Evidence is building that some foreign air fares will be lower than ever this spring and summer as airlines begin to take advantage of provisions of liberal air pacts signed by the United States and other nations in the past year.

The latest move came yesterday when Northwest Airlines proposed to cut regular fares from the West Coast to Korea by 27 percent under the terms of a new liberalized air rights agreement approved last week.

Korea and the U.S. agreed that neither country can unilaterally disapprove proposed fares of any airline, allowing a low fare to go into effect so long as one of the countries approve it.

With the Civil Aeronautics Board's penchant for low fares, Norhtwest's proposal to lower fares across-the-board is virtually assured approval. Effective April 298 Northwest is cutting the regular economy fare from its West Coast gateway cities to Seoul to $420 one-way from the current $581 for all coach seats, no strings attached. The first-class fare will be reduced from $927 one-way to $675. Reductions also will be offered on excursion tickets.

"It's a major price break," Donald A. Farmer Jr., director of the CAB's Bureau of International Aviation, said yesterday. Farmer and other U.S. officials are hospital the lower fares and opportunities for new competition the Korean agreement facilitates might have some effect on U.S. -Japan prices by adding some options for travelers to the Orient who may use Seoul as an intermediate point on the way to other destinations.

Northwest recently got word that the Japanese for the second time had turned down their "Orient Express" fare proposal, which would have reduced Seattle-Tokyo economy fares by 35 percent to $650 round trip, effective April 1. Northwest first proposed those fares more than a year ago.

In the transatlantic market , two formerly charter-only airlines with new scheduled authority announced new low-fare plans to take advantage of liberalized bilateral agreements with some Western European countries. Trans International Airlines told the CAB it would like to begin New York Amsterdam service May 1 with a regular economy fare of $289 round trip; the peak season fare will be $319 round trip. National Airlines has a $310 round-trip Amsterdam fare but is available for only a portion of the seats, not all them. The new fares compare to regular economy fares of about $800 round trip.

Trans International plans to institute New York-Paris service June 19 and New York-Frankfurt service a week later. The airline's proposed Amsterdan fares even during the peak season are lower than almost all individually ticketed fares-including advance purchase fares-in the market.

Capitol International Airways has a similar fare package, expecting to begin service May 5 from New York and Boston to Brussels for $300 round trip. Similar Frankfurt fares are expected.

Under bilateral agreements signed last year, fares to Brussels go into effect unless both governments disapprove them; fares to Frankfurt and Amsterdam are governed by country-of-origin rules, allowing them to go into effect if approved by the country where the traffic originates.

Fares to France are another matter. Yesterday, the CAB asked President Carter to disapprove a discount fare proposed by Air France on the grounds that France has been disapproving low-fare proposals by U.S. carriers, allowing them only to meet Air France's filings.

In a related development, President Carter yesterday announced he would go along with a similar CAB strategy to get Canada to be more receptive to the low-fare offerings of U.S. carriers.

While some airlines ar announcing new low fares, major international airlines are meeting in Geneva beginning today to discuss soaring fuel costs. The airlines are expected to seek higher worldwide fares to offset the fuel costs, according to International Air Transport Association spokesman Gordon Ruddick.

Besides making low fares easier to confirms the United States' right to confirms the United States right to designate multiple carriers to Korea. Northwest is currently the only one with authority. In addition, the agreement liberalizes charters, allowing charters from the U.S. to operate under U.S. rules.

Along with its low-fare proposal, Northwest announced it would begin nonstop service to Seoul from Seattle on July 1; the airline has 10 weekly flights now using various routings. In a related move, the CAB gave Korean Air Lines authority to fly between Korean and New York with a stopover in Alaska Beginning Friday.