NeverEasy (Chet Myrick) submitted his project for changing the Cab Air Bags in his 2001 Gen 1 Volvo VNL 660.

Chet used these parts and pages from Volvo manuals:  Enclosure CabAirSprings.pdf 

Gen 1 Volvo Cab Air Spring (Bag), Cross-member, bumper-stop, and Shocks Replacement. 

Here is a Youtube video that is what I used to get started.

https://www.youtube.com/watch?v=8Q7UKtZRaTs

My truck was a Canadian commercial truck.  All the bolts, cross-member, and shocks were rusted beyond service after 15 years.  When the cab air bags started leaking everything from the truck frame to the cab was replaced. 

The nuts are all locking nuts.  They should be replaced or use lock-tight when reassembling.

The bolts are expensive to the tune of $3.50 to $7.00 each.  Try to save them.

This is a link to torque specs for metric bolts.  Note at the bottom that lock-nuts are to be torqued dry to 50% of the dry torque value.

https://www.swtc.edu/Ag_Power/diesel_engines/lecture/Metric%20torque%20values.pdf

All the part numbers referenced here-in are from the data sheet for my Chassis ID.  That pdf should be available with this document.  There are some peculiar things about the pdf in that the frame mounts holding the cross-member are shown in two places and neither are like to ones on my truck.  Perhaps these are updates. 

A thought about this job.  If you know you are going to replace everything from the frame up then it is easier to remove the bolts from the cab end on the shocks, remove the supply airline tubing and control rod from the leveling valve, remover the air bags, and then remove the 4 bolts holding the cross-member to the frame.  Then slid the whole assembly out and work on it on a bench.   How to do all that is listed below.

1. Preparation two days before the job starts.
   
   1. Most important: If your air springs will still hold air and you are happy with the ride height, air up the truck.  At some point of reference, measure the height of ride, frame to cab, so that you can adjust it back to that after changing the leveling valve.
   2. At the back of the cab support stringers behind where the top of the air bags mount, there is a square plastic protection strip (Volvo part number 8191166).  It has a tab that sticks out at the top.  Grab hold of it with pliers or vice-grips and gently move back and forth until it comes out.  That will reveal a hole.  Just beyond that hole is the bolt on top of the air bag. Spray some rust penetrate through that hole on the top bolt of the air bag.  Inside is a captive nut that will be very difficult to replace or rig a fix if the air bag bolt will not let go of it.
   3.  Depending on what else you are going to replace (shocks, leveling valve, cross-member) use the rust penetrate on all the bolts.
2. Open the hood to prevent stress when jacking up the back of the cab.
3. Remove the side panels or not? If you can get right behind the cab with enough room to get the job done then you need not remove the side panels.  However, most of our beds are built too close for working on the air bags solely from the back. 
   
   1.  Remove all the bolts (T50, I think) at the top of the two panels and one at the rear on the inside of the panel.
   2. Remove the top step.  (13 MM bolts).
   3. The front part directly under the doors overlap two bolts at the front of the long back part and therefore, the front panel needs to be removed first.  Remove the front part by lifting out at the front and sliding it forward from under the rear panel.
   4.  Remove two bolts on the front of the back panel.
   5. Set out of harms way.
4. Lift the cab.  Caution: Do not jack on or use supports on the floor of the cab.  Only jack and block on the cab stringers that the air bags are attached to.
   
   1. Prepare a sturdy 4x4 or 4x6 by cutting it to 54 inches.
   2. Get four 6 inch long 2x4s ready for added height on the cab.
   3.  If the air bags will hold air, start the truck and air it up.  If the bags are completely shot you must use a hydraulic jack and lift the cab.  Get the long board on the frame from one side to the other.  Then block up the cab on the cab support stringers that run the length of the cab (the air bags are attached to them at the back) with two short 2x4s on each side. 
   4. If the air bags are not lifting the cab high enough, try detaching the leveling valve control rod (item 0230).  Carefully pull the leveling valve lever down to inflate the bags more.  On mine I had to put a 4x4 on top of a hydraulic jack and set that all under the middle of the cross-member 4x4 and jack the cab up.  It took two short 2x4s on top of the long 4x4 on the cab stringers by using the air bags for lift.  Then one 2x4 under the long 4x4 at the frame after the hydraulic jack lift to get mine off the cab bump stops (items 0335 and 0340).
5. Remove the air bags.
   
