Hi all,
Great site..been a lurker for some time, so I finally signed up.

Just bought a used TaoTao 125 for my daughter to learn on and cannot get the rear disc brakes working.  I usually pride myself on being a decent mechanic, but this has stumped me.  Tried a new master cylinder, tore apart the complete caliper unit and still nothing.  I seem to be able to good build pressure to the top of the ms, but if I put on the brake line and use my thumb and finger to "cap" the other end, I can feel slight pressure while pumping the pedal, and do get a little fluid, but not much.  Could I have a bad hose or banjo fitting?  I do not see any fluid leaks.  
Any help is greatly appreciated.  

been there-done that, you need to rebleed the system. i know you'll going to say i did it already, but you need to do it again, this time you need to keep pumping the brake lever until it builds up a lot of pressure then release the bleeder valve just a little until a small amount of fluid and air comes out, then close it, and pump it up again, need to do it several times. if you still can't make it work the other option is buy a complete brake system off of e-bay, cost is $25-$35  

thanks Bill, Yep, I've bled and rebled the system over and over.  I can get pressure to the caliper end of the line now, and do get fluid out of the bleeder when cracked.  But never seem to build pressure on the pedal.  With my thumb on the master outlet port, I can acheive pedal pressure, but once everything is connected, I loose it.  I may be leaning toward a small air leak in the calipers.  Gonna replace o-rings in them today.  

It doesn't seem likely to me that air is leaking in during master the cylinder retraction stroke (small forces), and not leaking fluid out during the pressure stroke (larger forces).  

I would try unbolting the caliper from the frame, pushing the calipers all the way open (purging any air trapped there), block the calipers open with a block of wood, raise the caliper above the master cylinder and try again.  By raising the caliper above the master cylinder any trapped air bubbles in the hose wont migrate back up (or down in this case) the hose in between bleeding strokes.

This isn't my idea - I read it elsewhere as the final solution to a similar problem on a buggy.  I haven't tried it myself.  Big grain of salt here...  

Thanks all, finally got it.  I bled the system in reverse, an old airplane trick.  I took an oil can/pump and some tubing and pumped thru the bleeder valve until she was full and voila, it worked.  Brakes are great now.

Also, on a unrelated note...I changed the oil in it, but do not have a manual.  On my suzuki, the oil level should be at half in the sight glass.  Is that the case with these atvs too?  It does seem to shift better, but shes no zuki or honda.  I got this one too cheap to pass on, but never again.  I'm too anal about the quality and its just not there, from welds to noises it makes
I'll keep it running, but lesson learned  

unholy89

no, do not fill oil half way in the site glass, you will over fill the engine and it will leak out of the case vent and burn oil, you need to use the oil dip stick on the oil fill plug, use the site glass to check oil only while the atv running, then you will notice it should be around the middle of the site glass.