Brake fluid weeping from overflow

[aston@ultrasw.c]

Ran hard at a higher speed track last weekend (76mph avg) with lots of long sweeping corners.

When I put the car back on the trailer I noticed a small amount of brake fluid weeping out of the reservoir overflow. The res was not overfilled so I am assuming that the awesome lateral G pushed the fluid out.

86 911

Can anyone confirm my hypothesis and offer any simple solutions?

Cheers

[surflvr911sc]

Another possibility is the reservoir overflowed when the fluid was hot, but once it cooled it no longer looked overfilled. How fresh is the fluid? The older the fluid the more water that is in it and that will make for more expansion.

[aston@ultrasw.c]

Originally Posted by surflvr911sc

Another possibility is the reservoir overflowed when the fluid was hot, but once it cooled it no longer looked overfilled. How fresh is the fluid? The older the fluid the more water that is in it and that will make for more expansion.

Good point.

I had just installed a new master cyl so the system was full of new Motul fluid.

I have set the level at about the same point for a while without issues, but the car is getting quicker and the tracks are getting faster!!

The orig prob was a loss of fluid with no leaks. I assumed the MC so I changed it just to be on the safe side.

[moneymanager]

Add a catch bottle so you're not leaving it on the track. This is pretty normal in my experience.

[175K911]

Both this '86 and my previous '86 did the same thing- I could drain almost 3/4 of the reservoir in a track day. When I converted to my 996TT brakes the problem stopped. My theory is that the stock brakes generated so much heat into the fluid that it expanded enough in the reservoir to hit the overflow tube, which then could set up a syphon effect. And this was with Motul 600 brake fluid changed for every track weekend. But the new brakes generate so much less heat that there's no fluid expansion. Again, just my theory.

[aston@ultrasw.c]

Other MC reservoirs simply have a small breather hole in the cap.

What if I closed off the overflow tube and drilled a small hole in the cap?

The res is protected by a fine nylon mesh filter that might stop fluid from sloshing up to the cap.

In fact I still have the old cap. I'll give it a try - watch this space.

[moneymanager]

The fluid doesn't "slosh", it is pushed out. I think you will find that drilling the hole just spreads brake fluid around where you don't want it. It takes 10 minutes, a short piece of plastic tube, and a plastic bottle from the refrigerator or medicine cabinet to solve this problem correctly.

[KTL]

I've seen the same situation Ed described- tiny reservoir gets depleted all the time. I asked others about this and they said the same thing- always gets drawn down. Seems silly for it to do this. I suspect Ed is correct that it's a siphoning issue and not a massive heat issue.

While the catch bottle is indeed the way to go to avoid needless dumping of fluid on the track (ahead of your front wheel if you've still got the factory hose plumbing in place), I think it'd be better to loop the overflow back into the reservoir. Because even with the catch bottle, the reserve still gets depleted. However nobody i've talked to ever had a problem with the fluid level dropping to a point that it caused a problem.

The reservoir vent is definitely the issue, because the later reservoir on the G50 cars does not experience this weird condition. I got the brakes plenty hot on my '87 many times, both with stock brakes and larger brakes, and I never had the reserve fluid get depleted like I see on my '79 racecar. I picked up a used 87-89 reservoir to address this annoyance. Plus it makes me one step closer to G50 conversion or 915 Tilton/Wevo race clutch (hydraulic) install down the road.... )

[RaceProEngineer]

Originally Posted by moneymanager

The fluid doesn't "slosh", it is pushed out. I think you will find that drilling the hole just spreads brake fluid around where you don't want it. It takes 10 minutes, a short piece of plastic tube, and a plastic bottle from the refrigerator or medicine cabinet to solve this problem correctly.

First of all, in our experience, NOTHING takes only 10 minutes. But other than that detail, we agree completely!

