The problem comes when bleeding brakes when a problem comes.
Almost all systems can be bled with standard procedures. The systems with accumulators will not be bled in this way but can be manually operated by backprobing.
We recently had a ASR version A6 Audi with a sticking caliper at a rear wheel. When bleeding we could get almost nothing from that circuit. We went to the hydraulic actuator and grounded the apropriate pins that the controller would to activate the valves. Each one of the valves made a distinct clunk when activated except the one we were interested in. An ammeter showed that the current was the same. This indicated to us that the oiston was jammed. Probably in the hold state. In new actuator fixed the car.
Getting all the old fluid out of the accumulator would probably require such techniques or a scan tool.
Bosch Master, ASE Master, L1
33 years MB technician