could be a blockage in the master cylinder not allowing fluid to return freely to the reservoir,or more likely either a brake hose deteriorating inside them and blocking fluid return.Since you say you have rebuilt the calipers, take it you did flush and fill the entire brake system with clean fresh fluid,if not,do it there may be moisture in the fluid.
These systems are notoriously hard to bleed,try pushing fluid back from the bottom with a syringe and watch for bubbles to come back up into the MC.
That should be enough to help you pinpoint the problem,one more thing make sure the calipers themselves can move freely and are not binding,good luck
I replaced all fluid with new brake fluid, system-wide. Only thing I did not do is disassemble and clean the m/c.
I do see bubbles coming back up into the m/c during the bleeding process but it was taking a LONG time to bleed them -- too long from my past experience with other bikes. I got a suspicion right there when I did not see a huge volume of bubbles coming back up into the m/c -- on other bikes after having the system wide open to re-do the calipers, there's usually a flood of bubbles coming back up into the m/c. On this one, just a few tiny bubbles every once in a while during the bleed process.
It was taking so long to bleed, I used a vacuum-speed-bleed attached to the bleed nipple on each caliper to hasten the bleeding process, to suck the brake fluid out of the m/c downward to the calipers. My vacuum speed-bleeding will go up to 30psi of suction. I do not think that air is the problem.
I am thinking it is your supposition of the blockage -- "blockage in the master cylinder not allowing fluid to return freely to the reservoir" -- so tomorrow I will yank and disassemble/clean the m/c with the intent on unplugging that blockage.
As to 'tightness' -- the tightness is consistent 360 degrees around the rotor and the pads have plenty of material on them. When I had the calipers apart, the pucks move quite freely in-and-out of the caliper bores so there is no binding there.
And when I installed the rubber o-rings in the caliper bores, and lubed the bore and orings with fresh Dot-3 brake fluid and went to install the pucks back in there -- I was able to do it with my thumbs and it was so dang easy compared to other calipers on other bikes I've done -- that experience was one of the grin-inducing part of my day. The pistons just got right back inside there with just the right amount of snugness. They're not binding.
The calipers are not releasing such that I have to pull hard with both hands to rotate the front wheel.
Normally, when you squeeze the front brake lever and brake fluid is pushed out of the m/c down to the caliper to push the puck out to the rotor -- when you then release the front brake lever, the big O-ring inside the caliper exerts a slight pull to pull the puck back away from the rotor -- and in so doing some brake fluid is pushed back out of the caliper bore back upward to the master cylinder.
In these disc brake systems, the big o-ring inside the caliper's bore will *not* normally pull the puck 100% clear of the rotor but will normally get it off the rotor enough so you can freely turn the wheel. Normally that big O-ring pulls the puck back into the caliper enough so that the brake pad will barely touch the rotor.
Since the big O-ring is fine, and the caliper bore is fine, and the puck is fine -- something is stopping the fluid from getting out of the caliper bore and returning back up into the m/c.
I cannot submerge the m/c in a bucket of cleaner and that tiny tiny return hole in the m/c is not accessible very easily -- what cleaning method will do me right on this one?
Obviously I will use my 120psi air gun. And I will have the m/c's piston and stuff removed from the m/c's body.
But what cleaning agent will open this apparently blocked tiny hole that I cannot get at directly with, say, a bobby pin to unblock it?