How does my 2001 4Runner Limited get "power brakes"?

Hi folks, just to clarify why I'm asking a simple sounding question, a little back story is in order.

I'm chasing a "Too Lean" trouble code, and suspecting a vacuum leak, so was checking all my vacuum hoses, expected to see one running to the power brake booster. Well, my 4Runner doesn't have a "typical" brake booster with the big diaphragm vacuum assist unit.

See the pictures of what I see where the brake booster is usually found. Can someone that knows, explain what I'm seeing? I'm sure the lower unit is the ABS, but how does the power brakes happen without the vacuum boost unit? Does the ABS unit combine the power assist function as well?

Thanks for entertaining a probably elementary sounding question, but it seems my 2001 4Runner has several features that are different from other 2001 4Runner models.

 

What I do know is the unit has a electric motor and a nitrogen canister that creates the power braking component of the system.

 

I've been trying to search online for any write up or video that explains how this relatively "old" Toyota brake system works without the vacuum booster unit, but every video I have found regarding 2001 year model 4Runner brakes has the vacuum boost unit and not the same unit mine has.

I'm drawing some conclusions about this system based on the dearth of information on this unit, it wasn't installed widely on 3rd gen 4Runners - perhaps only on Limited models with an optional package that included the brake system upgrade (?), and that they are pretty reliable units since there's practically no videos or writeups dealing with "common issues" or how-to videos on troubleshooting/replacing these particular brake units.

Thankfully I don't have any problems with mine (that I know of), the brakes work well, and I know the ABS works, it recently activated during a rainstorm and I had to brake hard just recently. But I am still curious how the unit actually provides the brake boost function.

 

It has a nitrogen canister on the top left side that is charged with a electric Motor that is underneath the ABS unit

 

Ya, I see the nitrogen cannister there, and all the electrical connections to it, but I guess what I'm looking for is how does it actually work, and what maintenance does it require if any, because frankly I've not done any maintenance on it except to change the fluid a while back.
Does the nitrogen cannister require recharging? What if it develops a leak, how to tell if its ok? If it discharges, do the brakes stop working, or do they become hard like old manual brakes?

Lots of questions.

 

Really no maintenance other than a fluid flush when it’s gets dark in color. Canister cannot be recharged so far that I found. To tell if it’s ok is kinda two fold if you have the hood open turn the key to on you will hear the motor kick in for a few seconds then turn off that’s normal operation. I have yet to have a canister fail but I am sure the brakes will become hard as it still functions like a vacuum booster.

 

For ATRAC to function, the system needs to have predictable, high pressure brake boost so Toyota uses an electronic boost system in lieu of a vacuum boost that you see on most cars. The booster system itself is zero maintenance and brake fluid in the reservoir and brake system should be serviced just like any other system.

You replace the boost unit when it fails and you'll know because braking will be even firmer than usual and it's going to feel like it doesn't stop as well.

 

Thanks for that information. Now you got me curious how the electronic boost function works, as that seems like a separate function from the ABS. The brake boost needs to regulate braking pressure based on the amount of foot pressure applied to the pedal, as opposed to modulating the pressure to prevent wheel lockup.

 

All three of the common booster types are controlled by pedal position and are just a force multiplier. They only really differ in how they build that pressure and how much pressure they can build. They're separate from ABS/ATRAC which actually modulates the line pressure from the master. ABS reduces pressure on the brakes to avoid locking while ATRAC adds pressure to clamp down on the brakes.

It's been like 15 years since my Toyota training so I don't remember exactly (I was the drivability/electrical specialist anyway) but there was something with how the ATRAC system worked that didn't like vacuum boosters... it's why in the Taco, for example, the ATRAC and MTS trucks have the electric booster while those without still use vacuum.

 

Thanks! I appreciate the more detail responses. Even if I still don't feel qualified to make a diagnosis on this newer unit, my Limited does have the ATRAC as well as the ABS, so it makes sense with the newer (at the time) ATRAC technology, there was a reason for the change.

Thanks again!