C1391actuator replacement

That sound there is the electric pump that raises the brake fluid pressure, and it should not need to run nearly that often. It only runs when the stored pressure drops below a limit, and then the pump runs to raise it again.

Normally, you use a bit of pressure each time you use the brakes, and after every few times, the pump will run again. It should hardly ever run on its own like this if you are not applying and reapplying the brakes. Repeated pumping with no use of the brakes means the pressure is leaking down rapidly for some reason.

The sound isn't really from the replacement actuator, assuming we use the established names. There are two things under the hood there;



The one on top ("brake booster assembly") is what's also called the actuator, and it presumably is what you replaced.

The one below ("brake booster pump assembly (accumulator0)") is what makes the noise. But that doesn't mean it has to be what's at fault.

Usually, these cases where the pump has to run often are caused by internal leakage by the valves in the actuator (the thing you just replaced). The fluid leaks back to the reservoir from there, and so the pump has to run repeatedly to keep the pressure raised.

It seems possible the eBay actuator may have an internal leak. It could also be possible that the pump/accumulator assembly needs replacement, but that seems to happen less often.

 

Luckily I'm not at this point in any of my generation threes and they're getting phased out as fast as I can get them out of here or people out of the vehicles. A mechanic stethoscope may help you here but you will have to be quick and try to listen in between pumps if you will You're listening for liquid movement try it on a few things first to see if you can understand what I'm talking about and then you want to listen to the actuator and then you want to listen to the accumulator and pump assembly below it or whatever you want to name them I don't care You're listening for fluid making a leaking sucking noise when the pump is not running now determining whether that leaking is actually in the accumulator pump assembly or whether it is actually leaking from the actuator above is very tricky That's why they want you to spend the 15 or $1,700 for both pieces It is recommended to replace them both I do believe. That's why in the generation 2 there's only one part Toyota basically had it right in that situation. But even the $1,700 is a bit much to stomach for a 20-year-old car almost Toyota and others have really got this right I think 16 on up this piece sits over on a fender well I see them on other cars the pump and accumulator it's rebuildable you can buy them from rebuilders for 150 bucks so on and so forth it seems to me the Prius is kind of been embellished with some nonsense as far as this brake part business goes so there's always that kind of fun business but once you replace those two parts it seems you're good again for the life of the car at least I mean you're putting these things on at 200k where do you think you're going to take this car a generation 3 to 600 I don't think so No about 425K and you'll be out of that car. Or sooner

 

Under these circumstances, it is most likely that the leakage has been in the actuator all along, which has not been replaced. The pump/accumulator is what vibrates and makes sounds when the pump runs, yes, but the reason the pump has to run is a loss of pressure (between runs of the pump) as fluid escapes back to the reservoir through a valve that is not fully sealing. The actuator (part not yet replaced) is the most common site where that happens, because it contains about a dozen valves.

Trapped air in the brake lines would have a fixed volume, and it can reduce the number of uses of the brake you normally get between runs of the pump.

But if the pump has to repeatedly run while no one is using the brake, the cause is not a fixed volume of air in some fixed location; the cause is fluid, pumped into the accumulator by the pump, circulating back to the reservoir through internal leakage.

The repair manual describes the specific thresholds that trigger a C1391 code. Probably the old unfixed leak, with a newer pump and actuator, is not quite reaching the threshold yet to re-trigger C1391. The reappearance of that code may not be far off.