We chased this problem on our SC racer, on which the rules forced us to use essentially "stock" calipers. Run a hose from the MC overflow port into an aluminum or clear plastic catch can. We used an empty Kraft Parmesan Cheese bottle that had perfect sized small holes in the top, and yet would seal up. The clear plastic bottle allowed us to monitor the volume of fluid being ejected. RUN THE HOSE all the way to the BOTTOM of the catch can. This helps limit the volume of fluid that is expelled from the MC, and once the fluid in the system cools, it allows a reverse-siphon effect, refilling the MC reservoir.

We approached Kraft for sponsorship, but they were unable to see how this related to cheese!
Ed

[fredmeister]

Originally Posted by KTL

I've seen the same situation Ed described- tiny reservoir gets depleted all the time. I asked others about this and they said the same thing- always gets drawn down. Seems silly for it to do this. I suspect Ed is correct that it's a siphoning issue and not a massive heat issue.

While the catch bottle is indeed the way to go to avoid needless dumping of fluid on the track (ahead of your front wheel if you've still got the factory hose plumbing in place), I think it'd be better to loop the overflow back into the reservoir. Because even with the catch bottle, the reserve still gets depleted. However nobody i've talked to ever had a problem with the fluid level dropping to a point that it caused a problem.

The reservoir vent is definitely the issue, because the later reservoir on the G50 cars does not experience this weird condition. I got the brakes plenty hot on my '87 many times, both with stock brakes and larger brakes, and I never had the reserve fluid get depleted like I see on my '79 racecar. I picked up a used 87-89 reservoir to address this annoyance. Plus it makes me one step closer to G50 conversion or 915 Tilton/Wevo race clutch (hydraulic) install down the road.... )

what is the difference in design between the g50 reservoir and the 79 reservoir?
Dont they both have a vent hose? If you have photo that would be awesome, picture worth a thousand words.

[924RACR]

I'm running the early 944 brake setup; has a vented reservoir cap. We've always had the issue of piddling some out, so we a) run at half-level, and b) run a "diaper" - a folded paper towel zip-tied on the cap.

[Eagledriver]

The main problem is that the vent hose is on the side of the reservoir. Every time you pull a 1G right turn the fluid goes up and out the tube. I run my reservoir about half full and that helps. Best option would be to vent the cap instead. You could even epoxy a tube on the cap if you end up with fluid comming out still.

-Andy

[KTL]

There's definitely something different about the G50 reservoir vent vs. the earlier smaller reservoir. Maybe its just the added volume of fluid, plus the air void above the fluid that doesn't allow the fluid to be spit out the vent hose as easily? Also, the G50 reservoir is a shared unit that provides fluid to the clutch plumbing. There's a dividing wall that separates the reservior into two, but the fluid level remains the same in both because there is some kind of opening in the wall that lets the fluid level equalize. When you fill the reservoir, you're filling the brake side. Then the fluid slowly flows over to the clutch side.

If you look closely at the vent tube, it's nothing more than a nipple sticking out the side of the top of the clutch side. However on the older small reservoir, it looks like the vent has a column that goes down the side of the reservoir and I think this is how it can drain or siphon its fluid level so far down?

[fredmeister]

thanks Kevin. I think the added height of the vent tube above the fluid level and internal wall keeps fluid from rising in the ven in a right hand turn.
I experienced the unpleasant mess all over my lower suspension and chassis during track days last summer. Only had this problem happen in my 79 930 turbo, never saw it in my 911SC, but I think i am cornering harder in the turbo.
My solution is a gatoraide bottle with a tube to the vent line to act as an overflow tank. I attach it when at the track and monitor it all weekend after every session.
I guess Porsche didn't always get it right from the factory on every car.

[KTL]

Fred,

Thanks for mentioning the overflow bottle. I forgot to mention I have one of those installed on my '79 too. If its going to overflow anyway, might as well collect it cleaniy instead of dumping it on the track & your underside!

[aston@ultrasw.c]

Thanks for all the suggestions, here is my solution: