Lots of bad information in this thread. Let me start by explaining how the hydraulic system works and my personal experience with poor braking in regards to my 2007 4wd sr5. This is not meant to step on the toes of previous posts, but should clear things up. I hope this can act as a reference for other members as theres a ton of assumptions as to how this system works, but not a whole lot of published information.
Here is a very brief explanation of how the hydraulic booster works: Brake assist force is created by pressurizing an accumulator (basically a small tank) with brake fluid. When you depress the brake pedal, pressurized brake fluid is released from the accumulator and helps with braking force. An outright failure of this system would result in a very firm brake pedal with very little travel. All the braking assist force lost would have to be supplemented by means of your leg.
My experience - About three months ago I noticed that my runners braking pressure and pedal travel became very inconsistent. There didn't seem to be any logic or correlation as to when the pedal feel would be ok or when it would require more force or the travel would increase. So I did the go-to first steps without any real diagnosis. I bled the brake system multiple times and activated abs on damp roads, but this didn't seem to help. I knew all of the calipers were fine as I replaced them all just less than a year ago.
I began researching how the hydraulic system works and decided to buy a m-vci cable and install techstream. Since there wasn't what I would consider a hardware issue from the brake lines at the hydraulic booster, down to the calipers, I assumed there must be something wrong with the hydraulic booster.
With techstream I began logging vehicle speed, master cylinder pressure sensor voltage, accumulator pressure, and brake pedal switch state. I logged this to and from work for a total of 3 hours and then parsed through the data. What I found surprised me, and taught me volumes about the system and logic used to control it.
The data showed the electric motor pressurizes the accumulator with brake fluid until the accumulator pressure sensor reaches 3.8 volts. When the accumulator pressure sensor dips below 3.2 volts the electric motor cycles again until the 3.8 volt conditional is reached. Therefore, we can deduce that any time the accumulator sensor voltage is between 3.2 - 3.8 volts we should have all the braking force the toyota engineers designed into the system. Additionally, if the voltage drops too low, as in the event an accumulator or booster motor failure, an alarm will sound indicating a brake system failure and a DTC will be set.
Well bummer, it appears everything is in range… hmmmm. Knowing that data doesn’t lie I had to revaluate my approach. During college, and for 3 years after, I worked as a technician for Honda. I began racking my brain trying to think of interesting braking cases. I distinctly could recall a pilot that came in complaining of inconsistent braking. I test drove the car and the right front wheel bearing was so loud it sounded like a helicopter. I racked the car and there was almost a quarter inch of play in the bearing. The advisor sold the wheel bearing citing it as a safety issue and told the customer the brake diagnosis would follow afterward. I certainly wasn’t going to be driving this vehicle until that bearing was replaced. So I replaced the bearing and much to my surprise the inconsistent braking issue was resolved. Play in the bearing allowed the pistons to be depressed and subsequently when braking more travel was required to push the pistons out of their bores.
Remembering this, I decided to check the wheel bearings and found the left front had failed. It had significant play in it. I replaced the wheel bearing an my braking issues were resolved.
Lessons learned or reinforced:
1.) A bad wheel bearing can manifest itself as a braking issue
2.) Wheel bearings can fail quietly. (I’ve replaced tons of wheel bearings and never have had a sealed unit bearing type fail quietly.)
