Brake bleeding problems, 1966 w dual mc

[snowman]

I have a 1966 Porsche 912 race car. Since the brakes are the same as the 911, I am posting the question here.

The car has a 1969 dual master cylinder in it. I have been racing it for over 10 years now and have had the car somewhat longer. I was preparing for a race at CA speedway this weekend and decided to change the brake fluid as usual, ie I change it at least 4 times a year and have done so for 10 years. I changed from the super blue to Motul 600 racing fluid.

I used my trusty mighty vac to change the fluid, as I usually do. Button up the car and go to leave for the races. But NO brakes! So I drag the thing back into my garage and bleed, bleed, bleed, bleed and my wife said something about divorce and I tried to bleed them again anyway. We are talking the old fashioned way of bleeding, a hose , jar and two people. 4 pints of fluid at $12 a pint and no luck. Well off the races anyway, don't need any stinking brakes to go fast do we?

At the track this weekend. 2 friends and the now almost ex wife with me. We get togather and knowing everything there is to know about everything, esp brakes, continue the proceedure, bleed bleed bleed. NO luck! Talk to friend at track, He a certified Porsche expert and is taking care of at least 7 or 8 Porsches for the race. He has nothing much to add. SO we scratch our heads and say it must be a bad master cylinder. Like most good racers I happen to have a new one with me, just in case. Change the MC and bleed, bleed, bleed. 4 more pints of $12 brake fluid. Every once in a while my Porsche friend stops by, you can tell he is saying to himself, Don't volunteer, anything. He is truly puzzled, told by his look. He walks away, to revisit occasionally, but just to see what is going on. He is saying to himself boy I am glad this is not me. Anyway go racing, no brakes needed. Maybe brakes will bleed after some use. Bleed some more..very slight improvement. Tried banging on mc while pumping, bleeding at every joint, only ever so slight improvment. Go racing again. A little better, but got to pump like hell for brakes.

Next day. at the track. Bleed, bleed, bleed again. more brake fluid. ever so slight improvement. Porsche mechanic friend stops by, asks about brakes and quickly leaves. Race again. better. But not good, first pump goes to floor. Could it be a bad mc as well? Not likely as original had same symptoms and worked before changing fluid. Is it the Fluid? Ask a bunch of people, not likely. Bleed , bleed , bleed some more, buy another pint of fluid, bleed some more. Almost have brakes again. Not quite right yet though. Will try to bleed again tomorrow.

Anyone had a similar problem with Motul 600 brake fluid or their 911/012?

[911pcars]

Sorry to hear about your brake problems. I'm going to assume fluid normally flows out of the caliper with the bleeder screw open. This indicates nothing is blocking fluid flow in the lines (flex lines especially). Motul is good stuff. It's not the fluid.

Did you bleed the MC before installing? If not, you will have lots of air in the system.

Did you/do you see air bubbles drifting up from the fluid in the reservoir when you started to pump the MC? Stroking the pedal gently usually allows air to escape from the reservoir supply tubes.

I'm going to suggest trying the old fashion, gravity bleeding method. Get some nylon hose from the local hardware store. I forget the ID but it should tightly fit the OD of the bleeder screw. You'll need about 2' attached to the bleeder screw and suspended vertically somehow. You can use friction, safety wire or a clamp to hold the tube onto the bleeder screw.

Open the bleeder screw. With the tube in place, you now have extended the hydraulic passage past the open bleeder screw. The fluid head from the reservoir will force fluid through the system and out of the caliper into the vertically-positioned tube. If there's air in the caliper, you'll eventually see it in this tube. You can increase the air/fluid flow by gently stroking the pedal (halfway to floor only) to push fluid through the system. Keep an eye on the reservoir to make sure there's always fluid in there. As the tube fills up, bend the upper end downward into a catch can. Close the bleeder after you're convinced no more air is in that corner of the brake system.

Repeat on the other calipers. You can even do this simultaneously on all four if you want. Encourage more air to escape by tapping the caliper with a wrench or equivalent, then close the bleeder screw. After performing this on all four brakes, test the firmness of the pedal.

Gravity bleeding, like the speed of an air bubble traveling through brake fluid, is the slow-motion version of bleeding using a power bleeder, vacuum pump or bleeding with two people. I'm not sure why it didn't work using those methods. If this doesn't work, I'd suspect the new MC is faulty as well.

With this slower process, you shouldn't have to use more than a quart of expensive brake fluid.

Are the calipers installed with the bleeder screw on top? Just checking.

Sherwood Lee
http://members.rennlist.org/911pcars

[snowman]

Yep, done all of those things. Its just got air somewhere that isn't being forced out, ie a high spot with an air bubble in it. Went thru 8 pints of fluid so far. thats the hard way with 2 people, a short tube connected to bleeder and open end in a jar. open end is submersed in fluid as well. Pedal is held down, bleeder screw opened and closed rapidly so no air can get back in. A short squirt of fluid always results, in fact all 8 pints have flowed thru the system. Pretty sure mc is not bad because of this. Pump the pedal and hold. Pedal stays up, never sinks.

[dad911]

How old are the rubber brake lines? I have heard that they can fail internally, but I have not experienced this. I change mine every few years on the track car.

[Chas White]

Originally posted by snowman
Pedal is held down, bleeder screw opened and closed rapidly so no air can get back in. A short squirt of fluid always results, [/B]

OK, my understanding has always been that you open the screw, then step on the pedal, then close the screw, then let the pedal up. If you're getting a "short squirt" of fluid, how can you tell when all the air is out of that particular caliper? Is there enough fluid in the tube long enough to see if there are bubbles using your method?
Regards,
C.W.

[snowman]

The clear tube is already full of fluid and the end immersed in fluid. Any air shows as a bubble, or bubbles. One problem I have found is that the fitting lets air in from around the outside threads, even with teflon tape on it. To preclude any possibility of air entering the system, a positive pressure ( pedal pressed first) method is employed. To further preclude any air coming back into the system the fitting is quickly closed, almost before the pedal is completely down. This usually works very well. Remember I have been changine the fluid in this car about 4 times a year for over 10 years without any problems. I also have 6 other cars, 2 more Porsches, 2 BMWs, a MB and chevy Denali Truck or whatever it is. I change the brake fluid in them without any trouble once a year.

This problem is beyond the normal. it is a real puzzle. The brake lines??? They are teflon, ss jacketed. Could this be?? I am just recalling a tale about a 93 Chevy Suburban, something about brakes pulling left, then right, turned out to be the teflon brake lines. I don't have the same symptoms but its certainly worth looking into. Its the only thing I haven't changed afterall.

[Vintage911Racer]

Does the cap on you Reservoir have a small pin hole in the top for air? It should, if it does not that could be your problem. I had a similar situation happen to me. Noticed fluid around the reservoir. Only place it could escapre from was the cap. Taped up the cap to cover the whole, went to bleed the brakes, no brakes. bleed the brakes again.... no brakes. cant figure this out. Tried about 5 times before I back tracked my steps of things I might have changed, and remembered Ihad taped the whole off the the cap. removed the tape. Everything was fine. Plenty of presure and brakes were back!

Why dont you race with VARA or do you?

[snowman]

Cap was off the entire time.

I do race with VARA, 66 Red 912 #1