Aggressive brakes?

It is often reported that the G2 Prius and maybe others as well, can suffer from "aggressive" brakes while stopping. After much reading on PC and elsewhere and with a good understanding of hydraulic brake function, I decided to embark on a mission to discover why this would be.

A few relevant facts:
- Prii do not provide stopping power based on brake pedal PRESSURE, but rather brake pedal DISPLACEMENT
- The HCU style of brake control is mostly electric
- Prii can suffer from the same wheel-end maladies as any other vehicle (loose bearings, stuck caliper, shoe adjustment, etc)

As our 05 Prius displayed this annoying behavior, but not overly so, I decided to make a few checks. All wheel bearings were found to be serviceable with no abnormal noise or play in them - jacking up one wheel at a time We use the parking brake regularly, and it holds solidly well above reaching the floor, so rear shoe adjustment is good (I should have verified this when I checked the bearings, but was not thinking ahead). Lastly, to eliminate the potential of the front caliper slides being sticky, I decided to make a FULL brake application while parked. Upon moving the car, the aggressive brake application was IMMEDIATELY remedied, it was obvious that what ever had been causing it was no longer in play.

I suspect that because most, if not all stops, are with a very light (low pedal displacement), the inboard front pads are doing most of the work. This is actually quite common on vehicles operated "conservatively" and something experienced mechanics could look for in terms of uneven pad wear.

One thing to mention about a FULL brake application is that if you have rusty brake lines, or degraded hoses, you WILL be exposing them to full system pressure - something not common except during emergency braking maneuvers. Therefore, it is possible to burst a line or hose, if it has been weakened by time and the elements. I however, would rather know this while parked in my driveway than as I am passing under the rear bumper of a big-rig

In the end, uniform braking performance has been restored and a closer inspection of the calipers and caliper slides is on the schedule for the next available opportunity.

 

To my knowledge, they are NOT cross split (Opposite F&R like Volvo and a few others), but have a traditional F-R split.

 

Exactly!

But it is not as bad as before ABS, because at least you can get the greatest effect from the rears possible, instead of just skidding uncontrollably to disaster.