If you take the four nuts out that hold the steering colum up and lower it you can then get to the servo bolts.
First of all try this:
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, use 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...
Don't 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.