nypost.com — REUTERS The Secret Service's seamy hooker scandal may have involved as many as 20 elite US agents - who are specifically trained to avoid such security-breach pitfalls, officials and sources said yesterday. "Historically, we've heard about these 'wheels-up' parties when the president leaves - well, this was 'pre-wheels down,' " said furious House Oversight and Government Reform Committee Chairman Darrell Issa (R-Calif.).