Government unions are finding it easier to sue Uncle Sam than to bargain with him over working conditions. Barred by law from strikes, and limited in the scope of negotiations, federal organizations are turning more to the courts to force agencies to do what they want, or try to block them from doing things member oppose. Examples:

The National Federation of Federal Employes yesterday went to the Federal Labor Relations Authority (a precourt administrative procedure) asking it to block the Carter administration from extending "merit pay" to all Grade 13, 14 and 15 workers. There are 65,000 people here in those grades.

Merit pay, originally designed for managers and supervisors, guarantees employes only 50 percent of the regular October federal pay adjustment. To get more, employes have to win approval from their bosses, a new concept in government. There are signs, however, that the administration wants to extend the merit pay blanket to cover non-supervisory personnel. NFFE says this is against the law.

The National Treasury Employes Union has gone to U.S. District Court here, seeking to force the government to pay higher mileage and per diem rates. The maximum per diem currently is $50 for selected highcost cites, $35 for most areas. Mileage payments now run between 18 cents and 19 cents per mile. NTEU says that is not enough, and the government should be more flexible to meet the expenses of employes who travel for it especially in an era where gasoline prices change -- upward -- almost daily.

The American Federation of Government Employes is seeking multimillion dollar refunds for federal workers forced to pay for parking. AFGE has taken its case -- that the unilateral decision to impose pay parking was illegal -- to U.S. District Court here. If it wins, President Carter's pay-parking program could be in real trouble and the government conceivably have to pay back fees workers have been charged since last fall.

A growing number of attorneys here are beginning to specialize in civil service personnel matters, to devote major portions of their practice to handling grievance actions brought by federal workers against agencies and individual bosses.

"Instead of having trial by combat through the negotion process," one union official said, "we have been forced into a situation where we have negotiation by trial."