This paper describes the basic process of using the proxmark3 to clone Proxcards and then introduces ProxBrute, a new tool for brute forcing valid proxcard values.
You can download it from the following link: https://packetstormsecurity.com/files/download/97706/proxbrute-proxcard.pdf
Source: https://packetstormsecurity.com/files/97706/ProxBrute-Taking-Proxcard-Cloning-To-The-Next-Level.html