Gasoline tax, nuclear waste, subjects like that. But it's also legislative happy hour on Capitol Hill, that time of another Congress when legislators belly up to the parliamentary bar to get one more for the home district. Call it one for the road. With two weeks to go in this lame-duck 97th, the imbibing has just begun.

A taste of it came Tuesday when Rep. Edward R. Roybal (D-Calif.), an Appropriations subcommittee chairman, took his fiscal 1983 Treasury-Post Office bill to the House floor. Among other things, it ordered President Reagan to restore 2,300 Customs jobs that had been ticketed for extinction.

But that simple restoration didn't solve the problem of Roybal's friend, Rep. Abraham Kazen Jr. (D-Tex.). It seems Reagan personnel cuts already have left Customs shorthanded and contributed to long backups of cars awaiting clearance to enter the United States from Mexico at Laredo, Tex., a major crossing point in Kazen's district.

Kazen said there was a two-mile backup at Thanksgiving, with the prospect of more at Christmas and New Year's. Even worse, he said, will be the traffic jams when the Nuevo Laredo Hippodrome, just across the frontier in Mexico, opens in a few months.

That's all Kazen had to tell Roybal, who is familiar with such problems. There is also a race track in faraway Tijuana, just over the California border in Mexico.

So Roybal gave his assent and the House added $5 million to the Customs Service appropriation for fiscal 1983, with instructions that it be spent to post more agents along the U.S.-Mexican border . . . to keep traffic moving.

They started out bravely enough, but after warning shots were fired, Reps. Vic Fazio and Julian C. Dixon, California Democrats, tippy-toed right off the House floor. With them went a nifty little amendment they had planned to offer Wednesday to the Labor-Health and Human Services appropriations bill.

The Fazio-Dixon amendment would have overturned earlier congressional action cutting payments to hospital pathologists, anesthesiologists and radiologists under Medicare.

The reimbursement rules were toughened earlier in this Congress, despite lobbying by the medics, after disclosures that many of them were collecting annual six-figure fees for handling Medicare cases.

Prompted by a new round of House-calls by the physicians, Fazio and Dixon were prepared to introduce their amendment until the Ways and Means Committee warned there would be a floor fight if they tried.

The issue could surface again when Congress considers a new continuing resolution to fund government operations, but for now the prognosis apparently isn't good. "It's dead . . . four feet under already and going deeper," said an aide to Fazio.

The energy and water bill picked up a provision or two on its passage through the Senate Appropriations Committee. It always does.

Sen. Robert C. Byrd (D-W.Va.), the minority leader, persuaded the committee to add $5.4 million for the Levisa and Tug Fork flood-control projects in his home state and neighboring Kentucky.

Sen. James A. McClure (R-Idaho) won unanimous approval of report language warning the Bureau of Reclamation it better live up to a promise to resurface the Black Tail Road in Idaho, and Sen. Quentin Burdick (D-N.D.) added language calling for the expenditure of $10 million on the Garrison Diversion irrigation project in North Dakota.

But sometimes the votes go the other way, even in lame-duck sessions.

The Senate yesterday decisively defeated an effort by Sen. Strom Thurmond (R-S.C.) to cut off a two-day filibuster against a bill that would relieve price-fixers of millions of dollars in liabilities.

The vote was 44 to 51, 16 short of the 60 needed to invoke cloture, and Thurmond agreed afterward to stop pressing for consideration of S. 995, the Antitrust Equal Enforcement Act, in the lame-duck session.

The vote was a victory for Sens. Max Baucus (D-Mont.), Warren Rudman (R-N.H.) and Howard M. Metzenbaum (D-Ohio), who had organized the filibuster.

The bill would spread liability on a market-share basis among defendants in price-fixing conspiracies. Under current rules of law, major conspirators are sometimes able to make "sweetheart" settlements with plaintiffs, leaving smaller conspirators exposed to enormous triple-damages judgments based on what the entire industry has done.

That part of the bill was not too controversial, but a lobbying war was touched off when a number of major corporations, led by Georgia-Pacific, Weyerhaeuser, Willamette, Milliken and Mead, sought to have the provisions apply not only to future cases, but to cases in which they have already been found liable. Mead recently settled its case out of court, but the others stood to reduce damage exposure by a total of roughly $400 million if the proposed law were to be applied to their cases.

Supporters of the bill and its application to pending cases said they would press for passage in the next session.