Make sure the valve on the brake booster is not broken, if it is, you will need to replace the entire brake booster valve/line. (They don't sell the valve alone) It is very easy to change and the part is around $20. Here is a picture of mines that was broken and leaking vacuum and also shows the new part. (See Step #4)

http://www.insidemiami.com/transmission/transmission.html

Thanks,
Adiel