Is the engine vacuum tied into this system in any way or location? If so maybe that is on wrong side of a check valve or otherwise creating back pressure against the electric pump ( fuse blows with engine run ). Or maybe you created this issue in how you plumbed a 2nd reservoir.
Typical, small, OEM like, vacuum reservoirs are for nothing more than compensating for very brief vacuum drops, holding the little blend door servos in place at best. A reservoir that makes any difference where a power brake booster uses a LOT of vacuum... That reservoir would need to be huge.
