What is the rule of thumb for equal pay

I am an African American man,an employee in good standing with a company ten years. I was promoted two years ago and asked to retain my job as Help Desk Manager and take on the responsibilities of Systems Administrator. The corporate office has since added more facilities and the job became too much to do in an 8 hour day. I began working from home on weekends and evenings. Additionally I was on call 24/7 to supervise the Help Desk. The workload was over the top. I replaced another full time employee. When I brought this to the attention of my supervisor he would ridicule me. In addition to my added responsibilities he would ask me to work on personal computers for himself and other high level employees. I recently wrote a letter to my supervisor w a copy to HR requesting a salary review as I know I make less than the person I replaced.He had one job, less facilities to monitor and me as a support staff. My work environment is stressful now and getting worse every day,