(This procedure covers GMT800 trucks with Hydroboost systems; (99-06); 07-up similar)
So I got the booster / module R&R'd. Some notes...
- Get a new gasket ($8), along with your new booster.
- You'll only need one socket for the entire job - a 15mm deep.
- 1/2" drive is a bit bulky, under the dash; 3/8" would probably work better.
- You'll definitely need some combination of wobble extensions (shorter the better), universal swivel, flex-head ratchet. Bring 'em all to the party. Some may work better for you in a particular location than others.
- A pick tool or small flat screwdriver will help with the retaining clip & rod removal.
- Some form of lighting is definitely needed. Flashlight, small spotlight, etc. Emphasis on small, because you don't have a ton of room to move around under the dash.
First, disconnect the underhood side. 16 & 18mm flare nut wrench, pliers to loosen the clamp on the rubber return hose, and the 15mm socket on the two nuts that connect it to the brake reservoir.
Don't disconnect the brake lines - not needed. There will be enough slack to pull the reservoir forward and off the two studs. But leave it as-is, for now, once you get the nuts removed.
Now take all of your sockets / extensions, and head inside the cab.
Remove the knee bolster / kick panel, with a phillips screwdriver & small socket (mine was off already... sorry I forgot about the other socket!)
Depress the pedal several times until the compressed gas reserve is exhausted. This is important to do, before you disconnect the booster from the pedal.
Move the seat back as far as it will go. If you want to recline the seatback, that's fine, but not a must.
If you have the power seat, disconnect the fuse, as it's easy to trigger the fore / aft switch when you lean under the dash. But truth be told, you better be ready to work on your back (really, your shoulders!), looking up at your work. And you better be fairly dextrous with your left hand, as it's going to be doing most of the work.
Flip the steering wheel all the way up, so you have room to extract yourself.
Looking from the cabin end, it's pretty easy to tell where the studs / nuts are for the booster. Look for the rod that connects it to the pedal. There's a retaining clip. Take that off first. Be aware that it may go flying (mine did). A replacement costs $8, most places. If you think I'm spending $8 for a clip, dream on. I'll use a zip tie for a few days, till I find another.
Now you have a brake switch surrounding the rod. Your job is to extract rod from pedal, without breaking the switch assembly. It's not too difficult, just don't try and gorilla it off.
The easiest way to get the rod and switch assembly out in one piece:
Take one hand (left, for most people) and grab the rounded portion of the rod, firmly. Take the other hand, and put it behind the far side ('back') of the plastic switch assembly.
Now, start wiggling the rod and switch toward you, alternating pulls between hands.
If this is the first time your HB is coming out, expect that the rod will be somewhat stuck on the pedal, and it may take a bit to get it to start moving. But once it does, both rod and switch should come off without too much trouble.
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The one side of the brake switch has a slot cut out on the one side. By turning it, it allows you to lift the one side while gently moving the other side and remove it from the rod. Note that the brake switch connector & wiring was pointing downward - it's easy to put it on backward (and wonder why it doesn't fit as well on reassembly). Ask me how I know.
Moving the brake pedal toward the firewall will make this process much easier. If you didn't discharge the residual pressure from the booster earlier, do so now; the pedal will be much easier to move.
Once you get the rod / switch off, take a look at your four studs / nuts. The one on the bottom LH side (as you face the pedal), close to a metal brace, is by far the hardest one to get out. You can remove that brace if you wish; a couple of bolts hold it in, and it slides out. I did NOT remove this brace, so you *can* get the nut off that stud, with it in place. But it might be easier, if you remove it.
There shouldn't be TOO much torque holding the nuts on. Once you get them broken, and a few turns in with the ratchet (which you won't be able to swing too far; a flex head does help), you can turn the socket with your fingers (short extensions may come in handy for this). It will seem like forever until you free the nuts from the studs. Keep turning!
Once you get all four nuts off (the last one will also be a pain because the booster housing will start moving / shifting), crawl out from under the floor and head back under the hood.
Now, move the three lines you disconnected earlier out of the way, and gently pull the brake fluid reservoir forward till it clears the studs. With this, you'll have enough room to pull the booster forward and out from the firewall. You'll need to twist / turn it a bit to get it out, but it's not hard. And you can easily stand on the ground / floor while you do this.
Take your new booster, and make sure the rod end matches your old one - there are two different ones. One has a squared-off end; the other is more rounded. If they differ, it may not fit properly.
Take your new gasket, and place over the studs on the new booster. It'll stay on until you can twist the new one into place. Or, put it on after you get it in the engine bay, but before feeding the rod / studs through the firewall. Whatever is easiest for you.
Get the booster in as close to the firewall as you can, but you *don't* need to have it exactly flush on all four ends. It'll stay in place while you head back under the dash.
Thread at least two nuts on their studs; this will keep the booster in place, but it really doesn't move. If one of the studs is crooked, or barely through the firewall, leave that one for last. Drawing the other studs onto the nuts will bring that odd one in. Or, you could use a needle nose pliers to pull it in, as well.
As with most things, reassembly is the easier part. I was able to get three of the four nuts threaded most of the way with just the socket and my fingers (left hand). Then hook up your socket wrench and tighten them down. Not really tight. 'Sortatite', if you're using German torque specs (I did).
Once you've got the four nuts tightened down, it's time to reattach the rod & brake switch.
With the wiring / connector facing downward, position the holes on either side of the rod. You can turn it a bit upward, to take advantage of the slot on the one side. Just make sure its 'finished' position will be with the connector facing downward.
Bring the brake pedal inward, until the stud portion is lined up with the holes in the rod / switch. Then push the assembly onto the brake pedal stud, making sure the stud on the pedal enters the 'blind' hole on the brake switch, before the rod (the two ends of the brake switch 'surround' the rod, on either end.) If the one end of the plastic housing starts pulling away from the 'open' side, back off a bit. You can break it easily, and if you do, a new switch will cost $15-30 or so, and you won't have brake lamps until it's replaced. No, I didn't break mine!
Once everything's in place, push the rod & switch onto the stud as far as it will go. You can swing the switch downward into its resting place, now.
Before putting the clip back into place, depress the brake pedal; you should see the reflection of the brake and CHMSL lamps. If you don't, recheck how you connected the brake switch, make sure the connector didn't pull out of the switch, etc.
Once you've verified you have brake lamps, then reattach the clip. You're done with this portion, except for reattaching the knee bolster panel.
If you had fluid in your system before pulling the HB, then at least top off your P/S fluid. Raise front wheels, bleed, check for leaks, repeat top off as needed, done.
If you're replacing the HB (or hoses, or P/S pump) because of a catastrophic leak... there's going to be a lot more air in the system. Better to do this... \/
Fill the P/S reservoir to the top and leave the cap off (don't worry about the 'full' mark; put the fluid almost up to the top of the neck.) Reconnect the three lines, the brake reservoir assembly, and tighten up everything. Start the truck and check for leaks at the connections. Then bleed the air out of both the brake pedal and steering, as per procedure (you can look this up, or follow the manufacturer's directions, if they provided them.) Add fluid as needed. If you start seeing foam in the P/S reservoir, shut off the engine and wait 30 minutes. Then check the fluid / level, restart the truck, and continue bleeding.
Note that when you take the truck out, you may find the steering is more difficult, especially when turning the wheel a large amount, like a 90 degree turn left or right (if so, it still has some air in the system).
Be prepared for this, so you aren't taken by surprise.
The system *will* self-bleed (provided there's enough fluid), but it'll take a bit of time / miles (about 20 -?) until you get the proper assist back. Or, head back home and bleed some more, if it's really bad.
Recheck your P/S fluid level, once all the air is out of the system, and you're done.