From the US Inspect website:
The full "S" trap or 3/4 "S" trap should not be used in plumbing installations. These traps are almost impossible to vent properly. The 3/4 "SÂ trap forms a perfect siphon. The problem with "S" traps is that the waste water may siphon out, causing the trap to lose its liquid seal. Every plumbing fixture, such as sinks, tubs and showers, should have a proper trap. Toilet traps are built into the bowl of the toilet. If a sink, etc. does not have a trap, trace the drain for other inequities that may be present (i.e. draining into a sump pump, a dry well or to the exterior surface). In older construction, there are thousands of "S" traps in use. Older building codes did not address this issue. In many cases it will be very difficult to change an "S" trap to a "P" trap.
Can you install a studor vent to alleviate this problem?

Randy,
Thanks for the information. I am going to install a p-trap with a studor vent.
Mark