"We will continue to work with our customers on their travel plans and apologize for any inconvenience the disruption in service has caused".
Alyssa Eliasen, a spokesperson for the airline, said Southwest had 'actively worked to manage our glycol levels (used to deice aircraft),' but that due to the severity of the winter weather it was forced to cancel the flights.
Southwest was forced to cancel almost all of its Sunday flights as it waited for the pump to be repaired.
No one was injured in the incident and the airport resumed operations normally.
"Those developments, coupled with a more positive weather forecast for Chicago position us to safely operate our more than 250 departures out of Chicago Midway". The day after Christmas, the airline cancelled nearly 90 flights because of delays over de-icing aircraft.
This is the second time Southwest has dealt with de-icing difficulties at Chicago's Midway Airport since December.
A winter storm that dumped about 9 inches of snow on the Chicago area by Friday evening was followed by another 3 inches of snowfall Saturday night into Sunday afternoon, according to the national weather service.