The Prince William County Board of Supervisors rejected yesterday a compromise on the proposed expansion of the Manassas Battlefield National Park, leaving the next move in a two year park fight up to Sen. Harry F. Byrd.

A spokesman for Sen. Byrd, who recently announced that he favoted " a reasonable expansion consistent with the views and the needs of the people of Prince William County," refused to comment on what position the senator might take.

The board's action came on a motion by Supervisor Kathleen Seefeldt to support a House-passed version of the expansion that would add 1,700 acres to the 3,000-acres park. Her motin was rejected on a 2-to-4 vote and the board refused to take any further action on the issue.

The park expansion, which has now passed the House three times, has been bottled up in the Senate by the refusal of Sens. Byrd (Ind. Va.) and William Scott (R-Va.) to allow it out of committee. Under "senatorial courtesy," a state's senators can usually block a bill that affects only their state.

However, the expansion was added this month to a major parks bill affecting more than 40 states by Rep. Herbert E. Harris (D-Va.) the measure has the backing of the House leadership, and is ensured of going at least to a House-Senate conference, thus bypassing the block by Byrd and Scott.