Why has the brake system internal check valve failed and the hybrid stopped?

@SolarDarren Did you even find out more about this and did the problem reoccur? I have exactly the same problem with a 2009 Escape Hybrid. In late 2016, I would get the check brake system error but not consistently. I took it to a Ford dealer repair shop who diagnosed the problem (and charged me to diagnose) as needing a new ABS vacuum pump so I had them replace it ($500 part and $230 labor). The problem reoccured a couple of months and about 2000 miles later. I had the vehicle at a non-dealer shop for unrelated work (new AC compressor) in April 2018 so I asked them to check on the ABS warning issue. That shop said there was a blown fuse in the for the ABS vacuum pump and replaced the fuse. Less than a year after that, the warning light now goes on again intermittently. This time I finally figured out that when it happens the electric motor will not kick-in for hybrid mode as you describe. It also tends to happen at low speeds or during stop and go traffic. Never at constant highway speeds. Also, actual braking has never been effected despite what the warning says. This has led me to believe that there is some issue related to the regenerative braking and the hybrid mode. This seems like an ambiguous issue I'l again have to pay a not insignificant amount of money to get diagnosed with a resulting repair that might not be to actual fix. Therefore, I'm trying to get information through online searches and came across this post. (I've not found any other similar posts so far but I'm still looking.) My hope is to be able to go to a repair shop or even a dealer with some more concrete information.