The market was 500 trades away from Armageddon on Thursday [September 18], traders inside two large custodial banks tell The Post. Had the Treasury and Fed not quickly stepped into the fray that morning with a quick $105 billion injection of liquidity, the Dow could have collapsed. According to traders, who spoke on the condition of anonymity, money market funds were inundated with $500 billion in sell orders prior to the opening. The Fed's dramatic $105 billion liquidity injection on Thursday (pre-market) was just enough to keep key institutional accounts from following through on the sell orders and starting a stampede of cash that could have brought large tracts of the US economy to a halt. Cracks started to show in money market accounts late Tuesday when shares in one fund, the Reserve Primary Fund - which touted itself as super safe - fell below the golden $1 a share level. By Wednesday, banks sensed a run on their accounts. They started stockpiling cash in anticipation of withdrawals. Banks, which usually keep an average of $2 billion in excess reserves earmarked for withdrawals, pumped that up to an astounding $90 billion, Lou Crandall, chief economist at Wrighton ICAP, told The [Wall Street] Journal. And for good reason. By the close of business on Wednesday, $144.5 billion - a record - had been withdrawn. How much money was taken out of money market funds the prior week? Roughly $7.1 billion, according to AMG Data Services. By Thursday, that level ... had grown to $100 billion.