04 620 rear brakes fading

[vipin620]

Hello,

I got this 2004 monster 620 from a friend with about 4200 miles on it.  He had not ridden it much in his 4 years and it mainly sat in front of his apartment and it sat for about 1.5 years on my driveway before I started to ride it in May of 2008.  I quickly noticed that the rear brakes were not gripping at all.  I bought a new rear master brake cylinder.  The day I went to install it, I decided to bleed the existing unit and see if that did anything.  It did.  The rear brakes were working correctly.  I thought that it was all that was needed.

But, within a few days (2 weeks max) I noticed that the brakes were fading again...So I thought that the rear master cylinder was weak and decided to replace it with the new unit I had bought.  I installed it and bled the system and the brakes were working fine again.  But then again, within a very short time, the brakes started to fade again.

I didn't ride it much in 2009 but I started to ride it in the summer of 2009 and bled the brakes (rear) and again they only lasted about a few weeks.

There are no leaks that I can see and the fluid level in the reservoir does not go change.  I don't know what is causing the rear brakes to fade without any leaks.  They work perfectly right after bleeding.

Please help!

What I'm wondering about is if the heat from the engine is causing the master brake cylinder to heat up...
I can bleed the system and come back to it in a month and the brakes don't work.

What gives?

Thank you for your help in this...