...I bot three bikes from them a few years ago, and you just might have my problem. The midsize Pitster Clone was/is simply superb, and the two 10" bikes are very good themselves overall, right out of the box neither had ANY rear brake---and I tried everything. I have been riding and wrenching for 30yrs, and it had me stumped, until I finally realized that the brakes on these china bikes are non-floating---which means that ANY misalignment in the relationship of the caliper/pads/disc will cause the disc to bind/drag. Or worse, in my case, not drag/touch the disc at ALL. In these two "Pitster SX90 Clone" situations, the binding was caused by some mis-manufacturing of the swingarm. I made spacers by slowly adding thin shims and reinstalling the rear wheel, until I had built up to the exact thickness needed to ensure perfect alignment of the caliper/pads/disc...if u have no access to a machine shop there, use automotive valve shims of various thicknesses...as with any metal frame tho, don't have any gap between the swinger and the spacer, as it will put the swinger under stress...