Unless you have some unusual local supplement or rider I've never heard of, there is no "max" pay rate, or "top rate" for preload for
you. Should you continue in this position, upon your 4th year anniversary, the contractual minimum for you is $12.87 (Article 22, section 5(b), of the UPS/Teamsters National Master Agreement).
At that time, you would start receiving the annual raises specified in Article 22, section 5(a). If your seniority date is before February 1, there is one remaining raise of $.475 before this contract expires. This assumes your local is not diverting any of your raises into your Health & Welfare or Pension Funds.
If there is a preloader working with you who started back in the mid-to-late seventies, he/she would be at what might be considered a top rate, about what a package car driver rate is.
There is a copy of the NMA here:
http://www.browncafe.com/forum/f39/ups-teamsters-national-master-agreement-335360/ , look for pages 64-66. Or, better yet, get your own copies of the contract, and applicable supplements, riders, addenda, etc, from your union shop steward.