More than 900 United Parcel Service employes have petitioned the Teamsters union to break away from their local union because they are due to receive less than half the pension benefits they believe they are owed, according to a dissident Teamster group that yesterday began picketing union headquarters.

"We are not saying there is anything illegal, we are just saying this is wrong," said Lester B. Varney, a 43-year-old driver at UPS's Laurel facility who led a group of 20 pickets in front of the Capitol Hill offices of the nation's largest union.

The group, which hired its own actuarial firm to study its pension funds, contends that based on their employer's current pension payments, they should receive $1,500 to $2,200 monthly upon retirement. But their maximum benefit is currently $735, a figure set by a six-member union-company board of trustees.

"We just want a retirement plan where we get what we have paid for," said Varney, a 17-year Teamster member who said his pension would more than double if he belonged to another UPS pension plan on Maryland's Eastern Shore instead of the plan administered by Teamster Local 639 in the District.

Teamsters president Jackie Presser, in a letter to the group last month, acknowledged problems in the local pension fund. But he said Local 639 officials were working to correct the situation, and he denied the group's request to split from the 5,500-member Local 639 to form their own local of 1,200 UPS employes at Laurel.

Yesterday, carrying signs saying "Teamsters Unfair to Their Own Members" and similar slogans, the picketers said they plan to return daily and will hold a mass rally Saturday to sway Presser to reverse his decision and allow a separate local or separate pension fund.

Local 639 vice president John Catlett, a pension fund trustee, said yesterday that some of the group's complaints are valid. "We don't disagree with a lot of their contentions. A lot of it is true," said Catlett. "We will have a raise (in pension benefits). The money is there to do it . . . . We wish we had made a change already, but we don't have the luxury of making snap decisions."

Pension benefits can be increased by votes of the trustees, which occurred in 1979 and 1982, and Catlett said another increase is imminent.

Part of the reason for lower-than-expected pension levels, he said, is that the trustees have had to pay the pensions for non-UPS employes who belong to the same pension fund but whose companies contribute far less than UPS.

UPS pays $1.73 per hour for each full-time employe toward pensions, which employes receive at age 60, according to the firm's Teamster contract. At that contribution level, a Teamster should receive $1,500 a month after 20 years' service and $2,250 after 30 years, according to the dissident group's pension study completed in August by Edward H. Friend & Co.

"Clearly, the UPS hourly employes would be substantially better off with their own plan" instead of the Local 639 plan, which pays $735 maximum, the study said.

The Local 639 pension, which has a balance of $65 million and a membership of more than 4,500, includes workers whose companies contribute as little as 25 cents per hour and who receive far less in benefits than UPS workers, Catlett said. In some instances, he said, "some of the (UPS) contribution might be used to pay for another union brother's pension," which he said is both a legal and acceptable practice.

While other Teamster pension funds in the West and Midwest have been scandal-plagued, the Local 639 plan, started in 1956, is "one of the soundest in the country" and has never run afoul of the law, Catlett said.