As a volunteer and not a paid employee, you probably don’t have much of a case in court or before the EEOC.  There are a few cases in which a volunteer position generally leads to employment or the volunteer role is so much like an employment relationship that courts have allowed Title VII civil rights suits to proceed.  (See Nonprofit Issues^®, 1/16/13.)  If you really want to sue, you should consult a lawyer, but it doesn’t sound like either of those theories would apply here.

Your best response may be to talk with the president and treasurer directly and then talk to any supporters you may have on the board if those talks don’t do any good.  If the conditions don’t change, you are not required to continue.  There are probably a whole lot of other organizations that would love to make use of your services.