When the peddle goes to the floor slowly its a sign that the one way valving in the master is not working.
When you take your foot of the brakes and the master cyl retracts, it is designed so that fluid can pass through the piston to keep the system full. If the cup/reed valve fails, or gets crud in it, it doesnt form a perfect seal and so your foot goes to the floor. In an emergency you may find that when you hit the brakes hard and fast the peddle stays up more but eventually slips to the floor.

As far as how to fix it, its an economic choice.
If the safety of you and you family is only only worth $2 to you, then get a second hand one from the junk yard for $2.
If you have no experience with rebuilding master cyl's and like Russian roulette, get a kit & try rebuilding it. Chances are the bore of the cyl will be scored & need honing as well.
If you value your families safety, then get a new replacement one. I am sure Phil will help you. For the cost of a feed at McDonalds, you get peace of mind as well. they are not expensive!
Its a no brainer really!!
Take snookwhaler's advice on bleeding. I find it best to put 2 foot of clear plastic tube on the bleed nipple so that you can see any bubbles. The fluid that is bled out should be thrown out, not kept for a rainy day!!

__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

Behind on my reading I guess...I had an identical problem and it WAS the master cylinder..course I waited an hour too long - i had the dam* new one on the seat next to me when the master went into a failsafe mode which meant the front calipers started dragging and burned up the pads...so those got replaced too....

Fortunately the MC isn't terribly expensive, make sure you bleed, bleed bleed...