now rear master cylinder....

so mine looking rather rusted and tatty I decided to forgo the £30 odd rebuild kit and buy this £30 odd new master cylinder.

http://www.wemoto.com/bikes/honda/cb_500_y_2/00-02/picture/rear_brake_master_cylinder/



aside from the reservoir being oblong and not the standard round will this part actually do the job sufficiently - is there anyway I can tell?

I remember that the original has an intentional bend in the tube..

Also, the lid is up quite tight to the rear fairing, so you might have to modify the bracket perhaps...

all sounds doable I was more concerned with the ability to stop the bike

Should be fit for purpose I would hope.

oddly despite being from Wemoto came in a David silvers bag

Perhaps respectable Silvers sold their stock to wemoto when they realised they weren't fit for purpose 

Fit the rear brake only, and then try a straight line skid when carriering towards a wall at 70mph...

I'm not helping am I....

As long as it locked the rear wheel it would be just as sufficient as the stock one

The only issue would be if it either was the wrong bore (so gave you the wrong ratio), so badly
made that it leaks or the mounting holes are in the wrong place.

I bet it has none of those things and if it did wemoto would probably have it back off you anyway, so you are probably in luck :)

EDIT: sullivj: I'll try out that rear brake test but only if you try it first ;)