Hello, I recently bought a 93 907ie. I have been going through making sure it is up to par. So far, all is well. New belts, and valve clearances are at spec, but I seem to be having trouble with the back brakes. On a few occasions I have lost all back brake pressure. So, I checked the fluid level in the rear resivoir, and it was fine, so just to be sure, I replaced th brake fluid not knowing the last time it was replaced. I bleed the brakes thoroughly until there was no bubbles and then some. It worked fine for four days, then stangely enough, on the same corner, which is a tight left on a steep downhill, I lost brake pressure again. I pumped it up about ten times, and it works fine again, but just to be sure again, I bleed them again and made sure the resivoir was topped off. This is all I can think to do. Any other suggestions. Oh and the brake lines are fresh as well. No cracking or anything of that sort. Any input would be greatly appreciated.

I have had dramas with my rear brake too - see the separate post on how I nearly killed myself...

on the rear brake the brake line loops up from the master cylinder than back down to the swing arm clip before reaching the rear caliper...
I believe air can collect at the top of the loop in the line...
you can remove the rear caliper and lift it high to bleed the air up

Im not happy with the brake yet - even after several bleeds it seems spongey.
I have even tried some fandangled brake bleed toys...
I'll be interested to see where this post goes.

check the lines by pushing the pedal HARD...they won't blow unless they were ready to blow...I discovered my hose had burst but you need to watch the hose under pressure to see the bubble.

above all - make sure you have free play between the pedal and the master cylinder

Thankyou a lot. I do believe that bleeding it with the caliper higher up than the resivoir allowed the bubbles to float to the bleeder, instead of just staying in one spot in the hose while the fluid was just going by. Thanks again

one of the things I want to do is to replace the braking lines as they are pretty aged by now.
So far, I faced no sponginess or what so ever.

the free play of the master cilinder piston is very important as Laddy wrote, to feed in additional oil as the brake pads are moving and to compensate wear of the pads.

I use to dismount the systems to allow airbubbles to rais to an bleedable point.
hanging up a mastercylinder by the lever, for instance results in that the reservoir is the highest point. sqeeze the lever several times and wait overnight will do the job.

Ducbertus

the free play at the master cylinder also allows any developed line pressure to be relieved.

when brakes heat up the fluid expands a bit but any trapped air will expand a lot - the line pressure will relieve back thru the master cylinder if the free play is available when the brake level is not depressed.
If there is no free play it will build pressure and continue to apply pressure in the caliper - so the brake drags and gets hotter and hotter...

I could have fried a steak on the disc...