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Well, I do believe you have my scooter's brother... LOL!

Your experience is exactly parallel to mine. You won't be sorry about replacing the rear caliper along with the master cylinder. After the old caliper's pistons have been extended WAY far out, I'm uncomfortable with attempting to push them all the way back in. With exposure to the elements, I think it's asking for a leak at the pistons... Some mechanics say it's OK to do that on a car, and some don't. And these scooters aren't as well made as most cars... LOL!

When the new caliper arrives, please do set it over the disk, and see if it has all that extra space (about 1/8") between the pads and the disk. If so, I strongly recommend you remove a pad from the OLD caliper, and grind off all the friction material, and place it BEHIND the inside pad on the NEW caliper, as a spacer. That is what I did, and it immediately solved the problem. (If you do that, and the caliper is too tight over the disk, just grind off a few thousandths-inch of the steel on the "spacer"). You don't want the new caliper pistons to have to extend nearly to their limit just to begin braking action.

If you follow the instruction-video from Alleyoop, you can't go wrong on the back-flush/fill operation. You do need a big syringe like the video shows. You can actually watch the master cylinder fill, and start and stop the fill at will without letting air in, by just closing the bleeder while maintaining pressure on the syringe. With the scooter on the center-stand, and the bars turned slightly right, the master-cylinder is nearly level, and easy to see and fill.

Once the system is full, you should have operating brakes immediately. (That may ONLY be true with the spacer in place.) If you just install the new caliper, and it has a lot of free-space, it will have to pressurize and push the pistons out, which may require more fluid to be added.

In addition, if your rear caliper is like mine, once the new pads wear just a little, you'll be right back to no rear brake again.

It's unlikely that your line is bad. I too questioned my lines, but as with yours, I saw no leaks, and now with the new caliper, all is fine. If, after all is working, you start to lose pressure, the line might be leaking. But... that is not too likely.

Do be sure to use DOT 3, or DOT 4 brake fluid. That's what these use. Also, the fluid is hard on paint! So... Stuff a big old towel all under the master-cylinder to absorb any fluid you may "over-squirt"...

I have noticed that the rear 2-piston caliper has a little different feel from the front single-piston caliper. The lever moves a little further, but everything works fine. Mine was always that way, and the new caliper is the same.

All this is a whole lot easier to do than it sounds when written out... REALLY... Removing and replacing the pads simply requires removal of the two long allen-bolts they ride on. The pads are under slight spring-pressure and need to be pushed upward to free the bolts, but it's self-explanitory when you look at the caliper.

I think you and I are "pioneers" in this weird rear-brake phenomenon, but as more riders get some miles on their rides (mine has 3,500 miles) the problem will become more common. I honestly feel that many riders get SO fed up with the never-ending problems with these, that they sell or trade off their Chinese scoots before the rear brake wears enough to cause this problem. I dunno, they ARE a real pain, requiring a lot of "wrenching" but for the money they're still hard to beat. I've ridden mine now for FIVE years, with the last FOUR being fairly trouble-free. Now, it's caught up with me... HeHeHe... However, even with repairs, I don't have quite a thousand-bucks invested.

Any help I can give, please let me know...

Leo in Texas

You're most welcome!

And you must have my scooter's twin brother... LOL! My first "fix" was also to check out the master cylinder. It actually WAS bad, as it was not pumping much through the line. It would still apply the brake though, but weak. It looked like it came out of a Chinese dumpster, and sat in the weather for a year... Probably it was. The FRONT master-cylinder assembly was shiny and nice... Go figger...

When removing the cover, one of the screws broke off, but at least I could see that the fluid was passing by the piston, so I too ordered a new assembly from eBay. I put it on, figuring THAT would fix it... NOPE! However, it was now properly pushing fluid to the rear caliper, and would give a solid "squirt" out the bleeder. But still NO brake action.

Since the line was not leaking, that left ONLY the caliper itself as the culprit. After examining the original caliper, and seeing the pistons were overly extended, I placed the new caliper on the disk and saw it too would require the pistons to extend FAR more than I was comfortable with. That led me to make the spacer from the old inner pad.

The new caliper might work OK, but I didn't chance it. I figured that after a little wear to the pads, it too would suddenly fail due to the pistons reaching their maximum-possible extension, and I didn't want to experience that while riding, OR go through all the hassle of repair again... Even if it was a couple of years down the road... LOL!

You won't regret replacing the master cylinder for sure. It might even fix the problem, but I'll bet the Chinese farm that it's still your rear caliper experiencing "full-extension" of the pistons, at which point the fluid just blows by them. I'm surely no expert on these often "marginally" engineered scoots, but I'm getting pretty savvy on the brakes... If I can be of any advice, feel free to PM me any time.

Wishing you the very BEST with this irritating problem,

Leo in Texs