2002 gsxr, bike sat for 3yrs. rear brake quit working. no pressure to peddle. unable to bleed if possible ? seems like a pressure problem in the master cylinder?

Of you seen any brake fluid when changing pads the the leak needs to be addressed and brakes need to be bled out of any possible air in system. If this was a problem before then you have a bad master cylinder or brake booster.

You've obviously still got air in the lines. If you can't remove it, take it to a shop for a pressure bleed.

When you say that the first master cylinder had no pedal, did that just suddenly happen?

A master cylinder has rubber seals inside - think of a bicycle pump - when the bicycle pump seal wears it becomes difficult to pressurise it. The same is true of a master cylinder.
A classic sign of a master cylinder with worn or perished seals is that it needs pumping a few times to pressurise but there is no leakage of brake fluid.

Prior to you changing the master cylinder had the level of the brake fluid dropped .. indicating a leak somewhere? Had you done any work on the brake pads/rotors?

Eliminate other causes of air getting into the braking system:

First thing to do is check the flexible brake hoses on each wheel. Get a friend to pump - and hold - the brake pedal while you inspect the flexible hoses. Check for leaks (obviously) but also look to see if each flexible hose 'bulges' anywhere along its length.

Also check each flexible hose to see if it is perished or wet with fluid. A perished flexible hose can draw in air and cause no pedal/spongy pedal. The same is true with a flexible hose that bulges/balloons when under pressure.

It only takes a slight 'weeping' from a flexi hose or rigid pipe connection to allow air into the system. Check the rigid brake pipes too - especially where there is a connector/join.

Then remove the road wheels and look for the slightest fluid leak around caliper pistons.

The problem, I think, comes back to the original fault .. no pedal when your car had sat overnight. This is rather indicative of air getting into the system somewhere though not necessarily from the master cylinder.

Check for the slightest leak in the entire braking system. It only takes a small amount of air to get in and cause problems. The fact that you fitted a second master cylinder makes me think the problem could lie elsewhere.

Try bleeding the braking system. It's a 2 man job, but quite easy.
Here's a link that explains how to do it:
How to Bleed Your Brakes

Good luck!

bleed the brakes again starting at the longest line at the rear and work forward . The indication is still air in the lines.

Could be ignition fuse. If not that then check the large block connectors which are usually located on the left hand side at the front of the bike behind the cowling beneath the handle bar. These connectors are notorious on GSXR's

Do you actually see brake fluid coming from the bleeders? Check for a broken brake line. If no fluid is being lost and the peddle has proper pressure, then the brake booster has failed. If their is no pressure at all, then the master cylinder has failed. Replace accordingly.

with new pads, bleed the rear brakes

Yes, try bleeding the brakes again, you may have to bleed the front brakes too. when the master cylinder goes dry from a broken line sometime air gets into the front lines also. otherwise the master cylinder could be bad when the pedal goes past it normal travel sometime it tears the seals inside and you be unable to build pressure in the master cylinder

Bleed them again, and don't leave the bleeder open, every time the person lets up on the pedal. close on the way down and open and close next time etc. Your letting air back into the system. Don't let the master run dry.

hey

bike brakes are a pain to do man!! id use a bleeder kit on it and if this doesnt work then your master cylinder at the pedal is gone!! iv a fzr600 race bike and thats ten times worse man