Investors have long pretended that money-market mutual funds are as safe as cash — even though they're made up of short-term IOUs that can drop in value. So last fall, when Reserve Management said its Primary Fund had lost three cents a share because of defaulting Lehman Brothers bonds, panic ensued. Institutional investors pulled hundreds of billions of dollars out of money markets, which threatened companies that depend on the system for funding. The Federal Government rushed in with an industry-wide guarantee. That backstop ends on Sept. 18. Meanwhile, the Securities and Exchange Commission is suing Reserve's managers for fraud. Reserve has asked a judge to dismiss the suit. The firm now says investors could recoup as much as 99 cents for every dollar invested. Even if Lehman's debt winds up being valueless, they should get back 98.75 cents.