Spongy brake syndrom - 1971 911T

[morini]

My 1971 911T has had spongy brakes since I bought it a couple of years ago. I think it has gotten worse over winter, but it is hard to say since I have not driven it since November. Snowy and salty roads here in Norway. The last thing I did before putting it into hibernation was bleeding the brakes and overhauling two of the calipers. After that the pedal travel was like before; very long. It feels longer and spongyer now, but I am comparing to my ´77 911S, which has very good brakes. These are the symptoms:
- Very long pedal travel
- Spongy feeling pedal
- Brakes OK if I give it enough foot, but far from optimal
- Pumping the brakes does not help, so I doubt that it is air in the system
- When blocking all four calipers (on the brake lines close to the calipers) the pedal is short and hard as it should be
- When unblocking on caliper at a time the pedal travel increases with the same amount for each caliper
- No brake fluid is visibly leaking anywhere

The symptoms indicates that there is nothing wrong with the hydraulic system and that the calipers are the culprit. (Unless the brake pads are made of chewing gum and are compressable - I guess not).

Theory: Brake discs and calipers look original, but could it be that these are upgraded to larger items while the master cylinder is kept original? I know from motorcycles that having a too small or too big master cylinder compared to the caliper can alter the brakes tremendously. The original master cylinder is 19mm. If this theory sounds realistic - would it help to change to the 23mm master cylinder 91135501202A or 91135501202?

Or could it be something else I am missing? I will try to bleed the brakes again, but my hopes are not too high…..

[Trackrash]

The two things that caused spongy brakes on my car were poor bleeding and soft rubber hoses.

I found that getting the brakes fully warmed up then re-bleeding helped get out that last bit of air.

After I put new rubber lines on my car I never had a firm pedal. I finally changed to stainless braided brake lines and now have a much firmer pedal.

[Hi_Fi_Guy]

You may also want to check the pedal assembly and brake push-rod adjustment. I did not believe it would make much difference on my 1971T but it did.

[porsche930dude]

Yes the pads can be spongy. Thats exactly what happend to mine. Happend on my jeep aswell. I suspect they got soaked in brake fluid or oil on them at some point. Also crimping the rubber lines is bad for them you should not do that. If you dont know how old the hoses are its probably a good idea to replace them aswell

[morini]

Thank you for your replys. Yes - new lines are ordered. Crimping them is not a good idea if they are to be reused. Unfortunately stainless steel brake hoses are difficult to get approved in Norway, since a leak is hard to see.

I know that the pedal assembly has a little play, but not much. I guess I will fit the new brake lines, new pads and bleed the system. I had never heard about spongy pads before my mechanic mentioned it, so good to hear that you have had the same experience.

If that does not solve it the next step is a bigger master cylinder.

[NYNick]

Bleed bleed bleed.

It took me at least 5 tries and different methods to get my 72's brakes to work properly.

Once (or twice) it was the overflow hose leaking fluid under pressure. I ended up replacing it with a simple clear rubber hose from a home improvement store. The new hose was a lot more pliable and easy to crimp.

I used a Motive bleeder a few times. The final time I pressurized the Motive to 20 PSI and let it sit for a few minutes to see if it held. Then, I would pump the Motive a few times to make sure it was at 20 PSI after bleeding each caliper AND I would also pump the brakes 10 times before bleeding the next one.

That process finally worked. Good luck.