Hydroboost bleeding process

[Bigbertha78]

Today I finally got around to flushing all my old brake fluid out and bleeding out all four calipers until there was clean fluid. Went out for a drive and even slammed on the pedal to see how the ABS activated and all is great. I have a 2004 Tahoe Z71 with hydroboost. While driving I can quickly tap the brake pedal without stopping or really slowing down and the pedal kind of has a lot of play in it, hard to explain exactly what it feels like. One question I have is how to bleed or flush the power steering fluid thru the hydroboost system without draining all the fluid out and making the power steering pump bubble up? I had to replace the hydroboost system back 3-4 years ago because it had a bad leak, never had any issues since then. Looked up a lot of different videos on you tube but not sure if it can be done without draining all the fluid.

 

[Scottydoggs]

you need to jack the front up in th eair so both front wheels are in the air. engine running pump the brake pedal 20 times, then turn off the engine, but leave the key to run and turn the steering wheel lock to lock 15 times.

the work truck has a small leak in one of its lines and air gets in the system on me. so i do this bleed out a few times a year.

lay some card board under the front of the engine under the pump, as the air coming out can push past the cap and of course blow fluid all over.

ive done it cap off and on, still makes a mess. if your pedal is mush this should fix you up to a nice firm pedal.

check and top off fluid as needed when done.

 

[Chubbs]

You should really consider buying or gaining access to a shop manual for DIY maintenance & repairs. bad advice, misdirection, and overlooked steps can compromise reliability of a vehicle subsystem & leave you both stranded and making a 2nd repair/replace.

Anyways, PS fluid flows from the belt driven pump to the brake booster. From there, it returns to the steering gear, cooler, and ends up back at the pump. At 3-4 years old, the PS fluid is due for replacement. Steering pumps usually fail due to poor maintenance that only entails replacing the fluid since its lubricating properties is not infinite and the fluid eventually becomes saturated with contaminants made up of metals from the pump rotor and pieces of inorganic materials lining the hoses. You will flush the PS loop by removing the return hose at the pump, plugging the pump return fitting, and then begin filling the pump resv with new fluid just as the old fill approaches the emptying out of the resv. as it drains from the PS pump return pipe.

Some people will extract the contents of the PS pump reservoir and just refill it but that doesn't remove all of the contaminates posing a threat to component service life. The orig steering pump could prob have run forever given a complete fluid replacement at the intervals spec by GM through their various literatures.

If you didn't completely flush the PS when you last replaced the pump, some might advise a more diligent approach at this point, before the replacement pump is damaged by the contaminates that remained in the system. These contaminants, in theory should inflict the same damage to the brake booster & seals as occurred with the original PS pump (underlying cause of spongy brake pedal maybe) so more importance should be placed on timely maintenance regarding a hydroboost steering system.


Power Steering System Flushing
Power Steering System Flushing


NOTE
Important

Do not reuse any drained power steering fluid regardless of appearance or condition.

1. Turn OFF the engine.
   
   
2. Raise the front end of the vehicle off the ground until the tires and wheels turn freely. Refer to Lifting and Jacking the Vehicle.
   
   
3. Place a large container under the fluid return hose in order to collect the draining fluid.
   
   
4. Remove the fluid return hose at the power steering pump reservoir inlet connection.
   
   
5. Plug the reservoir return hose inlet connection on the power steering pump.
   
   
6. 
   
   NOTE
   Important
   
   This step may require 4 L (4 qt) of power steering fluid until the draining fluid appears clear.
   
   
   
   With the key on engine off, turn the steering wheel fully to the left and to the right while an assistant maintains the minimum fluid level in the reservoir using new approved power steering fluid. Continue until the fluid from the return hose runs clear.
   
   
7. Remove the plug from the pump reservoir inlet connection and install the fluid return hose to the pump reservoir.
   
   
8. 
   
   NOTE
   Important
   
   Do NOT run the engine without the power steering fluid at a minimum system level.
   
   
   
   Bleed the power steering system. Refer to Power Steering System Bleeding.
   
