read this problem on a 306 , i know you said they checked the servo , so i just cut and copied this answer for you to look at -
I know exactly what that is. It is the seal on the back of the Servo unit...
What you need to do is climb under the dash and pull back the carpet then behind the brake pedal is a pin that goes through the bulk head.
Pull back the waterproof rubber seal and you should see the back of the servo and a small red seal...
Now what you have to do is seat it back into its original position using a blunt tool, i used the back of a pencil..
It happens after you brake hard then let off the pedal, the seal just pops out as it is only held in by the vacuum of the servo...
Dont worry too much about it, your brakes wont totally fail but you should really get it done!
If after you try this it still leaks then you need to replace the whole servo unit
as i said this is about a 306 , might not relate to you.
another 306 hissing brake problem answer was -
Took it into the garage, new Master cylinder and new n/s wheel cylinder.
can`t find anything on a 307 i`m afraid.