flush, using the power bleeder, I forgot to block the vent line and put a whole liter of ATE blue on the garage floor. Unfortunately my vent tube was routed to the front of the luggage compartment! What a mess.

I found a small catch bottle that is used to bleed brakes at the local auto parts store and connected the line to it. It has a magnet mounted on it so it’s pretty secure and now I can see when and how much fluid overflows. And it doesn’t get under the car or in the luggage compartment.

[911pcars]

Not sure if these are the same ones that Ryan describes. I got this from Harbor Freight. It's a polyethylene bottle with a round magnet attached to the vented screw top. It's $3.99 and comes with some hose/hose adapters. Bottle holds approx. 2.5 oz.



If your fluid loss needs are higher, you can use a bicycle water bottle and cage. Mount it where it won't spill and where you won't be tempted to drink out of it.

Sherwood Lee
http://members.rennlist.org/911pcars
no affiliation with HF, but I wish I was

[surflvr911sc]

That's it! Thanks Sherwood.

[dtfastbear]

Fredmeister,

Does this look familiar?

Brake fluid pouring out the overflow tube!!

Good luck - Dean