I have had this problem with my rear brakes for the last month or so, After ride for ½ hour or so my rear brake goes away (pedal all the way down and no brake) and then after a few minutes the pressure comes back and I have braking to the rear brake. I have blead the brake line and replaced the line with Stainless Galfer line but the brakes still just go away after riding for awhile.

Any suggestion.

Had a similiar problem with my 95 Monster 600 especially on a few hard rides.

If you have eliminated any air and leaking seal problems could it be the actual fluid that you are using if there is any moisture in it or it is of a poor grade then it may be overheating. Check also that the caliper is moving freely as when you bleed it, it may not be fully opening and after a few hard depressions while riding it may be causing an air lock or transfer of the heat back along the oil line if it is binding.

My 600 had a sticky pad and the fluid although new to me was also past it's best I believe - I had purchased it from my local motor factor and when we checked it's shelf age it had been there for at least 12 months with him I just happened to grab it when everything else had been sold in front. He took it back and I collected a fresher bottle.

I strongly suggest you read my post on how I nearly killed myself.

it was with a poorly adjusted rear brake [master cylinder]
I had adjusted the pedal height but also blocked off the fluid return to the reservoir.
The line pressurized under heat; brakes applied "automatically" and nearly locked the rear wheel while riding in traffic on the freeway.

the rear brake had been spongey from air in the system; I kidded myself that I never used the rear brake anyway...

DO NOT SCREW AROUND WITH DODGY BRAKES - even the rear.