   1. First, remove the air line.  Use an adjustable wrench.  Clamp it down on the plastic airline tubing and press the wrench against the brass retainer ring on the 90-degree air fitting.  Push in on the air line, push in with the adjustable wrench and then pull out the air line. 
   2. Remove the 90-degree air fitting.
   3. The air bags are attached at the bottom of the cross-member with a clip (item 0205).  It has fingers that dig into the housing part of the air bag.  Put a screw driver under the clip and pry the clip off.  If you want to re-use the clip be gentle.  The new air bags do not come with one so if you ruin the clip don’t forget to order a new one.  I highly recommend getting new ones.  Mine were too rusted out to save.
   4. Now you are ready to unscrew the top of the air bag from the cab.  You can grab hold of the air bag and give it a twist but it is unlikely to come loose.  If not:

i.Go through the hole at the back of the cab stringer that you should have revealed in the preliminary step and re-lube the top of the bolt to the air bag.

ii.With a sharp utility knife, cut the bag in the middle and then remove the bottom half.

iii.Now use the utility knife and cut the rest of the top piece away right at the top to get access to a cylinder inside. 

iv.Now, you must be very careful to not twist off the bolt going into the cab stringer captive nut or dislodge the captive nut.  With a pipe wrench, pull gently counter-clockwise (loosen) with moderate strain.  Reverse the wrench and gently go clock-wise.  Do this back and forth routine until the bolt lets loose.  Even after it lets loose, go back and forth to clear the threads as it come out.

6. If you will replace the shocks and/or the cross-member, proceed to those sections.  If the air bags are the only part you are replacing, put them back in in reverse order.   It is likely that you will need to lift the cab a little more to get the air bag in place.  The top air bag bolt should be torqued to 30-35 lb-ft but that would be difficult to do.  Tighten as best you can.   The retainer clip will go on by working it on with your fingers.  A 32-33mm socket can be used to push it on with the aid of a pry bar.    You may need an assistant to hold the air bag down while pushing up on the clip.
7. The leveling valve is straight forward.  Remove the three air lines making note of what they feed or are fed by.   Remove the height adjustment arm.  Remove the two bolts that hold it on.  I believe both the nut and head were 12mm.  The leveling valve must be removed to get the horizontal shock bolt out on the driver’s side.
8. Removing the shocks (leveling valve must be removed first because a bolt head is behind the valve).
   
   1. More rust penetrate on all 8 nuts. 
   2. Remove all 8 nuts.  The wrench sizes were 21mm nuts with 18mm bolt heads.   The top two bolts on the vertical shocks had the nuts on the front side.  These were the most difficult to get to as the air wrench would not fit up that close to the bottom of the cab on the nut side and would not get to the bolt head on the back side.  I had to use a ½ inch swivel and a long socket on the nut side.
   3. Remove the bolts. The top two bolts on the vertical shocks might hit the cab but they will come out if the bolt is pulled without the washer.
   4. Remove the horizontal shock anchorage brackets (items 0015 and 0020).  You can try to get the shocks out and back in without it but I was unsuccessful. 
   5.  Remove the horizontal shocks.
   6. Remove the vertical shocks by compressing them with a pry bar.
   7. If you will be replacing the cross-member proceed as below.  If not, replace the shocks in reverse order.
9. Remove and replace the cross-member (item 0180)
   
