Problem with my brakes.

In reverse at the speed you would be traveling only the hydraulic brakes would be working as regen braking only works above 7 MPH. If you hit the brakes hard the brake system does have an emergency brake assist and it is possible this is why you came to a sudden stop. If there was a problem with the brakes they should have set a fault code. The actuator pump sound is normal, it is possible if your 12 volt battery was low due to your being parked with some load on the battery (radio ect) the actuator pump is driven by the 12 volt battery and the pressure could have been low thus giving little assistance.

 

Interesting indeed.

You were reversing so as long as you were under 7mph, there would have been no regenerative brakes anyways as it is under the speed cutoff.

So the hydraulic brakes should have been engaged at various pressures the whole time. The system should be primed and under pressure at all times. The car even does checks on the pressure in the lines routinely while off and while on. it is also done everytime you open the driver door.

Just throwing out possibilities, but could your leg perhaps have fallen asleep? Then your sense of pressure applied would be wrong until you jammed your leg instead of your foot. I have heard of it happening before, and 2 hours sleeping in one position upright, would be enough for my leg to fall asleep.

If there was a recorded anomaly in the braking system there should have been a code thrown and a triangle sent. So either there was no anomaly and it was responding as normal, or there is a severe fault that prevented a warning even with all the check systems in place.

With the brakes you do have manual control with the hydraulic line even if there is no power. Of course this only applies if there is correct pressure in the lines or you are just moving air around. Have you at any point had the brakes bled? Done yourself or taken to a non-Toyota mechanic for brake changes or anything like that? The Prius system is unusual in that it is never dead when off. You have to pull some relays to kill it, then you can do your brake job. This conflicts with 99.9% of the vehicles out there, so your average non-Toyota mechanic and the majority of DIY'er don't realize this. Best case, nothing bad happens and you do your change inbetween random and live pressure checks. Worst case is it shoots a caliper through your hand.

 

To check the hydraulic brakes out, find a quite section of road, drive at 40 MPH select neutral and brake normally, "this will cancel regen braking". If all is well the car should come to a normal standstill under full braking control. If there is a problem you can shift into drive and brake again this will bring in the regen brakes.

 

OP's description seems to fit an infrequent but persistent pattern of reports here. It is as if the power assist / brake-by-wire system momentarily fails, without triggering any warning codes. This is different and far less common than the forward 'brake surge' or 'bump dropout' issue that prompted a recall of the 2010s (and still exists at a lower level), and the reverse grabbiness / over-sensitivity issue. But similar reports keep popping up here, without resolution. I had two events in my 2010, but resolved them by cycling power before thinking about observing or collecting enough information.

Federal safety regulations define how well brakes must perform when any portion of the system, including the power assist, fails. With a certain (brute force) pedal pressure, the vehicle must still stop within a certain (long) distance from a certain (highway) speed.

By standing on the brake pedal, OP did the right thing, manually pushing through to the underlying hydraulic system which worked as required. Other posters here indicate that the brake pedal fell close to the floor and felt dead, but didn't give a useful indication of how much pedal force they applied. Some sound as if they applied only regular braking force, which is quite inadequate for manual override braking. Others may or may not have been at or near the 112 pounds of the federal test.

In previous posts, I kept harping that in such events, drivers must apply at least 112 pounds of pedal force. In any car, not just in a Prius. Failure at that pressure is prima facie evidence of violating federal safety regulations, which would force a recall. Failure at lower pressure can be a legal operating mode.

Lesser failures could also trigger a recall or TSB or design fix, but reports and evidence are still too scarce to point to anything. So please consider filing a defect report with NHTSA.

When I have more time, I'll dig up links to previous discussions.