Can I bleed brakes old fashion way?

The dependable answer is no. Only with a scan tool can you trigger the opening of various valves in the actuator that are otherwise closed and create dead ends that cannot be bled.

If you're very sure the only air that entered the system is still localized back near the rear brakes, you might be able to succeed in bleeding air just from those lines, without a scan tool. That would be a specialized exception to the general rule. There's no guarantee you'll get away with it.

You can't do it by pumping the pedal with the battery disconnected. That only uses the fail-safe hydraulic system, which doesn't include the rear brakes. So the system has to be powered. When it is powered, with a steady pressure on the pedal, you don't have to pump; pressurized fluid will come out the bleeder as long as you have it open (or until the reservoir is drained and air is sucked in up front).

Without being in the bleed procedure or in invalid mode, though, that excess fluid flow is likely to trigger a trouble code that will leave that line valved off.

 

I've talked to several people who have successfully bled the brakes the old fashion way, even when replacing brake booster pump. But when I go down this rabbit hole I intend to use an Autel device's bleed procedure and if that doesn't do the final calibration I'll use Toyota Techstream on an old laptop.

 

If you had the rear disconnected at the beam? Fill the reservoir up I'm assuming you let it empty fill it up go to rest of car wait for fluid to come it is coming. Wen it do spin one caliper on bleeder open rt side . Go left add caliper close it's bleeder . Let rt side drip 20 minutes . Close same on left side . Whaddya ya got? No pumping .

 

Not 'start'. Just power to the brake system (what we often call "IG ON").

You don't get errors if you are using a scan tool to make the bleeding happen, or if you enter ECB Invalid Mode first, using a jumper wire.

If you enter ECB Invalid Mode using a scan tool, you do get a code (C1451) that can only be cleared with a scan tool—by going through the bleed procedure, which is what you're there for anyway, so that shouldn't be a deal-breaker.

 

Yep gravity fills up whole actuator from mid feed to bottom and top of the aluminum block do fluid get in front and behind those opening valves that are not yet . I've lotsa issues with brake bleeding not happening no matter the scanner . Fails on first screen alot Park brake set ign on or ready and so on . Push next and fails fire it starts . But vehicles scan fine 8 can clear all take info etc do other things but not bleed . Had happened 4 times 4 different vehicles battling one today .

 

It’s not that hard.

 

Just follow this video, please don't make it too hard

 

Well he doesn't know what he's talking about but yeah this is basically manually bleeding . Dot3. I run 5 but yeppers

 

I asked chatgpt to translate what @Tombukt2 is saying. Here is its response:

<begin CHATGPT>

Because of gravity, brake fluid fills the entire actuator—from the middle feed down to the bottom and up to the top of the aluminum block. Fluid does get in front of and behind the valve openings, even though those valves aren’t being opened yet.

They are having many problems getting the brakes to bleed properly, even when using a scan tool. The bleed procedure repeatedly fails on the very first screen. The scanner gives instructions like “parking brake set,” “ignition on or READY,” etc., but when they press “next,” it fails. If they start the vehicle, it fails again.

The scan tool itself works fine otherwise: it can scan the vehicle, clear codes, read data, and perform other functions—but it will not run the brake bleeding procedure.

This has happened to them four times on four different vehicles, and they are currently dealing with the same problem again today.

</end CHATGPT>

Edit: "they" == Tombukt2

 

I love it... I'm gonna use AI to make sense of his writings in the future too!

 

I wonder what ChatGPT would make of followup questions like "so, do the same actuator valves that reliably block fluid flow when the actuator is pressurized to ~ 2000 psi also happen to pass fluid because of gravity? and where does the trapped air go?".

 

Anyone have a cutaway diagram of this type actuator? Lexus LS and a few other cars have iterations of this gen2 assembly

 

I feel like we almost need a new thread for this. But here are the new chatgpt answers (surprisingly good, I might add):

--------- <start round 2 chatgpt>

Yes — I’m familiar with the 3rd-generation Prius (2010–2015) brake-by-wire system, including the brake actuator/accumulator, linear solenoid valves, pump, and the Techstream bleed sequence. Your questions get to the core of why bleeding these systems is non-intuitive.

I’ll answer them directly and precisely.

1. Do the same actuator valves that hold ~2000 psi also allow gravity flow?
Short answer:
No, not in any reliable or useful way. If gravity flow occurs at all, it is incidental and incomplete — not a functional “open path.”

