mg idles and goes lower and lower till dead apply brakes an it

How long has the car been sitting? Fresh fuel? Has the Carburator been cleaned or rebuilt? recently? Did you check for vacuum leak?

Power brakes ? Multiple ports in a carb can clog and it'll still run , one too many and it won't .

Sounds like the carbs are running rich and slowly flooding the engine and the vacuum from the brake booster leans it out. Probably needs the carbs overhauled with the ethanol resistant kits. Make sure all the rubber fuel lines are replaced with the new mulit-fuel hoses.

I would find and repair the vacuum leak on the brake booster first and then adjust fuel mix on carburator, otherwise u'll be chasing the problem.