Odd shifting behavior

If it wasn't completely bled you would have a different problem. Basically the system wouldn't create enough pressure to properly release the clutch so shifts might be hard but you would especially notice it when stopped going into first or reverse. Basically it would be hard to get it into gear.

It sounds like you've got something leaking, either the master or the slave. If you've already replaced the slave, which it sounds like you have then I would lean towards the master cylinder. Also, if you think about how the pressure is being exerted a slow, bleed off type of leak would be more likely in the master. The master generates and holds the pressure. A slave is having the pressure exerted on it so it's more likely to leak externally because there is no where for the fluid to go in the slave. If the master leaks it can just bleed past the seal and back into the reservoir. Any time I had a slave cylinder fail it just let go and died. Typically the seal blows out and fluid goes everywhere.

If it wasn't completely bled you would have a different problem.  Basically the system wouldn't create enough pressure to properly release the clutch so shifts might be hard but you would especially notice it when stopped going into first or reverse.  Basically it would be hard to get it into gear. 

It sounds like you've got something leaking, either the master or the slave.  If you've already replaced the slave, which it sounds like you have then I would lean towards the master cylinder.  Also, if you think about how the pressure is being exerted a slow, bleed off type of leak would be more likely in the master.  The master generates and holds the pressure.  A slave is having the pressure exerted on it so it's more likely to leak externally because there is no where for the fluid to go in the slave.  If the master leaks it can just bleed past the seal and back into the reservoir.  Any time I had a slave cylinder fail it just let go and died.  Typically the seal blows out and fluid goes everywhere.

I agree with spittinfire here. When I had to do a head gasket change 14 years ago I also changed out all the stuff that would be difficult or impossible to access every day--like all the (hidden) coolant hoses, the various engine sensors (coolant and knock sensors), and the clutch master cylinder.

Don't think you're screwed because it's no longer available as a new part. I would check salvage yards across the US (click HERE)--that part rarely goes bad (which is one of the reasons it's no longer available), and I only changed mine because it was then readily accessible with the cylinder head out of the car, not because it was failing. That link is for car-parts.com, a database accessing salvage yards across the US. I use this all the time.

Oh and one more thing--if you have the 1987 Ford shop manual, the information is incorrect. I don't recall exactly right now which it is--but the actual removal of the master cylinder is the opposite of what it says. If it says turn it clockwise to remove, it's counterclockwise. This cost me quite a few minutes. Also, you have to REALLY push in on the b*stard to get it to turn. Put a suitable socket on the end to help with that. There's a 6-point male "configuration" (bolt head) molded into the end which makes that much easier.

The 1988 Revision corrected those instructions.

I hope that helps.

Thank you both for the responses! They do make sense (unfortunately) as it relates to the master. I just went through similar "slow bleed off" symptoms with the brakes on my '75 MarkIV which ultimately pointed to the master. And FWIW spittinfire yes, when I was first directed toward the slave, it appeared to have blown out just like you experienced. The replacement seems better but performance still isn't at 100%. Anyway "What're the odds" that both cylinders would fail at about the same time tho?! Car is still a few ticks below 100K miles.

I did go ahead and obtain another slave just in case and to my surprise, this one is clearly more metal in construction whereas the other seemed plastic to me. I suppose it can't hurt to swap out again and hope for dumb luck.

anasazi4st thanks for the reminder about car-part. I've been looking everywhere for a new one with no luck; so if these are reliable enough and affordable then used maybe worth a shot. I used car-part to find a Tripminder last year (love that conversion!) and still tend to forget about them for some reason...

Was the car sitting for a long time? Hydraulics and seals can do some strange things after they've been sitting for a few years, especially if the fluid dries out. You could also pull the master cylinder apart and try to find seals for it. I don't think it would be an easy task to find those seals but there has to be some thing out there that would work.
If I had known a year or so ago what I know now about the master cylinder supply drying up I would have kept a couple of the bad ones that I threw away. I also would have bought the one that sat in my cart on Rock Auto for months. I just kept telling myself I'd spend the $40-50 on the next order.