Sales of heavy-duty Class 8 trucks fell 19.9% in August from the abnormally high levels of a year earlier, and were even down slightly from the recession period of August 2001.August 2002 was near the peak of a surge of "pre-buy" truck sales as fleets rushed to lock in older models ahead of untried new engine models required as of Oct. 1 to meet toughening federal emission standards.Last month's sales of 11,413 Class 8 units were down from the 14,254 units sold in August 2002, which had the most sales for any month since December 2000.