Brake Problem from He77.
 

1977 911 Targa

Background:

1. After sitting idle for a year, brake lines leaked and soft lines were replaced with new Braided Lines.

2. Rebuilt Calipers

Could not get a firm pedal after bleed regardless of effort and number of times. Used Vacuum bleeder, Power Bleeder and Two person method of pump and bleed.

Then:

3. Replaced Master with New Master.

Could then get a firm pedal halfway down. BUT when the car was turned on and the Booster kicked in, pedal went to floor. When car was then turned off, the pedal lost all firmness and went to floor again -- as if it was not bled just prior.

Had a local racing team mechanic recommend opening the lines at the Master only slightly under pressure from the Power bleeder. Did get a couple of bubbles and again got a stiff pedal about halfway to floor.

BUT again, turn car on, Booster kicks in, lose the pedal!

I have gone through this procedure of bleeding and regaining pedal, to only lose it when Booster is on too many times to count.

I have bled the lines at the Calipers each time and do not see any air coming out.

I do not see any fluid leaks and fluid levels are solid.

I am starting to believe that something is wrong with the new Master but am stumped as to what could be wrong. If it was not the master, I believe I would see bubbles at the calipers.

Had another racing team "brake expert" suggest that the diaphragm on the booster is possibly torn, which would explain why it would be sucked to the floor when it kicks in, but does not explain why the pedal does not recover when Booster is off.

Anyone have an experience like this or thoughts in general on the problem?:confused:

Hi,

I went through the same maddening thing on my 77 Targa this spring. I'm not sure how to make a link, but search for this exact title and there are a lot of good suggestions: " I Can't Get My Brakes to Work"

Good luck, you'll get it.

-Rutager

Same thing on my 88. Only thing that worked was the two man method round and round many times till it was firm. There are always a few bubbles. Seemed to help if some time elapsed between bleeds like a few days. Makes you think its the master but it isn't. Patience is a virtue in this case. Don't know why the brakes on these cars are so difficult to bleed but once done they are great. Just a thought, bleed sequence is right rear, left rear, right front, left front.

Appreciate the replies.

Don't think it is just a matter of re-bleeding as this has gone on too long and the symptoms don't coincide with trapped air. And especially as it will build pressure after bleeding and then totally lose it after the Booster is on. If it had a few spurious air bubbles trapped, then it would feel the same before the Booster was on and then again after the Booster is off. Booster shouldn't drag a stiff pedal to the floor.

The Master is the prime suspect and the suggestion of capping it off to see if it holds pressure sounds like a good idea to either eliminate or conclude it is the culprit.

I think my MC was the last hold out and took forever to get the air out of it. One thing I did that I thought helped was to hook up my Motive bleeder to it and with the bleeder pumped up and filled with fluid, I just kept gently pumping the pedal and I could see bubbles going up the bleeder hose while I was doing it.

The whole ordeal was crazy because I figured it was going to be a simple process to get my car back on the road after my suspension work was done, and it turned it to several weeks of on and off messing around- I feel your pain.

-Rutager

rwest - did you have the same symptom where you bled it, got a stiff pedal half way down, started the car, pushed the brakes, turned off the car, and pedal was no longer stiff at any point and needed to be pumped up to gain any pressure?

It sounds similar; I would get a good pedal, start the car and it would go soft and I could pump it up, but It would go soft again.

I have gone through the same issue except I have replaced my flexible lines twice, MC twice, rebuilt my calipers twice, and replaced the discs and drums but to no avail. I have done dozens of brake jobs on other cars. My 911SC's brakes are the worst of any car I have ever owned....in terms of performance and ease of repair. I'm thinking of replacing the booster and hard lines because there can't be anything left. If that doesnt work, its going to John Walker 8 hours away in Seattle.

I feel your pain.

I have had some very serious mechanics chime in on this issue who came away scratching their heads.

Was your second replacement MC the same brand as the first?

I may try one more bleed at the MC with some lite tapping with the rubber hammer along the way.

I had the same problem and I have bled countless brakes, but never came across a more difficult and time consuming bleeding job.

I believe it may be caused by fresh caliper piston seals pulling the pistons away from the disk. You push the pedal and piston moves but flexes back when you release pedal.

For me in the end I took the car around the block at a very slow pace and had a firm pedal within half a mile. I believe the rattling helped the pistons move and the new seals slide in the bores until pistons came closer to the disks.

Try it

Trond- Thx for the suggestion but have driven the car a number of times around the neighborhood. I probably have put 15 miles in laps around the neighborhood.

I have also bled lots of brakes without ever coming across this. The pistons settling in may be an issue but normally you would see either no change or some improvement as time went on. In my case, the routine is: bleed, 50% point firm pedal, start car/booster, pedal goes soft, turn off car, pedal remains completely soft.

Again, thanks for the input and any other thoughts are appreciated.

Another Data Point:

Bled brakes again tonight. Stiff Pedal until car is started.

Re-bled again. Disconnected the vacuum line on the booster and as expected, brakes were solid.

Something is happening when the booster (vacuum) engages which drags the pedal to the floor.