3rd Gen Prius/1st Gen Camry Hybrid Brake diagram

I'm just spit-balling here; but assuming no leaks or wet lines anywhere. Brake fluid level remains static - there must be either a sensor malfunction or internal leaky valve. If you find "crud", at the bottom of your brake fluid reservoir, There's a likely-hood some of it made it down into your ABS control assembly and is blocking a sensor or valve from doing its job.That's the mechanical issue side. I'm also assuming the electrical side is smart enough to diagnose itself for any internal errors on POST. Most of these things has "watch-dog" circuits built into them, but I haven't had a chance to take one apart and look at them.

 

I've asked if the mods can move #28 to some thread in the Gen 3 forum, because it is so specific to Gen 3 details, and the Gen 4 details are very different.

Morpheous will probably save a lot of time by looking in the New Car Features manual:

Toyota Service Information and Where To Find It | PriusChat

It doesn't have just the one diagram pasted above: it has versions of the diagram for all of the different normal and fail-safe braking conditions, showing what positions the valves are in and arrows for where the fluid is going, so it will answer a lot of those questions. An interesting detail about the SMC and SRC solenoids can be found here.

The stroke simulator, of course, is only in play in the normal modes: it lets your foot feel like you are working the brakes, while really the computer is working the brakes. In fail-safe mode, the stroke simulator must be blocked off, so that the fluid pushed by your foot actually goes to the brakes.

Gen 1, for what it's worth, did not have a stroke simulator cut valve. If it went into fail-safe mode, your braking effort just got split between the real brakes and the simulator. Oops.