MellonC00

Twice this week I had incidents in which I had to stomp on the brake from about 25 mph to mph 0 right away. Normally my car is perfectly good at coming to a dead stop with just a hint of tire chirping (abs kicking in). These two times, however, my foot was already slowing it down, say, with a 25% force, until I saw the car cutting in or turning into my lane and stopping in front of my car, the point in which I went to 100% force to come to a complete stop. But for a 1/2 second or less, the brake pedal went all the way to floor until it decided to brake. The first time it happened, I thought I hit the car. it was so close. The second time, the car that cut in had the sense to steer away to miss me. Otherwise, I would have hit him.

Is this a hybrid regen braking switching over to brake pads? Is this a known issue?

I'm not going to the dealer, btw.

703

salimshah

Pedal going to the floor is a sign of bad hydraulics. If you dont see a loss of fluid [either drop in the level or trail on the garage floor] then it is the master cylinder.
Salim

Droid13

There are a couple of known hybrid quirks when it comes to braking, loss of traction on gentle to moderate braking causes a very short loss of braking as ABS turns off regen braking a split second before the mechanical brakes are applied. The second is in the rare occasion where the brake pedal goes hard and braking effort is very high usually when backing up from stop after a start whilst having foot on brake the entire time from starting. Releasing and reapplying the brake makes it go away immediately. These don't sound that similar to what you describe.

ukrkoz

Your brake pedal is connected to master cylinder that has brake fluid pressure sensor in it. It's brake pressure stroke sensor. It tells computer how much braking force you request and THAT tells brake actuator how much force to apply to hydraulic pistons in calipers AFTER regenerative force was not enough.
IF the electronic component fails, only THEN master cylinder directly applies braking force. Safety feature in case electronics fail.
I say either your pressure sensor goes bad or MS goes bad. Either way, CAUTION and urgent assessment is suggested. It's brakes. Also, double driving distance to car in front of you, just in case.

MellonC00

Well noted. I'm going to try to recreate this problem this week and see if there is an exact pattern I can reliably duplicate.

salimshah

ukrkoz explanation is about how to divert braking from hydraulic to regen, BUT since you [MellonCoo) stated that "the pedal goes to the floor" ... you should not be try to reproduce. Remember the regen helps slow down and stop is by hydraulics only.

Salim

takeshi74

You should have someone take a look at it. It doesn't have to be the dealer.

mcomer

Try isolating the two braking forces, straight mechanical and the regenerative by shifting into neutral
while braking. (Clear road/no traffic obviously!) When working normally the brake pedal force needed
will increase because the regen has been disconnected. This will let you analyze just the hydraulic
and mechanical aspects of your system. Loss of brake boost will manifest itself as a very hard pedal
which is not what the OP described. Pedal to the floor is master cylinder or brake lines as posted by
others above. Are hard braking attempts also pulling hard to one side? Brake system warning light(s)
coming on?

salimshah

Hydraulic testing can be as simple as standing on the brake pedal with vehicle "off". You can try pumping the brake too. While running one perceives a drop in the stopping point of the pedal as the assist kicks in, but the pedal must stop 1-1/2" to 2" above the floor [one should be able to slide the left toe underneath the fully depressed pedal ... safety requirement]

Salim