Continuing their months-long feud, the Washington Suburban Sanitary Commission members from Montgomery and Prince George's counties yesterday vetoed builders' request for new sewer service in each other's counties.

For the moment, that stalls development of more than 700 new homes in Montgomery County and seven commercial projects in Prince George's.

It also indicates that little genuine progress has been made in negotiations between the two county governments to avert an extended sewer moratorium that would halt development in Montgomery.

The WSSC staff informed the Montgomery government last month that the county had exhausted its share of sewage treatment capacity in the regional Blue Plains plant in the District of Columbia. Montgomery is trying to persuade Prince George's, which has unused capacity it is saving for new growth, to share that capacity temporarily.

The continuing friction over development was further demonstrated yesterday when WSSC Chairman Johanna S. Norris, one of the Prince George's members, endorsed a proposal to defer for a week consideration of new sewage flow calculations that would expand treatment capacity allotted to Montgomery and, at least temporarily, allow new construction.

These calculations, based on water conservation and other factors, show that water consumption in single-family homes and town houses is no more than 350 gallons a day, rather than the 400-gallon-per-day standard now used by the WSSC, which provides both, sewer and water service to the two counties.

"I see no fault with the figures," Norris said, "but I have a problem with moving quite so rapidly."

The request for the delay was made by Bert Cumby, a WSSC member from Montgomery who occasionally is at odds with his two Montgomery colleagues. Cumby said he was worried that staff members had had too little time to study the new calculations.

Assured that the staffs were familiar with the new calculaions and that the county governments and state health departments would have to approve their application anyway, Cumby withdrew his request for delay. The commission agreed to send a report on the calculations to the two county governments for final action.

Norris relented, too, but not until commissioners from both counties engaged in exchanges that have become stock segments of their weekly meetings.

"Montgomery is in a critical situation. Our backs are up against the wall," said Montgomery commissioner David Scotton in one angry outburst. "We need capacity, we need allocations and I will continue to press and press and press or we'll be in a moratorium. . . I don't want to see any delays in moving this (proposed new policy forward."

Norris suggested that Montgomery's failure to reclaim millions of gallons of treatment capcity committed to builders who have no intention of using it is holding up the bicounty negotiation.