Recently we added a new master bath all 3/4/ feeds,reduced at fixtures to 1/2/.
Now whenever we flush toilet in [old] exsisting bath we have no pressure anywere

How old is your house and what material are the original pipes made of?
This will be a volume problem, pure and simple. Regards, Tom

You got that right Tom, sounds like some wanted more volume at the new bathroom but connected to a small pipe, then big, then small again. Does not work like that, try feeding the new bathroom from the main, hopefully it is 3/4 or bigger. If it is only1/2" run 1/2" lines.

Recently we added a new master bath all 3/4/ feeds,reduced at fixtures to 1/2/.
now whenever we flush toilet in [old] exsisting bath we have no presure anywere
THE main is 3/4 copper, the house was remodled in 1985, the new bath is feed directly from the mainl all with 3/4 I am baffled?we also added a new water heater for the new bathroom could this be the problem?

Are you on a pump or city water? When the house was remodeled did they replace the water service line? Let me ask again, How old is your house,(the real age) and what material were the original pipes made of? Regards, Tom