... medicine man say don't close-um door on Gen II front pad voodoo rain dance ~

I do not know about the Gen III, but with my 2009 Gen II, I find it best to close (and lock) all the doors just before I start driving, particularly if I'm in a bad part of town!

 

What happens when you open the driver's door on your second gen? Do you hear a whirring sound? That's what you want to avoid. If the caliper is off when that happens, it could well pop a piston.

So I'd say keep the driver's door closed. That said, on third gen, I've had the calipers off a couple times, and yeah: just disconnect the negative cable at the battery works.

Also, no offense, but maybe edit your thread title?

 

Actually fortuitous this thread coming up, because I had the same sort of questions as I prepare for doing a brake service.

From what I understand, and please correct me if I'm wrong—because I don't know, and I want to know—but is only necessary to do one of these, not both, correct?

Even though asjoseph elected to pull the neg off the battery, I'd rather not.

Also, I'm not convinced one needs to do 1, 3, 5 and 6, but I could be a victim of my own ignorance. If it is necessary to remove the negative, why is it also necessary to then remove the 2 ABS relays; and is there another fuse or relay to disable the accumulator pump, if this is not disabled with the removal of the 2 ABS relays?

So, can some of the wise sages here answer this conundrum? Much appreciated.

PS, I don't have SKS, so I don't need to worry about the key fob. But I would have thought turning it to the off position on the steering wheel switch would suffice for that one.

 

asjoseph - I like your artistic writing style

Are we saying, as conclusion, that the only thing you really need to do is disconnect the 12v battery? If that's done, we don't have to worry about the keyless key being in the next county, the door being opened or closed before or during the procedure, disconnecting relays, fuses etc.? Logic certainly tells me that if the 12v is disconnected, we need not worry about any 12 systems doing anything at all. Because, how can they? Right - they can't. Can they?

Might this be a good moment to ask about the rear pads? Is the procedure the same?

 

The Prius has a capacitor storage system at the aux battery for 12V braking power. Traditional power brakes have a vacuum storage system for a couple of brake applications if the engine dies, this serves the same purpose for the all-electric Prius system.

In the US, we have rear drum brakes.

 

How long does the capacitor hold charge? Is there a way to manually discharge it?

I thought it was only the earlier Gen 2's which had drums in the US. Do they all? If so, is there a special procedure regarding disconnecting 12v, relays etc., when working on the drums/shoes? I would assume the same procedure would apply to rear disk equipped Prii.

 

Don't know, but I guess that's why it's recommended to disconnect the negative battery cable for brake work. I haven't had to do that yet, so I haven't paid it much attention, but I think that should address all concerns and it's very simple.

My '07 has rear drums. I don't know if later US Gen 2 models have rear disks. I hadn't heard that.

With the regen braking and my driving profile, my friction brakes see hardly any duty, so brake work is not on my radar. I have replaced the brake fluid a couple of times, and that's a little complicated since you need to run the actuator pump.

 

Oh PLEASE tell me (us) you did not push that old nasty brake fluid backwards into the HCU!

One should NEVER EVER push caliper pistons into their bores without the bleeder screw being opened to allow that old and stagnant fluid OUT of the system. This is especially true on vehicles with ABS and an order of magnitude more important on those with HCUs like a Prius.

Unless you have Techstream and can VERIFY the brake accumulator pressures are ZERO, it would be most wise to assume that the caliper piston(s) can eject themselves with GREAT FORCE and velocity at any time. Yea, I know that with the 12v disconnected it is theoretically impossible.... yet Bees do fly.

 

With 3rd gen you can put the car in "invalid mode" for bleeding, circumvent the need to for Techstream.

I've pushed pistons back without opening bleed screw being opened, for donkey's years. Not saying it's good, just worked out ok, so far. I do keep on top of brake fluid changes: every 3 years.

 

You may have a point, but if the brake fluid is "old and stagnant", would you not just do a brake flush after you've reassembled the brakes?

Do you care to elaborate with some detail for those of us who don't really know what you mean? Like the ins & outs and whys & wherefores.

Thanks.

 

all u.s. gen 2 have rear drums. 2004-2009. the only thing gen 3 rear discs introduced were problems.

 

Sure.

Q: Why open bleeders when changing pads?
A: Part 1 - Everyone knows about gravity and settling of dirt in liquid, it will go to the LOWEST point - the brake caliper or wheel cylinder. Also, while fluid flows INTO these parts, it does not recirculate, so the fluid and any contamination in it stays in these parts. Brake fluid is Hygroscopic - it readily absorbs moisture from the atmosphere which lowers the fluid's boiling point from 400*+ to something close to 250*F. Brake temperatures of 250* are a lot easier to obtain than one might think, making this a dangerous proposition.

Part 2 - Because brake fluids absorb moisture and some components are made of ferrous metals (steel, iron etc), as the moisture content in the fluid increases, RUST becomes a potential problem. On almost every road-going vehicle, the calipers are made of cast iron, when combined with a moisture rich fluid, you get rust formation on the walls of the caliper bore. This rust is very slight and typically does not cause pitting, but when you push the pistons back into their bores, this rust coating is wiped off and forced backwards into the rest of the brake system (ABS unit, HCU and Master Cylinder). None of this is a good thing and unless you like very expensive repairs and so is something to be avoided at all costs.

Q: Why ensure the HCU pressure is "0" prior to brake system work?
A: Pressures in the HCU can be 2000 psi an more. These systems are designed to apply brakes ELECTRICALLY during NORMAL operation and based on brake rod stroke, not on the actual pressure applied to the brake pedal - the PRESSURE generated is not by your foot, but by an electric motor. The brake fluid is pressurized by a pump and retained in an accumulator at nominal system pressure AT ALL TIMES. This means that full, unregulated brake system pressure is available at the speed of electrons, WITHOUT moving the brake pedal. There are safeties built into the system to: Provide braking should the HCU system fail, minimize the risk to service persons, and provide alerts in the event of a malfunction. Not a single one of those safeties are "fool proof", therefore it is *possible* for an unintended application of FULL SYSTEM PRESSURE to be made while a caliper is removed for pad replacement. This would result in in caliper piston(s) being ejected at high velocity and under extreme pressure; a VERY dangerous situation that could easily KILL.

Now, if you add pushing old stagnant brake fluid backwards into the HCU, where the potential for minute rust particles to jam parts of the HCU's valving to having full system pressure stored in the accumulator, you have the potential for a VERY DANGEROUS situation.

 

Thanks John, appreciate the time and effort you have taken in relying.

Cheers.