New-Style (Short) Airpack Teardown & Repair with Photos

[silverstate55]

Since the TMs show where these new-style, short Airpacks are located and how to remove them, I'm only going to focus on what the TMs and other printed literature don't cover. I've searched high and low but haven't been able to come up with any kind of drawing, schematic, or instructions for taking these apart and repairing/rebuilding them.

I have obtained all of my rebuild kits from TNJ Murray, and have been quite satisfied with the kits as well as TNJ's service & shipping. The kits contain a lot of little parts, enough to replace all of the critical items that may dry out & allow air to bypass, or worse.

Once you disconnect the 2 brake wires, 3 air lines, and 2 brake fluid lines, remove the two lower bolts & skidplate with a 5/8-inch socket, then remove the rear bolt from the rear mounting boss with a 3/4-inch socket or wrench.

Once the Airpack is out of the truck, we can look it over to get familiar with it. I don't have any technical references for this, so the labeling is purely what I call it...if you know the official names, please feel free to correct my stupid self as I'm just guessing. Photos 1-3 give us a basic orientation.

Notice that this Airpack has both the old-style hydraulic brake light switch and the new air switch; I removed the hydraulic switch and plugged the opening with a 1/8-inch pipe plug.

 

[silverstate55]

Remove the J-pipe; it is only threaded at one end, the other end (that goes into the air slave cylinder housing) is a friction fit, with a small O-ring on the end of the J-pipe that will be replaced by one from your rebuild kit. Once the J-pipe is out, remove the 6 bolts holding the large can to the body and pull the can off. Inspect your can body interior; so far all of the ones I've disassembled haven't had a speck of rust, corrosion, nor other problems. A quick honing with White Lithium Grease was all it needed, followed by a thorough wipe-down with clean rags or cloths.

Now we can see the large piston cup, the pushrod, the large piston cup spring (the small end goes towards the body, the large end is against the piston cup assembly), and the O-ring where the large cup joins the body of the Airpack. The only parts that will be replaced will be the inner rubber gasket that holds the circular spring (sandwiched between the inner & outer piston cups) and the large piston cup spring. Typically these items will be covered in grease of some sort; I wiped these down completely to remove any potential contaminants and when done coated them with a healthy application of silicone grease (NAPA Sil-Glyde, great stuff and goes a long way).



The large piston cup assembly actually consists of several parts: the outer cup, the inner gasket that contains a circular spring, and the inner cup. These are retained by a 7/16-inch nylon locknut, backed up to a thick metal washer. BE CAREFUL WHEN REMOVING THIS NUT! The large piston spring is under a bit of tension, and if you're not careful, you will launch a lot of parts into orbit (or may be injured, or both). Use care! I secured the body in a vise so I could hold the large piston cup assembly downwards, which placed enough tension on the pushrod to hold it in place in order to remove the locknut. Once the locknut is removed, carefully remove the large piston cup assembly and large spring.

Notice that there is another, thinner metal washer on the pushrod that lies behind the large piston cup assembly. Remove this as well and set it aside. You should see that the threaded end of the pushrod (where you just removed the large piston cup assembly from) has a notch in it, so that the large piston cup assembly will rest in this notch to prevent spinning on the pushrod (and make removing/installing the locknut that much easier).

 

[silverstate55]

Turning our attention to the body, let's remove the pushrod assembly. It is held in place with a snap ring (a replacement should be contained in your rebuild kit); using your trusty snap-ring pliers, remove this snap ring and discard it. Pull the pushrod assembly out of the body, and be prepared for some brake fluid (and possibly other gunk) to come out of the body along with the pushrod assembly.






