The “leaky roof circuit” (also called the “frying pan circuit”) referred to the small-time racetracks in horceracing, but the term was also used in baseball and ice hockey. The “leaky roof circuit” was the opposite of the “Big Apple” (New York City racetracks), which was racing’s big-time.

“Leaky roof circuit” is cited in print from at least 1927 and is seldom used today.


Google News Archive
23 February 1927, Ottawa (Canada) Citizen, “Sport Leads and Counters” by Ed Baker, pg. 10, col. 2:
The present system of appointing unpaid officials recommended by each club, and therefore friendly to the club, is a relic of the days when professional hockey played the Leaky Roof circuit.

Daily Racing Form Online
28 April 1928, Daily Racing Form, pg. 18, col. 3 ad:
“Ohio’s Leaky Roof Circuit About Shot” by Tom Durkin.
(The April 28th issue of the Weekly Racing Guide—ed.)

13 May 1928, Springfield (MA) Republican, “Romance Romps at the Derby” by Henry L. Farrell, pg. 7D, col. 2:
HERE was a son of a fair sort of sire and the offspring of a gypsy mare from the outlaw tracks of what the jockeys call the “leaky roof circuit.”

Google News Archive
7 July 1929, Miami (FL) News, pg. 13, col. 5 photo caption:
CHICAGO, .luly — Vice President, Charles Curtis, himself a former jockey on the “leaky roof” circuit, is shown as he attended the first day’s racing at Arlington Heights, near Chicago, the other day.

Time magazine
Art: Horse Painting
Monday, Dec. 20, 1937
(...)
As a horse trainer, Townsend sometimes races his own horses, sometimes goes on shares with other owners. He travels with the horses, in a truck. His affection is not for the bigtime tracks but for the half-mile county fair circuit in Pennsylvania. Ohio and Illinois which horsemen know as the Frying Pan or Leaky Roof circuit.