Brake Booster/Accumulator Troubleshooting

Hi All,
I just finished putting an engine in a 2011 Prius I bought with a blown engine. First startup was this afternoon and I quickly realized that the brake accumulator is running much more frequently than my other Gen 3's. When the car is turned on initially, it will begin cycling every 7 seconds or so, gradually increasing to about 20 second intervals over the course of about 10 minutes. I can hear an audible hissing sound in the brake accumulator/master cylinder area, but I can't determine exactly where it is coming from. The fluid level will drop about 1/8" every time it cycles and will slowly fill back to the original level before the accumulator cycles again.

I cleared the codes a number of times after the first startup to deal with some of the issues caused by the batteries, etc being unplugged or inactive. There was a brake code present at one time, but I can't remember exactly what it was (1391, maybe?). Whatever code might have been there, it isn't coming back. There are still no codes showing after about a 10 mile drive.

With no codes present, I'm tempted to just leave it, but I know the frequent cycling and leaking sound isn't normal and I'd like to take care of this while I'm still getting this car ready for the road. A few questions:

1. I've read that some leak down of the accumulator is "acceptable". What is the generally acceptable frequency for accumulator cycling?
2. Based on what I described above, is it more likely that the leak is in the brake master or the accumulator?
3. Can anyone help by pointing me to some previous posts that break down the troubleshooting method for how to determine if its the brake master or accumulator?

Thanks in advance