You should have a flat, black-colored nylon washer and an anodized aluminum cylinder you can slide right off of the threaded end of the pushrod. Notice that these photos show 2 different-colored cylinders; one is blue, one is gold. Both are identical, so I don't think that colors matter. Once you remove the flat nylon washer from the anodized aluminum cylinder, you should see an orange-colored U-cup gasket; remove this U-cup gasket & replace it with the new one from your rebuild kit (make sure the open end of the U-cup gasket faces out)...same thing goes for the O-ring around the anodized aluminum cylinder body. The other end of the anodized aluminum cylinder contains a staked-in U-cup gasket; there's no replacement in the rebuild kit, so I assume that is must be OK to leave it as-is. So I did.

 

[silverstate55]

To remove the end piston, you will need to use roll-pin punch & drive-pin tools. Rest the pushrod against a block of wood or other soft item while you drive the roll pin out & back in, so that you don't mar the pushrod.



Once the end piston is removed, there is a small end-cap gasket you can remove from the end of the pushrod & replace with one from the rebuild kit, and the piston body has a U-cap gasket as well as a split flat washer. Make sure that the open end of the U-cup washer faces towards the flat, closed end of the piston body, and that the split flat washer is against the closed end of the U-cup washer. Using a new roll pin from the rebuild kit, reinstall the end cap piston on the pushrod.

Carefully slide the pushrod back into the Airpack body, being sure that the U-Cup Gasket on the end of the pushrod doesn't roll back on itself when installing the pushrod. Slide the anodized aluminum body back onto the pushrod, followed by the flat nylon washer. Slide these fully into the Airpack body, and secure them with the new snap ring.

Secure the Airpack body in a vise with the pushrod facing upwards, and place the large piston body spring in place with the small end against the Airpack body (the large end of the spring will rest against the large piston cup assembly). Place the thin metal washer over the threaded end of the pushrod, then slide down the inner large piston cup body piece to compress the large spring in place, and make sure that this piston body seats in the slot on the pushrod. Now install the new rubber gasket with circular spring, the outer piston cup piece, the thick metal washer, and the locknut. Tighten the locknut until snug; if you hold the inner piston body piece, it should prevent the pushrod from turning while you secure the locknut.

Don't forget to replace the large O-ring on the body, to seal it against the large can. Give the inside of the large can a light coat of lubricant, and put a coating of lube on the large piston cup assembly as well. I also put a light coat on each metal spring just to prevent potential corrosion. You can now reinstall the large can body or save it for later; it doesn't really matter.

 

[silverstate55]

Now we can dig into the air slave cylinder housing. Remove the 3 bolts holding the air slave cylinder housing to the body, but BE CAREFUL to not hold the air slave cylinder housing vertical when you remove it...otherwise the upper piston & spring may take off running on you. You should be able to use your fingers to remove the upper piston body and spring; looking inside the air slave cylinder housing, you will see a snap ring securing the air slave piston.

Using your trusty snap-ring pliers again, remove the snap ring and discard it (replacement should be in rebuild kit). Remove the air slave piston (this one in Photo 17 is covered with White Lithium Grease), as well as the 4-prong small flat cap and small spring. Replace the O-ring on the air slave piston body, as well as the small spring (discard small spring; replacement is in rebuild kit). Inspect the pieces/parts for any scratches, marring, or deformities and remove any debris. Once cleaned, I lubed them with a light coat of NAPA Sil-Glyde and installed into the air slave cylinder housing in the following order: new small spring, 4-prong small flat cap, air slave piston, new snap ring.

 

[silverstate55]

Remember this orientation/layout carefully! When reinstalling, the new small spring goes in first, then the 4-prong small flat cap, followed by the air slave piston and snap ring.

During this step, be careful with the O-ring on the air slave piston; I found that when I had to slightly pull out the piston, the O-ring would stay trapped in the housing body, and subsequent attempts to seat the piston would push the O-ring to the end of the opening and prevent the piston from seating. If this happens, remove the O-ring, place it back on the piston, and reinsert the piston body into the housing.

On my first rebuild effort, I mistakenly installed the 4-prong flat cap first, then the small spring, followed by the piston and snap ring. This caused air pressure to exit immediately through the Airpack vent line, and the truck wouldn't build air pressure at all. If this happens to you, take the air slave cylinder housing off and re-assemble these parts in the correct order.

Then remove the U-Cup Gasket on the Upper Piston and replace it with a new one...make sure that the open part of the U-cup gasket faces upwards, towards the new spring that slides over the Upper Piston body. Install the Upper Piston with its new spring into the air slave cylinder housing, and set aside. If any remnants of the old air slave cylinder housing gasket (triangular shaped gasket) remain, remove these and clean the areas where the new gasket will be installed.



Now remove the Lower Piston body, and notice how it has 2 U-Cup Gaskets on it...both face away from each other, and each open face of each U-cup gasket faces towards each end of the Lower Piston body. Discard the old U-cup gaskets and install new ones from the rebuild kit.

Clean out any remnants from the Airpack body where the Lower Piston goes into; you'll probably find lots of brake fluid and debris here. Clean this out thoroughly, if you didn't already when the pushrod assembly was removed. Coat each new U-Cup Gasket with either a good silicone/di-electric grease or in BFS. Do not use petroleum-based lubricants as this may damage the U-Cup Gaskets and O-rings.

Reinstall the Lower Piston body into the Airpack body, with the slot in the Lower Piston body facing up (see photos).

Install the new triangular gasket and bolt the air slave cylinder housing back to the Airpack body with the three 1/2-inch bolts. Be sure not to overtighten these bolts, I simply tighten until snug.

Your Airpack rebuild/repair should be complete at this point. Reinstall it back in your truck, and bleed it thoroughly followed by a good full brake bleeding. My truck stops with little pedal effort now, and doesn't pull to either side.

 

[silverstate55]

This is where I need your help: for the life of me, I am always left with this O-ring and I have no idea where it goes...I've done several of these now, and can't find where this darn O-ring goes.

 

[welldigger]

Great write up. I wish I could help with the mystery o ring.

 

[Heath_h49008]

Thank you for the write up! This will be one thread to watch for me.

 

[silverstate55]

Thanks fellas! I hope this helps, I just did several of these so I had one to use as a model for proper orientation/order.

I just completed my edits, so I hope that the information is complete and accurate...if not, please let me know.

 

[wildbill88]

I'm glad I found this I'm going to have to go through my rebuild everything again

 

[Kasper31]

Thank you for this great write up, I'm about to rebuild mine, great timing with your post.

 

[Another Ahab]

Great write up. I wish I could help with the mystery o ring.

Ditto; textbook thread (photos and narrative). How's that credit card ad go? "Priceless"?

Did you ever ID the mystery O-ring?

 

[Dipstick]

Great write up with pictures! Thanks! Now I have to crawl under my truck to see if I have a long or a short airpack.

 

[silverstate55]

Ditto; textbook thread (photos and narrative). How's that credit card ad go? "Priceless"?

Did you ever ID the mystery O-ring?

Thanks!

No, never did find out what the mystery O-ring is for...I even rebuilt both master cylinders for the 2 trucks' Airpacks in the photos, and no clue what that O-ring goes to.

 

[ckid]

Just dropped by to say in 2021 this write up is still priceless. Kudos for the time & effort. Made my own rebuild easy.

 

[yolner]

About to start on one and thanks again for this thread

 

[Another Ahab]

About to start on one and thanks again for this thread

Good luck, yolner!

Please let us know how it shakes out.

 

[yolner]

Have it all apart now. So far looks like a much more refined design than the long airpack. No trouble at all opening it up with such detailed pics. Mine probably didn't need to be rebuilt. All the seals are in great condition. Going to save all the parts I'm replacing just in case.

 

[Another Ahab]

Have it all apart now. So far looks like a much more refined design than the long airpack. No trouble at all opening it up with such detailed pics. Mine probably didn't need to be rebuilt. All the seals are in great condition. Going to save all the parts I'm replacing just in case.

No harm; no foul