Why:

• The Gen 3 Prius actuator contains normally-closed linear solenoid valves that:
  
  • Isolate the master cylinder
    
    
  • Control pressure to each wheel
    
    
  • Separate low-pressure reservoirs from high-pressure accumulator circuits
    
• When the system is pressurized (~2000–3000 psi accumulator pressure), those valves are designed to seal extremely well.
  
  
• When the system is not pressurized:
  
  • The valves remain mechanically closed
    
    
  • There is no designed gravity-bleed path through the actuator
    
    
  • Any fluid movement due to gravity is:
    
    • Through tiny clearances
      
      
    • Around valve seats
      
      
    • Or via internal leakage paths
      → Not enough to displace trapped air
      

So while someone might observe fluid appearing at multiple ports and assume “gravity fills everything,” that does not mean air has been purged from internal chambers.

Key point:
A valve that can seal against 2000 psi does not suddenly become a meaningful open conduit just because pressure is removed.

2. Where does the trapped air actually go?
This is the critical issue — and the answer explains why Techstream is mandatory.

Where air gets trapped
Air can be trapped in:

• Accumulator chamber
  
  
• Pump inlet/outlet passages
  
  
• Stroke simulator
  
  
• Internal valve galleries
  
  
• Pressure control chambers upstream of wheel circuits
  

These areas are hydraulically isolated unless specific valves are actively energized.

What gravity bleeding does not do
Gravity bleeding:

• Does not open the isolation valves
  
  
• Does not cycle the pump
  
  
• Does not move air out of internal dead-end volumes
  
  
• Does not flush air past normally-closed solenoids
  

So the air just stays put.

What Techstream bleeding does
The factory procedure:

1. Actively energizes specific solenoids
   
   
2. Runs the pump
   
   
3. Cycles pressure repeatedly
   
   
4. Forces air:
   
   • Out of sealed internal cavities
     
     
   • Into wheel circuits
     
     
   • Then out through bleeders
     

Without that:

• Air remains trapped
  
  
• Pedal feels inconsistent
  
  
• ABS/VSC faults may occur
  
  
• Bleeding may appear to “do nothing” even though fluid flows
  

3. Why the scan tool often “fails” at the first screen
What the forum post you shared is really describing:

• The scan tool checks preconditions:
  
  • IG-ON vs READY
    
    
  • Parking brake state
    
    
  • Battery voltage
    
    
  • Actuator pressure state
    
    
  • No active brake DTCs
    
• If any condition isn’t perfect, Techstream aborts immediately
  

Common hidden causes:

• Weak 12V battery
  
  
• Slightly incorrect ignition state
  
  
• Residual pressure not released
  
  
• Parking brake switch misread
  
  
• Brake ECU detecting an abnormal pressure response due to air
  

This makes it look like a tool problem when it’s actually a system-state problem.

Bottom-line answers

• Do the actuator valves pass fluid by gravity?
  ❌ Not in any designed, reliable, or complete way.
  
  
• Does gravity bleeding remove trapped air?
  ❌ No.
• Where does the air go without Techstream?
  ➜ Nowhere — it stays trapped inside the actuator and galleries.
  
  
• Why does Toyota require a scan tool?
  ➜ Because only active valve control and pump cycling can evacuate internal air.
  

------------- </end round 2 chatgpt>

 

What people need to understand is that these Language Learning Models that we call AI are based on a massive amount of mathematical calculations to predict the best word in each sentence it compiles... Towards that end it's important to frame your question with the type of expert you want responding to your query so the math is as accurate as possible.

For example if you want to make sense of someone like @Tombukt2 who often posts incomprhensible rambles with zero proofreading and no punctuation and likely some kind of really bad speech to text system that turns his messages into nonsense you'd want to preface your query with: "Respond as an expert in deciphering poorly written, possibly voice to text gibberish, so I can understand what this person is trying to say."

Or in the case of your query you'd want to say something like: "Respond as an expert who has thoroughly studied every aspect of a Gen3 Prius brake actuator/accumulator who is also knowledgeable of all the different methods that people have used to bleed this system of air when replacing it."

 

This was my prompt which generated the answer I pasted in #15:

<begin prompt to chatgpt>

For your information, this discussion is about brake bleeding procedure in a third generation Toyota Prius. Not sure how much you know about that brake system, but are you able to answer the following questions: do the same actuator valves that reliably block fluid flow when the actuator is pressurized to ~ 2000 psi also happen to pass fluid because of gravity? and where does the trapped air go?

<end prompt to chatgpt>

For the original response from chatgpt, I simply pasted Tombukt2's post and asked it to translate it.