I'd like to share my experience with my brake accumulator noise problem and how I got it fixed in an unconventional way so it may benefit someone in the future who may encounter the similar issue:
My SUV: 2007 4Runner, 2WD, SR5, 142,000 miles
Problem: Brake accumulator making constant LOUD whining noise (with a frequency of every 2-3 second) whenever driving. This whining noise has been persistently bothering me for a couple of weeks day-in day-out. No warning light on the dashboard and the braking power actually feels normal.
Potential root cause as I learned from this forum and other online resources from the most obvious to the least likely:
1. excessive air in brake lines or brake system components;
2. a broken o-Ring (it can be ordered with under $5 from any Toyota dealer) between brake accumulator and the ABS boost pump.
3. a jammed spring and bent pin (parts can be ordered with under $5 total from any dealer) between the accumulator and the ABS boost pump;
4. a faulty accumulator (~$380 from dealer as of July 2017);
5 a faulty ABS actuator assembly (basically accumulator plus ABS boost pump) ($1600 from dealer);
6. a faulty Brake Master Cylinder (extremely unlikely and very expensive to replace. as of July 2017, the quoted price from dealer for this BMC is $2700).
Action Taken to have it fixed for $150:
1. I flushed the brake fluid and bled the lines - no help at all.
2. I then ordered the cheap parts (O-ring, Spring and Pin mentioned above) from the dealer and prepared myself mentally for the repair procedures (it's not an easy undertaking for me as an entry level self-taught cheap handyman)
3. I reluctantly followed my wife's advice to have a Toyota dealer run a formal diagnosis for my 4R without knowing the diagnosis is not free (I thought it's free!)
4. I Rejected dealer's proposal: The dealer concluded there's a kind of internal gasket leak in the ABS actuator assembly which must be replaced at the very minimum ($1600 part and $750 of labor). They did recommend to replace the entire BMC ($2700 part plus $750 labor) just to be sure to take care of the problem.
5. I declined dealer's alternative proposal: the dealer is kindly willing to replace just the accumulator per my request under the condition of my acceptance that it may not fix the problem. The cost is $1130 (part $380 for the accumulator, labor $750). I was struggling with the thought "What's the point to pay this much money without any kind of assurance?"
6. I Paid $140 diagnosis fee to the dealer (who informed me I can actually use this $140 toward any future bill if I come back later for this issue) and Decided to order a new accumulator online or a used ABS actuator assembly online so I can do the replacement myself. I will start with the cheapest option 1st and don't mind going through the replace procedure multiple times.
7. The Unexpected happened: On my way driving back home from the dealership, my 4R was whining/squeaking/buzzing as usual. But after about 5 miles of driving, the noise all the sudden miraculously stopped!! I thought it's just temporary but after 4 days of extensive or aggressive drivings, I am convinced that the technician who performed the diagnosis have "accidentally" made the noise go away. My brake is as good, as effective, as confident as before. Crossing my fingers...
Speculation:
The technician at the dealership must have done something during the diagnostic procedures to make the noise go away. I could think of two steps he may have performed during the 1 hour work: bleed the brake fluid again and re-set the ECU to just root out these two obvious possibilities with his own expert hands. It might be the ECU freaked out in my case so it keeps sending signals/commands to charge up the accumulator constantly. What else he may have done? It doesn't make too much sense to a logic mind, but doesn't matter at all as long as the issue is resolved and as long as it doesn't come back any time soon.
Well, that's my real experience with this "expensive" problem "resolved" for $150 ($140 diagnosis fee + $10 parts I never used). This is a rare case where a "formal diagnosis" may actually solve the problem being diagnosed. Lucky me, it actually saved all the toils I have planned to go through myself with the repair work in the coming weekend. Hope it helps. Good luck to all.
