rear brake

Guys, trying to sort out my rear brake as it doesnt seem to have much bite, with the pedal right down i can still move the rear wheel slightly by hand, pads seem okay and im pretty sure i have all the air out and its not a spongy feel just feels like the pads are not biting on the disc as they should, i have checked the fluid level and can see the piston is moving okay.
Any help/advice is appreciated
many thanks

I  had the same issue - the morning of my first ride this season.  Frustrating and no ride !
  My issue was that after 43 years the bore had developed some flaws. 
.
Several inquiries within in Canada were responded to with, " We can't sleeve that type of M/C" . 
One forum member ended up sending his to California(?).
 I made a hone, removed the flaws and it's now working fine.
A 1/2" S/S tube cleaner with the "handles looped-end" cut off.
 
Wrapped with a piece of man-made chamois. 
 
Roll the material into the bristles.
 
Wrapped with Wet/Dry sandpaper


You will have to experiment with how much Chamois and sandpaper.  The bristles and chamois provide the sponginess to allow the sandpaper to stay under some tension and in full contact when in the bore. The bore is S/S so I started with 160G and used 3 pieces before I noticed the flaws being removed.  Then went to 400G and used 4 pieces of this.  With the flaws almost gone I wore-out the last piece of 400 for a finer finish. Then washed out the bore thoroughly and rewrapped the cleaner bristles with a longer piece of chamois to get a larger diameter so it fit snuggly in the bore.  I found it best to put the tool on my bench and with the palm of my hand progressively roll the chamois. 

Success will depend on how deep the flaws are.  I figured: There's nothing to loose by trying this because at this point the M/C is essentially junk.  Re-sleeving would then be the only option, in order to  keep using the OEM M/C.  If the seals aren't damaged you may be able to reuse the piston/seal assembly.


 

Thanks Scotch, i will investigate further to see if theres anything more obvious and if i cant find anything will try your ingenious idea , by then ill have nothing to lose