   1. More rust penetrate on the 4 bolts holding the cross-member to the bracket assembly (item 0030).
   2. The bolts heads are welded to the bracket assembly(0030) on the bottom.  Use a flashlight to make sure yours are.  Therefore, it is best not to twist the bolts off or you will find yourself (as I did) torching the bolt heads off and welding in new ones.
   3. Examine the threads at the top.  If they are all rusted, consider heating the nuts with a torch before trying to get them off.
   4. The cross-member should now come out easily.
   5. Now, take all the parts off the old member and make the new cross-member look like the old one.  The bolts on the bump-stop (item 0340) were difficult to get out and required heating with a torch.   After removal, the bump-stops were found to be rusted beyond service so I replaced the bump-stops, too.  However, the bolts were still useable with new nuts.

i.Attach the bottom of the air bags loosely.  You will need to be able to twist the air bag to attach the top bolt to the cab.

ii.Attach the bottom of the vertical shocks also loosely.

iii.Install the horizontal shock anchorage brackets (items 0015 and 0020) and tighten bolts.

iv.Install the horizontal shock in anchorage bracket loosely.

v.Install the leveling valve to the front anchorage bracket.    There is a diagram that comes with it to show how to mount it but I could not get it to work.  I had to cut out a new mount for the replacement valve. 

vi.Run airline tubing from leveling valve over to the air-bags but do not attach at the air-bag end.  Also, do not install the 90 degree push connector as it will get in the way later. 

10. Remove and replace bump-stops, upper (item 0335)
    
    1. The 2-upper bump-stops are held on by a bolt (item 0290) through the middle.  The head of the bolt is an 8mm allen head.  The nut is a square captivated nut on the cab side of the upper bump stop.  This is another one of those bolts you do not want to be too aggressive with.
    2. Apply liberal amounts of your favorite rust penetrate to the top of the bolt.
    3. Remove the allen head bolt.  I used a 3/8 drive ratchet and 2 feet of extensions on an allen socket.   I had to use heat on mine to get it lose and then work it out by reversing directions many times.  After breaking the bolts lose, I replaced the 3/8 ratchet with an air-impact due to the number of times I had to reverse it to get it out.
    4. Bolt the new bump-stops in place.
11. Now it is time to reassemble.
    
    1. Put the cross-member in place.
    2. Get the air-bag’s top bolt started into the cab nut.
    3. Put the top two bolts/nuts in the vertical shocks loosely.  Note:  On mine, these are the only two shock bolts that were inserted from rear towards front. 
    4. Put the outer two bolts/nuts in the horizontal shocks loosely
    5. Attach cross-member with 4 nuts to the bolts sticking up from the frame mounts.
    6. Finish tightening the air bag at the top.
    7. Finish tightening the bottom air bag clip.  This is more difficult than you might expect.  If you have some help, have them hold down on the bottom of the air bag while you push up on the clip with a 32 or 33 mm socket and a pry bar.
    8. Insert the 90-degree push connect air fitting in the air bag.
    9. Attach the airlines to the bags and truck air source.
    10. Before tightening the shock mount nuts, the cab ride height should be at its nominal height.  It is time to connect the height leveling arm to the valve.  If you used the old valve, then it should just connect back to the height adjustment arm.  If you replaced it and ran into my problem with the new one not being the same as the old one, then some work is required.  I had to remove the upper attachment bracket and extend it out 1 ¼ inches to get the vertical alignment right.  Those top two bolts did not want to come out and I feared twisting them off.  I had to heat the heads of the bolts with a torch.  It did not take a lot of heat so a very small torch tip was used.   Then I found that the operation of the valve was backwards.  To resolve this, the new valve arm was removed and put back 180 degrees from where it was and then rotated 180 degrees to match up with the upper mount. 
    11. Now, get the cab height where it normally rides based on the measurements you first took.  This was done so that the shock mounts are bolted at their normal riding level so as not to twist the mounts in their rubber mounts too much.  Now that the airlines are connected, you can air the truck up and use that or use a jack and shims.
    12. Tighten the vertical and horizontal shock nuts.  The bolt head behind the leveling valve will require an 18mm open-end wrench.
    13. Everything should be ready to go.  Air up the truck and check for air leaks.