Does your Umarex Walther CP99  leak out the front or rear?  These are the most likely seals to fail.  After a lot of experimenting and wasted Co2 we have found these to be a superior upgrade to the original seals.  ATTENTION understand that this is for the pellet version of this gun and NOT the BB.  If you are comfortable servicing your own gun this can save you time and money.  There are online guides to assist you.  GOOD LUCK!