This weekend I will be replacing the PS pump (again) and all of the lines. I would like to do a complete flush again as well. Since I will have everything disconnected, I would like to flush the Hydroboost and Gear box separately so as not to pull the fluid in in them through the new pump and lines. I have a vacuum pump with reservoir - is it possible to flush just the gearbox and then just the hydroboost?

 

It is possible but also unnecessary.

Get your new pump installed but leave the return lines from the hydro-boost and cooler (or just the gear box if there’s no cooler) unhooked from the reservoir. If you got a new pump with reservoir, it should already have caps on the fittings for those lines - if you’re swapping the reservoir to the new pump, cap the fittings.

Get the front wheels off the ground and Route the return lines into a 5 gallon bucket or other container. Perform manual bleeding (stop to stop with the engine off), making sure to keep an eye on the fluid level - as fluid works through the system it will expel contaminants into the bucket.

It sounds like you recently flushed the system already, so if you don’t see much contamination, about 3/4 gallon of fluid is enough to adequately flush the system out.

Once you’re satisfied with the flush, hook the lines up to the reservoir and complete bleeding manually and with the engine running.

If you don’t know, most PS systems use ATF instead of PS fluid. I’d recommend also adding about 1 cup of Lucas Transmission Fix (it’s a very heavy ATF - similar to their oil stabilizer).

 

I just read through your other thread (didn’t see it at first otherwise I’d have read it too before responding) and I’m gonna suggest a different method for bleeding the system.

And let me just say this as well - the reason using vacuum to pull clean fluid won’t work to flush is because even at 30” of vacuum, there’s still only (in theory) an equal amount of pressure pushing fluid through the system. Pressures in PS systems get upwards of 2,000 PSI - a mere 30 PSI isn’t gonna dislodge anything - it’ll only replace the fluid.

I’ve replaced several PS pumps, with and without hydro-boost, on at least 4 different makes and this bleeding method has worked every single time - you just need to be patient.

So, after you’ve flushed the system and are ready to bleed, try it this way:

1) Engine off.

2) Reservoir to the full mark on the dipstick, turn stop to stop 5 times, recheck fluid level and add as necessary. Stop to stop another 5 times. Repeat 2 more times.

3) By now the fluid shouldn’t be dropping too much. Pull vacuum on the reservoir and turn stop to stop 10 times, recheck fluid and add as needed, then repeat 2 more times.

4) You’ll know when you’re ready to begin power bleeding when the effort to turn the wheel from centered to full left or full right is about the same. As air pockets are bled out, you’ll feel varying effort as the wheel is turned.

5) Make sure the reservoir is to the full mark. Start engine. Stop to stop 5 times while pumping/holding the brake pedal. Recheck fluid and repeat until fluid level remains steady.

You can pull vacuum on the reservoir during power bleeding if you want - I’ve never found it to be necessary but you’ll have to decide if it is.

 

Thank you. Both of your replies make sense. There are so many methods on the internet but your explanation is easy to understand. I don’t mind how long it takes, I just want to do it properly.

As far as the fluid, my manual calls for GM PSF #89021184. I’ve always used ATF in old cars but I’ve found conflicting information on the newer models, much of it saying to use what the manual calls for. I know that some newer models specifically call for ATF so I assume this one calls for PSF for a reason, but again, I’m newish to the newer vehicles. Most of what I’ve worked on was 80’s and 90’s cars which were much less complicated.

 

If you prefer the GM fluid (or equivalent) there’s no reason not to use it.

At the base level, ATF and PSF are essentially the same thing - hydraulic oil. The primary difference is ATF has a lot of detergents in it.

 

You will know you are done when the colour of the fluid in your system matches the fluid in the bottle.

I don't know what the correct fluid is for these vans but watch out for mixing ATF with real hydraulic fluid. Going back to Mopar again, I had a van with OEM clear hydraulic fluid that I switched to ATF briefly. The fluid turned black over a weekend as it cleaned everything! Back to hydraulic fluid after that.

While you are under there, have a look at your oil pan gasket, oil cooler lines, and transmission lines. The ones on my 2004 were just terrible, hopefully GMC improved them over the years.

 

I think the previous owner mixed it at some point because it definitely had some red in it despite being super dirty. At least it will get a second flush now, hopefully everything will be nice and clean for once the new fluid is in.

I’ve been through every part of this van, it’s in really good shape. Unexpectedly good shape actually, but then again we don’t have salt roads in the south. The steering lines are either leaking or soaked from a previous leak so they are all getting replaced.

 

I just finished swapping in brand new OEM PS pump, lines, and cooler. I used your method but added on a few more lock-to-lock repetitions out of paranoia from the last time I did this wrong. This is by far the simplest method I’ve found, thanks for typing it all out. I’m going to print it and keep it with my paperwork in case I ever have to do this again (hopefully not). Feeling the varying effort when turning the wheel corresponded with the air bubbles popping up in the reservoir. This process definitely makes a lot more sense to me now that I’ve done it. After I went through your steps, I applied vacuum and did a few more reps. Then did 5 minutes on each left lock, center, and right lock. This did bring up some more air. Then I started the engine and went through 20 reps, then 20 under vacuum, then 20 more. I can’t imagine there’s any more air in there. Everything is super quiet now, no more growling every time I use the brakes or turn. I guess that reman was junk, this new pump is nice and quiet. I used Royal Purple synthetic PSF (I’ll be in Northern winters soon) and did a full flush then installed a filter. I feel a lot better about this now and hopefully I don’t have to deal with the power steering system for another several years.

 

Good to hear it worked out for you.

The basic process is the same for every PS system but has varying methods between manufacturers depending on the setup (with or without hydro boost), some of which call for special adapters for vacuuming and/or filling the system. I work at an independent shop and we only buy the special tools we really need - adapters for the PS systems aren’t one of them.

When I was a noob professional mechanic, I tried to follow GM’s procedure for bleeding with hydro boost on 2 different trucks and it didn’t seem to work that well because I had to repeat it 3 or 4 times. That’s when I noticed how the turning effort changed during manual bleeding, so I started making notes to keep track of that and noticed it was the same for other makes too. Eventually I developed the process I outlined for you and I’m yet to see it not work. Depending on what repairs I make (hoses only, for example), the time to bleed can be cut down if I pull vacuum on the reservoir from the start.