rear brakes werent working.. pads are fine so i bled the line.. filled it up and then pumped it a few times to get pressure.. finally built up pressure and pumped it a few more times then lost all pressure... could it be a bad cylinder? or maybe just the gasket on top of the fill box lost the air - - any suggestions on what i should do?

 

OMG sometimes those brake calipers can be a pain in the *** to bleed right. Best thing to use is a vacuum bleeder. Even then it take a while. On my Commando the gasket where you put in the fluid has a small leak. I know just what you are talking about. I can see it thought. When the system is sealed and I press the brake, I can actually see the gasket get soaked with brake fluid. My leak isn't bad enough to cause the brakes not to function. If you have a bad enough seal with that gasket it could cause a lack of pressure. You have to carefully look at that gasket from all angles while pressing the brake. You will see it if it's leaking.
When I bled my caliper I actually had to loosen its mounting bolts and rotate it so the bleeder valve was pointing straight up. It sounds silly but it worked, all the air in the caliper went up and collected at the bleeder valve, then it just came out nice and easy. I tried for days to bleed the freakin thing until I did that I just couldn't get it done. I tore the master cylinder apart 2 times and cleaned it and put it back together and was at wits end until I simply rotated the caliper a bit.