poor idle

Help, Any ideas, anyone. 
When I press brakes the engine either cuts out or runs proper rough,  there's a new  servo,  also all vacume pipe in place cheers for that thread, running an 80, cabriolet with weber and manual choke 1.6 gls. Don't know why it started about 4 months ago bread all brakes after putting new servo on, absolutely mint then after 3 weeks back to square one ,  any ideas anyone cheers Chris 

has it got the non-return inline with the servo pipe, and have you checked it works?

have a very good look at all the vacuum connections, and check the flange the carb sits on especially if you havent got the steady bracket from inlet to airbox as it will allow carb to flap about and split the flange quickly