A failure in the anti-lock brake system (ABS) causes the ABS pump to continuously run, which will eventually wear down the battery. An improved ABS relay is available to solve the problem.Google+
Car Problem Reports
Subaru Legacy Failure in ABS causes pump to run continuously and wear down battery
Subaru Legacy Problem
Average Mileage: 189,000 mi (135,000 mi - 227,000 mi)
Visitor, 1994 Subaru Legacy, 205,000 mi
I just figured this out by reading THIS, but I inadvertently fixed the issue when I unplugged the ABS unit because it was pumping air into the brake lines and leaking.