Motorcycle Repair: rear brake spongyness, vacuum pressure, air bubbles

Question
I have been bleeding the rear brake on my 96 fsxtc for an hour. no bubbles are coming out. I have been pressing the pedal down with open niple then closing the nipple and releasing slowly.
the spongyness had been improving up to the point where the

master cylinder bottoms out If I push with  my foot moderately hard. (Is this acceptable?)Possibly some miroscopic bubbles remain since I was recycling fluid. I repaired the bike and have never ridden it so I dont know what feels normal for the bike but it feels spongy compared to other bikes. I have also used a vacuum pump and pulled through almost a pint  of fluid, still no change or bubbles
I cant help thinking there is air trapped somewhere. There is no fluid leaking anywhere. When I used the vacuum pump, some small air bubbles appeared but only at higher vac pressure. Mabey there is a small leak under vacuum that I cant detect normally. It could just be leaking at the bleeder screw under vacuum pressure.
I replace the master cylinder, should I have bench bled it(there was no bleeder screw)


any thoughts would help.   thx bruce

ps would a leak in the reservoir gasket increase spongyness?

Answer
If you have a new master cylinder there shouldn't be a problem within that part. A weep from the top gasket has no effect on the operation of the master cylinder piston. It sounds as though you have thoroughly bled the system. I would check the brake line when you press the pedal to see if maybe the line is weak. Look to see if the line swells anywhere along its length. A little sponginess is normal it all depends on how well it stops on the road test. There should be an adjuster on the end of the master cylinder piston to adjust for play in the pedal. Be sure that the front brakes work before you ride it. Let me know what you come up with.