Well, you're definitely a step or two ahead of me!  Glad you've got a hard pedal now after all that effort.  Hopefully I'll be there soon, but I took a step backward tonight.  Removed the M/C and the proportioning valve.  I'm going to re-bench-bleed my M/C and also make sure the lines and calipers are full of fluid before I go on.  

Dave be very careful on the material of the O rings. Just make sure it's compatable with American brake fluid. Years ago I had an MG and used American brake fluid. The British used natural rubber in the MC, which turn into gum. Not long after the fluid addition.............I was without brakes. It takes a lot of white knuckle driving to make it home with no brakes. I then rebuilt my MC. It's not very hard to do. The only special tool needed is a hone to reface the cylinder. 
Buna N does work with Dot 4 fluid. Just make sure they are not real rubber, and also make sure to use the proper ID OD and thickness of the rings 

 Don't know if this could be your problem or not but I replaced my leaking master cylinder with a rebuilt O'reilly's a couple of weeks ago. Could never get a firm pedal after bleeding 3 times. It would almost go to the floor, sorta like stepping on a ripe plum (courtesy of C W McCall. lol). Drove the vette over to close to Dallas for a bunch of major work. Almost there, traffic stopped due to an accident. I hit my brakes...and guess what! NOTHING..TO THE FlOOR! Luckily there was a median strip between the concrete wall an the two lanes of stopped traffic in front of me. Long story short... the master cylinder was CRAP! Might want to consider that as possibly your problem

  Dang, Dwain! I guess you took it to Rich's place?

Another wild idear I have on it....take the bleeders out of the front calipers completely, and see if the holes for the fluid where the bleeders thread in are stopped up. I have definitely seen this in calipers, both rebuilt, and old ones on the car. Crud builds up there over time, and sometimes, it just won't allow fluid to pass thru the bleeders. I'm not talking about the bleeders themselves, but the holes in the calipers. But...I have also seen bleeders that were crudded up enough to not allow flow.
Just because the calipers are rebuilt doesn't mean they were cleaned really well before.
It's worth a look-see, anyway.

It's interesting to see the research tangents I get to go on for this car.  Tonight I got to research o-ring material types that are compatible with DOT 4 brake fluid, thanks to Corvette440!  I came up with a little bit different info, though: most sources said to stay away from Buna N (also known as nitrile) and use something like EPDM, which I have no clue where I'd get a "variety pack of" those.  I'm using Valvoline synthetic DOT 3&4 and interestingly, they say that fluid is actually compatible with natural rubber!  (http://www.valvoline.com/pdf/valvoline_brake_fluid.pdf)  I think most of the o-rings you get at the parts store or hardware store, though, are just regular old nitrile.  The existing o-rings look like they're still OK so if I can't find any EPDM I may just put it back together with the originals (after I blow out the passages)

EPDM rubber (ethylene propylene diene monomer (M-class) rubber),^[1][2] a type of synthetic rubber, is an elastomer which is characterized by a wide range of applications. The E refers to ethylene, P to propylene, D to diene and M refers to its classification in ASTM standard D-1418. The M class includes rubbers having a saturated chain of the polymethylene type. Dienes currently used in the manufacture of EPDM rubbers are dicyclopentadiene (DCPD), ethylidene norbornene (ENB), and vinyl norbornene (VNB). EPDM rubber is closely related to ethylene propylene rubber (ethylene propylene rubber is a copolymer of ethylene and propylene whereas EPDM rubber is a terpolymer of ethylene, propylene and a diene-component).

The ethylene content is around 45% to 75%. The higher the ethylene content the higher the loading possibilities of the polymer, better mixing and extrusion. Peroxide curing these polymers give a higher crosslink density compared with their amorphous counterpart. The amorphous polymer are also excellent in processing. This is very much influenced by their molecular structure. The dienes, typically comprising from 2.5% up to 12% by weight of the composition, serve as crosslinks when curing with sulphur and resin, with peroxide cures the diene (or third monomer) functions as a coagent, which provide resistance to unwanted tackiness, creep or flow during end use.

EPDM rubber is used in seals, glass-run channels, radiators, garden and appliance hose, tubing, washers, belts, electrical insulation, vibrators, and speaker cone surrounds. It is also used as a medium for water resistance in electrical cable-jointing, roofing membranes, geomembranes, rubber mechanical goods, plastic impact modification, thermoplastic, vulcanizates, and many other applications.^[3][4]

EPDM exhibits satisfactory compatibility with fireproof hydraulic fluids, ketones, hot and cold water, and alkalis, and unsatisfactory compatibility with most oils, gasoline, kerosene, aromatic and aliphatic hydrocarbons, halogenated solvents and concentrated acids.

The main properties of EPDM are its outstanding heat, ozone and weather resistance. The resistance to polar substances and steam are also good. It has excellen

Typical properties of EPDM vulcanizates are given below. EPDM can be compounded to meet specific properties to a limit depending first on the EPDM polymers available, then the processing and curing method(s) employed. EPDMs are available in a range of molecular weights (indicated in terms of Mooney viscosity ML(1+4) at 125 °C), varying levels of ethylene, third monomer and oil content.

it has electrical insulating properties.

Typical properties of EPDM vulcanizates are given below. EPDM can be compounded to meet specific properties to a limit depending first on the EPDM polymers available, then the processing and curing method(s) employed. EPDMs are available in a range of molecular weights (indicated in terms of Mooney viscosity ML(1+4) at 125 °C), varying levels of ethylene, third monomer and oil content.( This was copied from wikipedia)

I was also wondering if Vitron (green) O rings would work. Vitron is sold at auto parts stores. They have variety packs. These O rings are used for Aircondioning seals that use sythetic oil (POE) for R134A. I purchased a packet at Advance

Fingers and toes crossed for you Dave. If it don't work go for the pressurized MC setup..........I think that's plan C.

Thanks, Larry.  The digit crossing didn't work out for me - still have a soft pedal.  BUT, I think I'm making headway.  I found a serious problem on the fronts: I did not install the hoses with the copper washer at the caliper.  With my vacuum pump I think air was being sucked in through that interface.  There is still another problem, though.  I'm pretty sure my prop valve is off-center again.  When I was working on them earlier today I did the rears first.  I actually had some pedal after I was done with those.  But then I moved on to the front and with the lack of pressure in that circuit I think the valve got off center.  I can tell by the fact that I have a full M/C, lid is loose, and no fluid is even dripping out of the disconnected hard line where it would attach to the front left hose.  So anyway, tomorrow I will buy some copper washers, put everything back together and try again!   I'm hoping I can get the valve re-centered by opening the back bleeders and slowly pushing the pedal.  We'll see.  Still realize that guy might need to be replaced.