Update on Brake ABS master cylinder replacement on 2016 Prius. Installed nearly new used master cylinder due to actuator pump running too much in "ready" mode sitting still, and while braking. (Thinking it's similar to Gen 3 in how it works). I did not replace the accumulator/pump yet, just master cylinder. Afterwards, now in Ready mode in PARK..sitting still no braking there are no actuations from the pump after a minimum of 10 mins which I think seems good, but stepping on brake it activated every 2 presses and while moving more than average pump cycles a lot still compared to other Gen 4's of the same vintage I've tested. With car turned OFF the pump actuates almost EVERY press of the pedal. Again..seems too much! I can't believe this is so hard to diagnose here, the cars have been around for 9 years now. Could the system is still leaking pressure if it actuates that much and how much is too many?Other Gen 4 cars pumps are nearly silent and not re-pressurizing as much. These apparently are not the same as Gen 3 as the car gets 62 mpg easily, and there's no codes for the ABS...safe to drive it and forget about it? Just seems excessive.
How confident are you that you got all the air out in bleeding? Air bubbles cause more brake fluid to move every time you use the brake. Unlike the fluid, the bubbles are compressible, so the target pressure isn't reached until more fluid has been sent out from the accumulator. When you release the pedal, that much extra fluid gets returned to the reservoir as the bubbles expand again. Because of that effect, an incompletely-bled system uses more fluid from the accumulator with every use of the brake, and it takes fewer uses of the brake to use up enough accumulator pressure to run the pump again.
We used Autel with Tech Stream to do all the tests and bleeding after install, not sure how that would fail to do the job?
Autel and Techstream? That's dedication. I'm not saying it failed to do the job, but of course there's human judgment at every step (did I get all the air out at this bleeder? am I ready to go to the next one?). So it's definitely possible to bleed brakes incompletely.
Which did you use? Autel makes some standalone devices, and also some Bluetooth devices that work with an Autel app on your phone. Techstream is Toyota's official software, runs on a Windows laptop. As far as I know, they both can do brake bleeding ok. I have used them both (at different times) to do the job.
I'm not sure there is such a thing as "as thoroughly as possible" for brake bleeding. It's like body work (which I also dislike). You could always have spent more time, run more fluid through, prepped and sanded and polished longer, ....
Use your scantool to watch ABS data. The ecu uses the ACCumulator pressure sensor to control when to turn the pump on or off. How much does that sensor drop in 10 minutes? Press the pedal until the pump turns on. How much time does it take until it turns off? Push the pedal part way and hold it still - does ACC pressure now stay steady or does it continue to drop? If you have access to another Gen4, compare all of this data with the same tests performed on your "known good" car. You could have an internal leak downstream of the normal high pressure valves, or the accumulator can have "low capacity", or something else is "using more fluid" (think air in the system or seized caliper pin- too much caliper movement). Posted via the PriusChat mobile app.
I'll have to dig into the software and read those values to see what you mean, but there could be something there. Had the dealer (ahem) tell me it sounded normal and only will "chirp" when the pump is going bad. I sat in 2 other Gen 4's and they both were as frequent under the same conditions as mine "Downstream of the normal high pressure valves" which are at the pump on the passenger side or further down like as in where? Just did all the brakes (rotors and pads) and regreased the guide pins, highly doubtful the issue is at the hubs but MAYBE..
I use the term "normal high pressure valves" to describe the parts of the brake system that contain high pressure fluid when you are not stepping on the pedal. Many actuator faults are due to fluid leaking somewhere in that section of the system - resulting in pressure bleed-off (all the time) and pump cycling. It is also possible to have internal leakage in the hydraulic circuits that are only pressurized when the brakes are applied. This would "use" more fluid and cause the pump to cycle more often, but not when just sitting there. Posted via the PriusChat mobile app.
It seems to cycle more after coming down a long hill on the brakes and then quickly parking and listening for it. Much more cycling than if it were flat ground. The "stealership" "service advisor" said his master tech said that these will chirp when they are going out and about to fail...I'm more frustrated at the lack of knowledge as a whole on this topic for the Gen 4, someone out there must know and have been a tech who can answer definitively. I appreciate trying to trouble shoot.
