Several things come to mind:
1. Make sure that all moving parts, such as the rod from the master cylinder
to the brake pedal, and the slave center rod are not worn. Inspect the clevis
pin holes on these rods to make sure that they are not egg shaped. The pins
should fit with only minimal play.
2. This is not a common problem, but I have seen metal clutch hydraulic lines
that have tiny tiny tiny rust pin holes in them. So small that air can pass,
but brake fluid does not. (amazing, isn't it........). If your line does not
look great, you may want to replace it.
3. More common are tiny holes in the rubber hydraulic line that attaches
between the frame and the slave cylinder. Again air will pass, before fluid
really begins to leak.
4. If you are sure that all moving parts are ok, and that the system is
properly bled, the next question is: How old is your clutch? The MGB uses a
carbon block throwout bearing. Over time, this wears and decreases in length.
They last a long time, but eventually can wear to the point that significant
play is introduced. I am not sure about the B, but on an MGA, if you pull
aside the clutch throwout arm rubber boot, and use a powerful light, you can
get a look at the throwout bearing. (Not a good look, but it is better than
nothing). I guess that you know that the only way to replace the bearing is
to pull the transmission from the car. (depressing thought).
To answer your original question, I don't know of any specifications
regarding the length of the operating rods. In fact, on an MGA the master
cylinder operating rods are adjustible. Good Luck!
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