Folks,
It is an unfortunate fact of life, but a very real one, that the bleeders are on the bottom on B wheel cylinders. The bleed screw and the feed line are different diameters and threads. You cannot exchange them, so that remedy wont work. And, to make matters stickier, there are locating pins on the cylinders. They will only go in one way.
When bleeding MGB rear brakes it is necessary to use sufficient force on the pedal downstroke to force air and fluid to go through at a rate which overcomes this design inconvenience. Also, do not dawdle in gettingt the bleed screw closed at the conclusion of the pedal downstroke. In fact, it is better to close the screw just before the stroke is completed. You can tell this because the fluid will still be flowing. Don't worry, it can be done.
For a more complete story on bleeding MGB brakes (or any brakes, for that matter) View my post number 4804, or better still, the entire thread titled "Leaking master cylinder?" of which it is part. Good luck.
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon