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Old 12-08-2012, 01:25 PM   eBay Motors  #1 (permalink)
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How To: Hydroboost Rebuild / Leak Fix - Discussion

Do you have power steering fluid leaking from the weep hole where the hydroboost unit bolts to the brake master cylinder?


If so, you have a worn piston seal that is allowing fluid to escape. This seal is Bendix part number 129484. If you have a local hydraulic repair shop or some other place that sells oil seals, you may first try to see if someone local can get you a replacement. Your other option is to purchase the repair kit from Piratejack.net "2771004X Hydro-Boost Repair Kit" for $33. Unfortunately you can't just buy the 1 or 2 seals you need, you get the whole kit of 25 or so seals. I only used the 129484 and 2770532 seals. But it beats purchasing a whole new hydroboost unit!

Here is what arrives from Piratejack:
A copy of their instructions and a pack of seals.


The instructions are also available in PDF format from their website.

Before you begin unbolting anything from the hydroboost, with your truck turned off, apply the brake pedal fully a few times. This depletes any reserve pressure from the accumulator.

Disconnect the brake master cylinder from the hydroboost unit by removing these two nuts. I use a 15mm wrench:


Here you can either just crack these connections loose, or completely disconnect them. The steel lines can be tight if they have never been removed before. You'll want some rags/paper towels as these three lines will leak some fluid when they're disconnected.


Now move into the cab and look where the brake pedal connects to the hydroboost. There are 4 nuts to remove. The threaded stems are long so you'll need a 15mm deep socket.



Look straight up at the end of the brake pedal and find the metal clip to remove. I use a flathead screwdriver to to pry it over top of the pin, then slide it off.


Once that clip is off, you can simultaneously slide the electrical connector and the hydroboost rod off of the brake pedal.


Now from the engine bay, you can pull the hydroboost unit away from the firewall. You'll have to push the brake master cylinder out of the way. I pulled up on it slightly and pushed it towards the fuse box, while pushing the hydroboost towards the engine and pulling away from the firewall. Once the threaded studs were out of the firewall, twisting the hydroboost counter-clockwise 90 degrees helped to get around the other obstacles in the engine bay. It should come out without too much hassle.




Take the unit somewhere decently clean to work on. You're dealing with hydraulics and parts that have tight machining tolerances. You don't want dirt/debris getting into the unit. I chose my dirty garage floor.
If you have the appropriate Torx socket, use it. Otherwise, use a 12-point 3/8" box end wrench to remove the 5 "star-shaped" bolts. These can be fairly snug. I had to tap on my wrench with a hammer to crack them loose.



Once it starts opening up, a good bit of fluid trapped inside the unit will spill out. So prepare with paper towels or whatever you use to soak up the mess.


Be cautious of any parts springing out.

Here is the unit in halves:


Take the half on the right, and look into this hole. About 2 inches in is the bad seal and all you can see is the inside lip of it.


I use two small flathead screwdrivers to get the seal out. You dont need to be gentle on the seal since you have a new one, but be gentle on the housing as to not scratch or ding it up.


Just to check, I slid both old and new seals onto the piston to see if I could feel a difference. The new one thankfully fit much more snug and felt solid!


Install the new seal into the housing. Be sure you put it in the correct direction with the lips facing the piston!


It can be tricky getting the seal fully seated. Just be sure when you have it in that it is all even and smooth. You should be able to see about 1/16" of the lip all around.


Once the new seal is in, you can start re-assembling. If any parts sprung out, be sure to put them back correctly. The yellow spring goes into this small hole. There is also a small valve that can dislodge itself, my red arrow is pointing to this. My middle finger is covering some of it up. If it fell out, be sure to put it back in the correct way with the three dots facing out.



Continued in next post due to limit of 20 images per post!
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Old 12-08-2012, 01:26 PM   eBay Motors  #2 (permalink)
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This part goes in the same hole after the yellow spring:


I set the push rod and the green spring into the piston. This helped me keep it all centered when putting the halves back together.


I also put this new seal on just because a new one came in the kit and it is an easy swap.


When you begin to slide the halves back together, pay special attention to this picture.

It is critical that those two pins are within the groove on that spool as shown. They will not fall into place by themselves.

Keep pressing the halves together. You may have to help center the push rod through that star shaped washer:


I also had to give some extra push to the piston to get it slid inside the new piston seal we installed. It is a snug fit so it required a little extra push.

When you mate the two halves together, put in two bolts on opposite sides to hold them together. You can check if you did everything correctly by the rod on the right side of this picture


If you can push it in about 1 inch and pull it back out smoothly, then everything inside the unit is actuating properly. If it binds or does not move in, then something is wrong.

