Refilling front brakes

Hi guys. Been a while! I finally gave in, re-ringed the pistons, and put her back together, and she started perfectly first time. So all was good. Just finishing getting her ready for the MOT, and went to refill the front brakes, but im having all sorts of problems.

The back brakes is fine, refilled and bled that no problem, but the front one isnt having any of it.

I figured the problem was the piston seals in the master cylinder, so i bought one of the rebuild kits, but it doesnt appear to have made any difference.

There are slight wear marks on the inside of the bore, but they are only very slight, so for the minute i am discounting that as being the problem.

The jist of the issue:

I fill the master cylinder with fluid, connect it all, open the bleed valve on one of the calipers, pump the lever (There is no resistance to pumping), and some liquid/bubbles come out of the caliper nipple.

I then close the nipple, and release the lever. I then pump the lever a few more times, and re-open the nipple, but this time the bubbles in the hose stay exactly where they were. If i wait for ~10 minutes, and pump, then a few more bubbles come out, but then rinse and repeat the issue.

I removed the master cylinder, and tried filling the cylinder again, but if i pump the piston over the sink, very little if any fluid comes out of the bottom, and certainly the level in the reservoir doesnt go down at all.

If i fill the cylinder, and the hose with liquid, and put my finger over the end, you can feel pressure build up when you press the piston, and if you release the pressure, and then release the piston, you can feel a slight vacuum built up in the hose.

What am i doing wrong here?

It is an '85 K100RT, non-ABS.

ToGGoT

[Edit] - Just to add, i have tried pulling fluid through from the caliper with a big syringe, and that doesnt really seem to work. Have also tried priming the system with fluid from the bottom with the syringe, and that only works if the master cylinder is disconnected at the top - when you reconnect the cylinder, the lever still has little or no resistance.

ToGGoT,
The procedure I use is to connect clear tube from bleeder to a bottle or pan, then top off master cyclinder. I then pump the master cyclinder lever slowly four or five times then while holding lever fimly down I open bleeder until bubbles stop coming out or the lever reaches hand grip then close the bleeder before releasing the lever so as to not suck air into system, I repeat this process until there are no bubbles and the lever pressure feels firm without spongy feel. A note, that sometimes the lever does not have any pressure or resistance and goes all the way to the grip for the first few tries. good luck .

Two approaches I use:
1.
a. pump the lever and hold.
b. open the nipple with a ring spanner make sure to close the nipple before you release the lever.
c. repeat above.

2.
a. get a 50ml syringe and connect to the nipple using a suitable clear plastic tubing.
b. fill the reservoir and pull the syringe all the way to 50 to create the vacuum.
c. repeat until no bubbles come through (keep the reservoir full!)

You may want to use the procedure 1 after 2 jut to make sure.

Cheers guys. Sorry, should have given a bit more information!

I have all new braided brake hoses all the way through the system, and have tested all the fittings - they dont seem to be leaking.

When i pump the lever, there isnt any resistance to pumping, but the first time i open and close the nipple (I have a length of clear hose connected, leading into a jar of brake fluid), some air/fluid comes out. Subsequently it doesnt. It is almost as if fluid is NOT feeding from the reservoir into the system, even when there is a vacuum, although i cant understand why that would be :?

If i use a syringe to create a vacuum at the bottom, exactly the same happens - no fluid seems to feed down into the system :?

How exactly does the fluid get from the reservoir past the piston? Is it through the little holes behind the rubber seal?

ToGGoT