strange brake problem on SC

[Peter Roberts]

I finished the restoration several weeks ago and am now having the enjoyment i deserve after all the hard work. However i have a strange problem with the brake pedal.
During restoration i had the calipers professionally overhauled and fitted a new master cylinder. All brake pipes were replaced except the one that runs through the tunnel.
From the outset i had to bleed the brakes several times before getting a good pedal of about 20 mm movement. As the whole system had been drained i did not think this unusual. Even then the disc brakes were not brilliant in stopping the car even though it well exceeded the MOT requirements. Within a few weeks the pedal movement increased to more like 50 mm with no drop in performance. Pumping the brake pedal did not noticeably reduce the travel.
I checked for leaks, rechecked the rubber bush sealing the pipe from the fluid reservoir and found no problem. Also checked i had free play between the push rad and the master cylinder. Also it is not losing any fluid.. I bled the brakes again and it brought the pedal back up suggesting air in the system. . Just taken it out after a 2 week gap and hey presto the brake has gone back to 50mm movement even though its not been used since the last bleed.
Before i bleed them again any suggestions as to what may be the problem.

[neilbardsley]

Interested to hear what others have to say as you seem to have replaced everything. That fluid are you using and was it complete changed went the new master cylinder was replaced? Very strange that the travel changes while sitting?

[210bhp]

Pinhole in the system somewhere sucking in a tiny amount of air with use ?

Regards
Mike

[Peter Roberts]

Yes the system was completely drained and filled with DOT4 fluid.
The only place that I may be sucking in air is the supply pipe from the master cylinder but I have checked this and it has a new grommet. If I had a pin hole elsewhere surely I would be losing fluid under pressure not sucking in air.

[210bhp]

Depends where it is but you seem to have checked the obvious. The master cylinder would be my next port of call. Did you fit it straight out of the box?


Regards
Mike

[Ashley James]

If it doesn’t get worse I’d drive it for a bit until it’s settled and then carefully bleed the system again. Often if you have trouble bleeding a system, the fluid aerates and you get symptoms as described. Obviously make sure there are no leaks and see how far the pads move back.

The brakes on old Porsches are heavy, it was the German way so you didn’t loock the wheels too easily.

[Peter Roberts]

I am using a Sealey pressurised unit to bleed the system so not constantly pumping the brake pedal to cause airation.The master cylinder was new out of the box but would hate to think such an item is faulty.
I will be bleeding the brakes again this weekend so will keep you posted.

[POR68]

Sounds odd, but MCs can be sitting on a shelf for a while and the metal surface corrodes or the rubber components degrade. How much play is there between your (I assume) new pads and the rotors? Are the pads able to move freely? If the initial gap is too wide that can contribute to the problem. Are the flexible hoses also renewed? As for bleeding, I also have a pressure bleeder but never relay on that to do the job perfectly. I always finish off with an 'assistant' in the car doing the pumping trick while I work on each bleed screw in the correct sequence.

[Peter Roberts]

All brake components are new including pads and flexipipes. If I bleed the brakes pedal comes good again but then deteriorates over a few weeks so sticking pads does not seem the culprit

[POR68]

That's good Peter. I am suspecting that the inner components of the MC maybe an issue then. The gap between pad and piston at the calliper should not be too great, however. I bought a sizer tool to set that on my early 911 with similar brakes after many re-bleedings with the same symptoms as yours (falling pedal). When you re-bleed do you get air? Small bubbles can be retained in the top of the callipers and I did some tapping with a rubber mallet on the calliper to get those out when bleeding.

[210bhp]

Are your calipers the right way up as regards the bleed nipples?
I once bought a brand new master cylinder (not for a Porsche) many years ago and fitted it never thinking it could be faulty but it was. Caused many hours of frustration bleeding brakes until I took it apart and found a nicked rubber (hence my comment above).
It really does sound like air in the system somewhere. Keep at it, you will get there.


Regards
Mike

[Peter Roberts]

The calipers have been on the car since i had it. They were professionally rebuilt and replated as part of the resto by a company that advertises in Porsche Post. I do not think they can be fitted upside down but in any event the bleed nipples are positioned at the top not the bottom. A good friend advised me that the first place to bleed is at the master cylinder by pressurizing the system and backing off the pipe connectors on the master cylinder. He had a point as i was having trouble getting a good pedal on my old 911 after fitting a new master cylinder and this cured the problem. I will be getting it back up on the lift on Monday and give it a bleed. i will be looking at all options and if there is air in the system and will report back.

[Peter Roberts]

Think i have solved the problem. The issue was something i would never have thought of and probably few owners would even think of.
Bled the brakes again but no air came out and still had extra travel the next day.
A person i now refer to as my "356 Guru" advised me of the problem. He stated that the seals in the calipers have a pull back memory profile. This is particularly prevalent when car is not used much or when seals are new. Eight big pistons pulling back slightly make a big difference to the small master cylinder travel. The solution is the use the car more to get the pistons moving or jam the pedal down with a bit of wood if the car is not going to be used for a while. Jammed the brake pedal down for a week and the problem seems to have disappeared as i have been on a good run with no dropping of the pedal.
This solution sounds a bit off the wall and I would never have come up with this but my Guru has forgotten more about these cars than i know so I will always take heed of his advice.
I have invested £15.00 in a carpenters clamp/ spreader which i will use to jam down the pedal when not in use untill i get some movement and wear in the seals.

[210bhp]

Good to know. Thanks for passing on the tip. Every day is a school day.


Regards
Mike

[neilbardsley]

Think i have solved the problem. The issue was something i would never have thought of and probably few owners would even think of.
Bled the brakes again but no air came out and still had extra travel the next day.
A person i now refer to as my "356 Guru" advised me of the problem. He stated that the seals in the calipers have a pull back memory profile. This is particularly prevalent when car is not used much or when seals are new. Eight big pistons pulling back slightly make a big difference to the small master cylinder travel. The solution is the use the car more to get the pistons moving or jam the pedal down with a bit of wood if the car is not going to be used for a while. Jammed the brake pedal down for a week and the problem seems to have disappeared as i have been on a good run with no dropping of the pedal.
This solution sounds a bit off the wall and I would never have come up with this but my Guru has forgotten more about these cars than i know so I will always take heed of his advice.
I have invested £15.00 in a carpenters clamp/ spreader which i will use to jam down the pedal when not in use untill i get some movement and wear in the seals.

I think Ashley recommend this to me once!