sorry its a cougar, but its roots are mustang. the car is a friend's 68 cougar xr7, he says when he hits the brakes rpms drop and the car dies, any ideas? it does have a power brake booster.

 

What kind of float is installed in the carb? When I had a plastic one installed in my carb it would do that when I took right turns.

 

i'd say it's related to the booster.
sometimes this can be 'normal' so you just need to increase idle rpm, however it could be related to where he picked up the vacuum for the booster. where exactly is it connected to (picture) and what else is connected to the same pickup (should be nothing)

 

the carb is an edelbrock, i would imagine 600cfm, so copper/brass floats? its a mild 302, its possible there is a bunch of gunk in the carb, who knows when the last time somebody was in it, the car even has original M/T valve covers. i would imagine hes drawing vacuum from the tree on the back of the intake manifold, he said he checked the vacuum lines and had no leaks. i dont know how thoroughly he checked for vacuum leaks, he is rather lazy when it comes to stuff like this, that and just about everything on the car is controlled by vacuum, hide away headlights, heater flaps etc. he thinks its electrical related, brake lights killing the ignition? does not compute