   
9. Inspect the power steering fluid for the following indications of contamination:
   • Milky fluid – water
     
     
   • Brown fluid – burnt
     
     
   • Plastic debris or dirt chunks
10. If the fluid is contaminated, repeat steps 2–9.
    
    
11. Lower the vehicle and check the Power Steering system for leaks.

Power Steering System Bleeding
Power Steering System Bleeding

1. 
   
   NOTE
   Important
   
   • Use clean, new power steering fluid type only. See the Maintenance and Lubrication subsection for fluid specifications. Refer to Fluid and Lubricant Recommendations.
     
     
   • Hoses touching the frame, body or engine may cause system noise. Verify that the hoses do not touch any other part of the vehicle.
     
     
   • Loose connections may not leak, but could allow air into the steering system. Verify that all hose connections are tight.
     
   
   
   
   
   NOTE
   Important
   
   Power steering fluid level must be maintained throughout bleed procedure.
   
   
   
   Fill pump reservoir with fluid to minimum system level, FULL COLD level, or middle of hash mark on cap stick fluid level indicator.
   
   
2. 
   
   NOTE
   Important
   
   With hydro-boost only, the oil level will appear falsely high if the hydro-boost accumulator is not fully charged. Do not apply the brake pedal with the engine OFF. This will discharge the hydro-boost accumulator.
   
   
   
   If equipped with hydro-boost, fully charge the hydro-boost accumulator using the following procedure:
   
   1. Start the engine.
      
      
   2. Firmly apply the brake pedal 10–15 times.
      
      
   3. Turn the engine OFF.
      
3. Raise the vehicle until the front wheels are off the ground. Refer to Lifting and Jacking the Vehicle.
   
   
4. Key on engine OFF, turn the steering wheel from stop to stop 12 times.
   
   Vehicles equipped with hydro-boost systems or longer length power steering hoses may require turns up to 15 to 20 stop to stops.
   
   
5. Verify power steering fluid level per operating specification. Refer to Checking and Adding Power Steering Fluid (Without Hydroboost) | Checking and Adding Power Steering Fluid (With Hydroboost).
   
   
6. Start the engine. Rotate steering wheel from left to right. Check for sign of cavitation or fluid aeration (pump noise/whining).
   
   
7. Verify the fluid level. Repeat the bleed procedure, if necessary.
   

 

[Chubbs]

Brake Assist System Description and Operation (Hydraulic Assist)
Brake Assist System Description and Operation (Hydraulic Assist)




Click to Enlarge
1 Master Cylinder

2 Power Brake Booster Outlet Hose

3 Power Brake Booster Inlet Pipe/Hose

4 Hydraulic Brake Booster

5 Power Steering Gear Inlet Pipe/Hose

6 Steering Gear

7 Power Steering Cooler

8 Power Steering Pump

System Component Description
The brake assist system consists of the following:


Brake Pedal


Receives, multiplies and transfers brake system input force from driver.


Brake Pedal Pushrod


Transfers multiplied input force received from brake pedal to brake booster.


Hydraulic Brake Booster


Uses power steering fluid pressure to decrease effort required by driver when applying brake system input force.


Power Steering System


Supplies pressurized power steering fluid used by hydraulic brake booster to decrease brake pedal effort.


System Operation
The hydraulic brake boost system uses fluid from the power steering pump (8) through the hydraulic boost inlet hose (3) and to the brake booster assembly (4). In addition to supplying the brake system with fluid for increased apply, the power steering pump is also connected to the power steering gear (6) and cooler (7).

The steering pump (8) is a continuous flow device supplying approximately 3 GPM at 1000 RPM, at the pump shaft. The fluid flows through the booster spool valve, then the return hose (5), and then to the steering gear (6). Pressure is produced by applying the brake pedal, resulting in the spool valve being actuated, internal to the hydraulic brake booster. The spool valve produces a marginal restriction that causes pressure in the booster (4) to rise and forces the power piston and therefore the output rod forward to generate force and motion at the master cylinder (1). The brake corners are actuated by the fluid impulse generated by the master cylinder (1). Internal blow by, or pressure relief, is directed to the low pressure return hose (2) back to the power steering pump (8).