Diagnose bad master cyl?

[cstreit]

Gang,

I think this is classig bad master cylinder, but wanted your opinion.

1970 911S with 3.0L. Pedal is firm most of the time, but occasionally will drop. 1) If I hold solid pressure on the pedal for 30 seconds, then press harder, it softens. 2) Randomly goes soft, especially after hitting runble-strips at track.

No air in system... (really)

Opinions appreciated.

Chris

[john walker's workshop]

it does sound like a bad master cylinder. usually you can feel the pedal go down slowly as you hold firm pressure.

[cstreit]

Actually it's not so much a slow reduction... More like if I press with x pounds of force for 30 seconds it goes so far. Then if I add a little more pressure it will go way down... DOes this make any sense?

[Zendalar]

My cars pedal does the same, thinking of airing the brakes today and checking if that helps, but I am supposing the trouble lies in the master cylinder.

//Zen

[Jens Wendorff]

One obviouis sign of a failing master cylinder is when you are loosing brake fluid.
remove the cardboard at the pedal assembly and check for any moisture which should not be there.
Of course, this has nothing to do with an internal fault. Just ONE possibility.

Jens