'82 Plymouth Gran Fury cop car, stock brakes - when I come to a stop the pedal slowly sinks to the floor. Not losing any fluid and no leaks that I can see anywhere; system has been thoroughly bled. Is this a failing master cylinder or perhaps something with the power boost unit?

Normally that's a sign of a master cylinder bleeding by. Time to get a new one or rebuild the one you have.

Totally agree, most likely MC internal leak between seals.

as has been stated, it's time for a master rebuild or replacement.

ASAP before it goes right to the floor in traffic.

Thanks for the replies guys - confirms what I suspected.

As rebuilt units go, any difference between the cast iron and aluminum versions? I currently have a rebuilt cast iron MC that I installed around 2002.

I'm kind of torn on this one - NAPA has brand new MC's (Aluminum), but I'm sure they're made in China; all of the rebuilt ones are cast iron - better to go with a rebuilt American piece or new offshore piece?

I would go with the alum. MC's are a pretty trouble free item