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Hydraulic pressure doesn't take time to build like air pressure does. If there is any delay in building pressure I would think it would be in building max pressure if your brake lines expanded, hence the braided lines to prevent that. Please correct me if I'm wrong Wrider.

Nope, you're 100% correct on that Doc.

Wrider? you changed your story,,, NOW you say back bleeding is an effective way of bleeding brakes,,, and you backed off of saying the perpose of the check valve is to bleed off return pressure,,,,
the rubber hose brake lines expand with pressure, much of that pressure is wasted by expanding the rubber lines,,,,, when you hit the brake lever the master cyl builds up the pressure first and it takes time for that pressure to transfer to the brake caliper,,,, but when the brake lines have a "slight" amount of pressure on them all the time the rubber hose brake line is already expanded and it takes a much shorter time for that pressure to get to the caliper from the master cyl,,,,, but its a check valve that is designed with a spring loaded ball and stop and the spring is designed to hold only a s-l-i-g-h-t amount of pressure less than 45 psi,,,, and 45psi isn't enough pressure to drag the brakes,,,, I had one in my hands once,,, its a replaceable part on many Honda motorcycles and if you would look hard enough you can find one on their parts list

There's no "Edited by" part of that post. I never changed my story, you are unable to read.
What you're saying is that it holds fluid in the lines. Yes it does that, as does gravity. Go out to your poor neglected vehicles, hold onto the front brake lines, and squeeze the brake lever at the same time, you'll feel them expand as you do. The check ball has nothing to do with "preloading" the lines. The lines don't expand until they have significant pressure on them, which comes from the cups being unable to move (the pads are pressed against the rotors).
45 PSI??? That's enough to give you some decent stopping power right there.

It takes between 600 to 800 psi of brake fluid pressure to act on the caliper,,, 45 psi is minor,,, like I said before and repeat again the perpose is to keep pressure on the lines for a much quicker reaction of the brakes such as is required on high speed racing bikes such as crotch rockets,,, not all bikes have this check valve.
I hope you already know this brakes on a bike that can go 160 mph needs better brakes and a MUCH quicker reaction to stop,,, so charging the lines with a little bit of brake fluid pressure all the time will shorten the reaction time and make the bike stop quicker.
tell me?? cruiser bikes without this check valve,,, can you feel the delay when you hit the brake lever till you feel the brakes grabbing? I can,,, but never feel that brake delay on a high speed crotch rocket, or on my scooter that has the exact same brake system as a R6.
The check valve is a saftey device so the people that built the bike doesn't get sued, so in court they can say the brakes have an added saftey feature such as a check valve.

I know what you're thinking,,, you are thinking how a check valve works in an oil filter,,,, nooo it doesn't work like that

Yet more misinformation.
Sigh...
If 600-800 PSI was needed you'd never be able to get the cups out with an average air compressor, and definitely not by hand.
Find me a hydraulic disc brake system without a check valve in it. I dare you.
Completely different valves. We're talking check valves, that's a bypass valve.
Oh, and PN for 2009 R6 L front brake caliper is 13S-2580T-10-00
PN for 2009 TMax L front brake caliper is 4B5-2580T-00-00
Not the same.

Well, if your master cylinder had a surface area of 2sq inches, and you applied 400 pounds of force to the cylinder, you would create 200psi. Run that psi down your skinny tubes and act on a caliper of say 4sq inches, and you get a force of I believe, 800 pounds, but the psi is still only 200. The psi never changes if the force applied never changes, and the psi is the same throughout the WHOLE system.

So, what's the most powerful part on your bike, oops, I mean scooter...???

Lol well done with the math, but keep in mind that when you're rebuilding a caliper, the easiest way to get the cups out is to remove the pads, put a rag where they were, and use compressed air to blow them out. Your typical compressor has approximately 125 PSI in it, and if you don't have the rag in there, the cups can (and will) go flying. I've seen them go flying across a shop floor before when someone forgot a rag. Dented and scarred them up to the point they needed to be replaced.

it seems like the problem is worse now that i don't use the gixxer very much.

im gonna trust my mechanic. he said, it'll be better to change the master cylinder 1st before rebuilding the caliper.

he already changed mine into a braided brake line before.

also, he recommends the master cylinder from a ZX14 2008 and up IIRC. he says it should dramatically improve the braking as well with that master cylinder.

before i do this though, he recommends another 2 day, then 2 weeks to check if it will get air again. if yes, then i'll go ahead w/ his plan.

hopefully this fixes the problem. he says he had very good success from this method.