BIG problem with Power-assisted steering and brake servo

My Mustang has got a rare and wierd problem with the power-assisted steering and brake servo.
The problem is :
*when I brake hard the brake servo stops working, when I brake smooth and light the breake servo works.
*If I brake and then turn both the brake servo and power-assisted servo won't work.
*If I turn whitout braking the powe-assistes steering work in most cases
*If I turn and then brake the brake servo dosen't work

What is wrong ??
Is there any connection between the power-assisted steering and brake servo ??

I have checked the drivebelt and it looks OK, there is fluid to the breake servo and steering. They don't share pump.
I thought about a electric problem but there is no electric that controlls the brakes right?

PS: I hope you understand my english, I know it's not the best

Post the year.