Track Brake problems

antaircooled · 09-07-2006, 11:38 AM

I have a '74 911S with standard brakes apart from Mintex 1166 compound pads and Precise Lines braided hoses. Everything new-ish and front alu calipers rebuilt. I did about 70 laps testing a few weeks ago and everything was fine - new RA1's got scrubbed in real good. First race of the season on Sunday and after practise got back to the pits and noticed a pool of brake fluid under left front fender - overflow pipe comes out there. I checked fluid level and it was down but not much - anyway this happened again after the first 8 lap race - brake light inside cabin came on. Got back to pits and pool of fluid again. Did a bleed and replenish fluid after race one but same thing happened again etc.

Any ideas you guys?

jaydubya · 09-07-2006, 11:45 AM

Sounds like time for a new master cylinder. Have you ever replaced it?

antaircooled · 09-07-2006, 12:05 PM

Thanks Jay - the MC is the only part of the brake system that hasn't been touched in my ownership (about 3 years). My car runs a '78 SC MC with booster so I was thinking of going to a 23mm MC.

cstreit · cstreit's Garage

Obviously it would help to be sure where the fluid is coming from first.

One thing I've seen though is that when brake fluid heats up, it expands. In my resevoir it'll raise up 1/4" or more. If it's overfull, it'll overflow a lot. It should never overflow to empty though!

dfink · 09-07-2006, 02:49 PM

Congratulations you are now making enough g's to force the fluid up the side and out the overflow. Mine does the same thing. I make a catch bottle out of model airplane gas tank. The brake fluid goes in one line of the tank and the vent attaches to the other. I hold it to the master cylinder with a magnet. I know I shouldn't but I pinch off the vent line and squeeze the fluid right back into the master cylinder. I thought maybe I was boiling but I never have fade. On R compound I don't even make it a session. On street tires (not as fast corners) I can make it 2 or three sessions. I have tried to overheat in a straight line and you will never lose any fluid. Also helps to keep the fluid down a inch or so in the reservoir.

JP911 · JP911's Garage

Just a thought, but a 23mm master cylinder with stock 74 brakes will just make the pedal harder to push. Do a search on the 911 tech forum on the subject. In particular, Bill Verburg has quite a bit of knowledge on the subject.

antaircooled · 09-07-2006, 10:11 PM

Thanks guys - I am doing 1min 16 at this track instead of 1:19 (since putting on the RA1's) so maybe faster means more temp in the brakes and more G's. The brakes were still good but just concerned about the fluid loss and the brake light flashing on and off. Dfink you da man - great photo. I will fab something close to that and see what happens.

KTL · 09-10-2006, 10:16 AM

You don't want to switch to a 23.81 930-sized master cylinder. It will push too much fluid and make your brakes much more pedal travel sensitive, as well as increasing the effort.

RaceProEngineer · 09-11-2006, 08:48 PM

antaircooled,

As Chris has suggested, first make absolutely certain that the fluid is coming from the overflow ONLY.

You do not mention, and no one has inquired, what fluid you are using. Average street brake fluids do not stand up very well on the track. They tend to attract moisture, and then expand, and eventually boil. Maybe try a complete flush and replace with a racing fluid like ATE "Blue".

We also use a closed system overflow catch tank like DFink's, and as the fluid cools, the master cylinder reservior replenishes itself.

Ed LoPresti

antaircooled · 09-11-2006, 09:36 PM

Hi Ed,

Absolutely sure the fluid was from the overflow. I was using good DOT4 fluid that was fresh and the system had been bled. I have gone to Elf Race fluid as I can't get the ATE 'Blue' down here. Race season has just started downunder so will find out this weekend how the R fluid and overflow catch tank go. Thanks everyone - you guys are so knowledgable and helpful.

dfink · 09-13-2006, 12:31 PM

Just curious about how things worked out.

antaircooled · 09-13-2006, 01:33 PM

Hi Dfink,

I am all set up with fresh Race fluid and an overflow bottle - I have the racing this weekend coming up. I race in a series for pre-'77 European cars - lot's of MG's, Ford Escorts, Marcos, TVR's Jaguars, BMW's and of course Porsches. Will let everyone know how it goes.

antaircooled · 09-17-2006, 11:10 PM

Hey Guys - thanks for all your help. I got through practice and four races over the weekend with no problems. The overflow bottle worked great and I had about 250ml go in there by Sunday lunch time. Just tipped it back in after each race and didn't need to bleed the system like the last time.

RaceProEngineer · 09-18-2006, 06:46 AM

antaircooled,

Thanks for the update - and those are the results we like to hear! "We love it when a plan comes together . . . . ."

Ed