Hi Folks! 2009 Prius, 90,000 miles When braking, as I pass through 7MPH, the brakes seem to release momentarily, then reapply with a clunk. This always corresponds with the point at which I understand that the car switches from regen' braking to friction braking. It never happens when the transmission is in neutral, disabling the regen' braking, and it's not the Traction Control issue - I'm used to that now I've connected a Code Reader, and there's nothing showing, (except a misfire on Cylinder 1). The brake pads are virtually untouched. I've tried cleaning the discs by braking from speed in neutral, to no avail, and now I need help! I have the feeling that this is probably a "stock" fault, so I'm wondering if you clever folks can point me in the right direction. Thanks a lot
Is the phenomena easy replicable? If so, maybe get a dealership mechanic to test drive? What's the service history on the brakes? Are they regularly inspected, say every couple of years? Has the brake fluid ever been changed? The above may or may not be related, just good to know.
That is about the speed where that transition happens, but it should normally be just about imperceptible. I would check for free mechanical motion of all the brakes, and also try a bleed and see if any bubbles can be removed. Bubbles in a Prius brake system can cause somewhat different symptoms than in an older car. It used to be, you could only push the brake pedal so far, and if there were bubbles, you would use most of the travel just compressing the bubbles, and only have a little bit of brake force left with the pedal down near the floor. In a Prius it's not like that, because there's an accumulator full of high pressure brake fluid at the ready. The car will just hold a valve open long enough for enough fluid to whoosh into the line and compress any bubbles and reach the target pressure anyway. So you don't necessarily have a different pedal feel or loss of braking, but there might be some extra fluid whooshing that you can sometimes hear, and the timing for the target pressure to be reached might be a split second longer, and that could make the regen-to-braking handoff feel less seamless. With large enough bubbles, the ECU will notice it needs to hold the valve open too long to reach pressure, and it will set codes. But there's a certain threshold to cross before it will actually complain. -Chap
