My 1992 940 Turbo brakes seem to stop just fine, but when I sit at a stoplight, the brake pedal slowly goes down ... I have no fluid leaks, the fluid reservoir is full, it seems to happen when the engine is running and in gear ?

 

That is quite normal with most cars i have driven with power assisted brakes.
Usually can be felt at the pedal 10-15 seconds after one has stopped and maintained a consistent pressure on the pedal with ones foot. (i.e. at traffic lights)
It is always well worth however checking ones master cylinder reservoir to ensure the fluid levels within the correct range and to confirm no fluid loss by leaks in the hydraulics. I note that you have already done that anyway.

 

This just started being a problem in the last month, the pedal goes very close to the floor when it fades, starts out at normal height ...

 

quite possible you have an internal fluid leak inside the master cylinder or you have a leak in your brake booster or vacuum line/valve. Do you hear any hissing when pressing the brakes?

 

If I had any king of fluid leak it would cause the fluid level to drop, but it's not dropping ! Also no hissing...

 

Not necessarily, It can be an internal leak, as was mentioned above. If your master cylinder seals are leaking, the piston might be bypassing slowly.

 

But even with an internal leak, the fluid has to go somewhere ? The level is NOT dropping !

 

The pedal should certainly not go down close to or near the floor for sure.
You may have a diaphragm problem in the brake booster.
Usually though a hissing sound occurs with that but it may not always be the case.

 

Think of it like a french press coffee maker. The fluid just goes from one side of the plunger to the other, instead of being forced to the bottom and through a line to the brake.