I'm hearing my brake actuator/accumulator leak down after the accumulator motor stops. I'm hoping to understand the system a bit and to fix it myself. The questions I can't find the answers to are: What component or where is the valve responsible for keeping pressure up? Could the sensor responsible for measuring fluid pressure fail? If so, is it possible the sound I'm hearing is excessive pressure being released?

The actuator was replaced before at the end of 2011 and 90k miles ago.

 

The first thing I would try is a brake fluid flush.

 

I'll give it a shot.
I think I just got my TIS working (Windows 10 X64).