Still having the pedal drop on my 1978.Ps Pb car. The light would come on once or twice on my way home. Seemed like the master cylinder. I replaced it, still had the problem, found a drop on  the front line so both front lines and both front hoses were replaced. Still the pedal would drop under light foot pressure. So I thought maybe it was a bad mc the first time and I replaced it again. Still dropped so I had both wheel cylinders replaced they needed to be done anyway. But its still doing it.

 Now heres the best description I can give.

 The pedal drops so slow under light steady pressure it takes 30-40 seconds to drop, and once it gets to below the gas pedal level it drops faster. When the engine is off it doesnt drop far enough to activate the switch/light. When it is running it will eventually. It still never goes completely to the floor.

 After it drops so far, the next time I press the brake pedal I can hear air rushing in or out somewhere around the pedal area.

 Anybody feel like offering any advice suggestions experience?

 It looks like the levels staying pretty much the same in the reservoirs. I dont see any on the ground on  the rotors etc. The lines and fittings are dry.

 I did read somewhere the booster can fail to seal off the brake lines but I never heard of that.

 The weird thing is when the engine is off the pedal wont drop far enough to make the light go on. It will when the engine is running though. ARgh mysterious brake issues I hate them.

another approach you could try is to gravity bleed the system.  when i've had stubborn brake systems, that's what i've done.  how you do it, is to remove the master cylinder cap, make sure the M/C is FULL of fluid.  start at the right rear wheel, the one furthest from the M/C...crack the bleeder screw and wait.  keep an eye on it and DEFINITELY pay attention to the M/C level.  it is critical to keep it filled.  once you see fluid coming out of the bleeder, close it off and move to the left rear.  fluid will most likely come out of it fairly fast.  then move to the front right...it might take a while, but when fluid finally starts coming out, close it off and move to the left front.  again, fluid will most likely come out relatively fast.  close that one off.  again, DO NOT let the M/C run low on fluid during this process.  once you're done, put the M/C cap back on and test your brakes out.  you should have absolutely NO air in the system. 

that's how i've always dealt with stubborn-to-bleed hydraulic systems...be it clutch/brake systems...good luck bud!