Install and tighten all 5 torx head bolts.

Reverse the steps you took to remove the unit from the firewall.
4 nuts inside the cab to mount it to the firewall.
Reattach the brake pedal to the hydroboost rod and install the electrical connector and the metal clip.
Attach all the power steering lines back to the unit.
Mount the brake master cylinder to the hydroboost.

Bleeding Hydroboost (taken from Piratejack instructions)
1.After you have everything back together properly, fill your power steering reservoir.
2. Start your engine.
3.Turn the steering wheel all the way to the left stop, then to right stop.
4. Shut off engine and discharge accumulator by pressing the brake pedal 4-5 times.
5. Repeat steps 2 thru 4
6. Check reservoir and fill to "cold fill" line.



If you have any questions or need any tips on any particular step, please comment and I will try to help out.
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Last edited by 2300Hz; 12-08-2012 at 01:29 PM.
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Old 12-08-2012, 02:20 PM   eBay Motors  #3 (permalink)
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Great write up! I dont need it ATM, but I am glad that you did it! I have always read on here that it is NOT rebuild able, and I am glad that you went and proved it is possible and not even too hard.
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Old 12-08-2012, 02:43 PM   eBay Motors  #4 (permalink)
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Thanks for the info. I think mine is leaking. Rather rebuild than buy new one. How long did it take you?
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Old 12-08-2012, 03:33 PM   eBay Motors  #5 (permalink)
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Took about 2 hours this morning and that is with knowing what to do. Before the seals arrived, I went through all the motions last week with a junkyard booster so I knew what to do with my truck's booster. 3-4 hours is a realistic time to allow yourself.

Just be careful with everything. Your truck is stranded completely without a hydroboost unit so you want to be sure it all goes back together and functions properly.
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Old 12-08-2012, 04:46 PM   eBay Motors  #6 (permalink)
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All of the other pics out there that show a disassembled hydroboost suck...nice to finally see some decent pics.

A few more tips/observations/rambling from someone who did one:

It is a real pain in the ass to do a complete rebuild.

I bought the 'complete' kit. There were a lot of parts in the 'complete kit, some of which aren't used in a duramax unit. In addition to the parts used by the OP, the 'big' kit came with some other parts that I used. Kit had a new dump valve (little round metal thing with 3 holes as seen in pic 16), spool valve seat, and input shaft seals.

I chose the big kit because it came with the input shaft seals. I knew the older Hydroboost units were prone to leaking there, the Duramax HB design isn't much different from the 1980s design, and I didn't want this one to leak.

The input shaft seals are what make it difficult, as they have to be slid on over the end of the pushrod. The Duramax Hydroboost uses what Bendix calls a 'non-serviceable' pedal pushrod, and some of the Bendix manuals state that if you feel the need to replace those seals, just buy a new unit.

Other Bendix manuals require removing the factory pushrod and replacing it with an adjustable one when replacing the input shaft seals. After a lot of phone calls, I found out that this is what some of the rebuilders do, and the parts are available from streetrod places.

Here's the problem with that idea: they tell you to just pop the pushrod out of its' socket and push everything out the front. Well, the pushrod ain't going to just 'pop' out. It is VERY well crimped to the socket.

After spending a couple of hours trying to pry the pushrod apart, I may or may not have cut it apart with a sawzall and welded it back together.

Finally got input shaft out of the housing and tore off the old seals. Then, I found a piece of tubing the same diameter as the shaft, stuck the new seals on the tubing, put the tube over the input shaft, and slid the seals from the tubing to the input shaft one by one. Took lots of lubrication (clean PS fluid) and getting the 2nd seal over the first one without destroying it was fun, but I got it done. The rest was no big deal. Pried the old dump valve out and lubricated the new one and tapped it in. The plastic seat for the spool valve has a screwdriver slot in it. Easy like Sunday morning!


It has worked beautifully for the past few weeks and NO MORE LEAKS!!

ALSO: Don't try this at home! The procedures described above may cause people with engineering degrees to cry themselves to sleep.
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Old 12-08-2012, 05:37 PM   eBay Motors  #7 (permalink)
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Awesome write up and pics! I wish I had seen this two months ago before the stealership raped me for $800.
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Old 12-08-2012, 05:56 PM   eBay Motors  #8 (permalink)
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Awesome write up. Thanks man.

I'm gonna order my kit in a little bit! Hopefully it don't take long to come in. I'm off until the 19th and hope to tackle it before I go back to work!


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Old 12-08-2012, 05:58 PM   eBay Motors  #9 (permalink)
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Nice work 2300HZ !
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Old 12-08-2012, 07:55 PM   eBay Motors  #10 (permalink)
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Great write up man.
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