I have a general question for the gen 3 drivers. How active is the pump on your brake system? I was getting pumping every 20-30 seconds even when not using the brake when I bought it. Now with a new accumulator it pumps up when I open the drivers door and might pump again after unsung the brakes a bit but it’s not like before. I test drove the car around 11 am and now 4 hours later it pumped up when I opened the door all I can compare it to is my original system in my 2007 that hardly seems to pump I was warned about replacing both pump and accumulator and took the risk. Thanks
I may be weird, but I'm counting seconds after opening the driver's door for the first time, as the pump is building vacuum up. If I count something around 15 seconds, as long the pump works, I consider it "normal". Sometimes I come to the car after parking it for, say, 2 hours. The pump starts pumping, but it takes less than 15 seconds to stop pumping - again - normal, as some of the vacuum remained in the accumulator. My two cents Mat
Hmmmm... If it wasn't you, I would be sure, you are not right... But it is you, ChapmanF I always thought, the brakes work with underpressure from the intake manifold on "normal" cars. Am I totally wrong? Or is simply only Prius or hybrides using positive pressure? Mat
A Prius can't use intake manifold vacuum, because it would be unacceptable to have brake power assist only when the engine is running. In vacuum-boosted cars, you see a large unmistakable booster drum behind the master cylinder. It has to be that big to have a diaphragm with great surface area, because the maximum pressure a vacuum-boosted system has to work with is the outside atmosphere, 14.7 pounds per square inch (less than that above sea level). The Prius, by contrast, uses an electric pump to force brake fluid into an accumulator that has a trapped high-pressure nitrogen gas charge in it on the other side of a bellows. Forcing the fluid in compresses the nitrogen even further, until the fluid is at a pressure around a couple thousand psi. With that kind of pressure to work with, the booster doesn't need to be a big drum anymore. It's just a small chamber in the master cylinder bore behind the piston. You would overlook it if you didn't know it was there. (I've posted that picture before but I'm having trouble finding it at the moment.) Another nice thing about the positive-pressure approach is the amount of reserve. In a vacuum-boosted system, if the engine stalls, you may get about two or three uses of the brake before all the vacuum in the drum is used up and you have non-assisted brakes. In the Prius, if the electric pump fails to run for some reason, you can get 20 or 30 uses of the brake before the accumulator pressure is all gone. (Normally, the pump will run again to replenish it after just a few uses.)
Sometimes it's nice to be mistaken I learned something new about Prius - thank you! Additional question - are the diesel cars out there all working with positive pressure too? Mat
