1. Air Chamber
  2. Guide Sleeves
  3. Tappet Heads
  4. Carrier
Maintenance of air disc brakes has been a part of regular workshop practice for more than 20 years, and most workshops will feel confident with all service aspects.
Even with well established ADB servicing practices, water ingress to the caliper remains an issue for many end-users and most of these cases are due to water ingress through the air chamber.

Air chambers have a pressure side and non-pressure side. The non-pressure side (closest to the caliper) has to be vented to atmosphere, and hence water can be present here also. On a drum brake this is not an issue as the pushrod is detached from the internals of the brake. However, for an Air Disc Brake, the pushrod has to be fully sealed against the non-pressure side of the air chamber so that water is kept out of the brake. If this seal is damaged, or ineffective, the brake will quickly become permanently damaged (through corrosion).
Note the face sealing point and pushrod sealing point in the layout of air chamber parts below.

English Water Ingress 900

So, what happens if the air chamber seal is damaged or ineffective? The following photographs show the resulting corrosion after brakes have been fitted with an air chamber with a damaged seal:

This brake had a reported fault after less than two weeks service – water entered through the air chamber seal, which created an electrical fault, highlighting the issue as a ‘worn-out pad’ signal in the cab.


Prevent Water Ingress Fig 3 web

More normal is for water to enter the brake and be undiscovered for weeks or months, until brake efficiency reduces, which is then picked up on rolling road inspection or test. This brake is completely beyond repair and had been fitted with an air chamber with a damaged seal.

Prevent Water Ingress Fig 2 web


Preventing Water Ingress

So, when fitting an aftermarket caliper to a truck, how does the mechanic determine if the air chamber is still serviceable? Checking that the pushrod operates correctly with no air leaks at the diaphragm is only one part of the process, the mechanic must also ensure that the caliper is not subject to water ingress through the air chamber by checking the push rod boot/seal:
1. First check the flange seal that is visible on the air chamber mounting face. The protruding seal should be at least 3mm proud of the non-pressure chamber (air chamber body), and without any damage or nicks.


air chamber 3mm

2. After checking the flange seal, carefully inspect the visible part of the pushrod. If water or rust staining is present on the pushrod, it is a clear sign that the pushrod seal/boot has split or is not seating correctly.

Air Chamber pushrod web

3. Next, examine the pushrod and the inner part of the seal for any damage or evidence of water entry through the boot. (See red boxes in the top diagram.) It is difficult to see fully inside the boot, but a torch will help. Remember, if the pushrod shows signs of corrosion (after wiping away any grease), boot damage is the likely cause.

Old Air Chamber web

If water or rust staining is present on the pushrod (see image illustrating point 2), it is a clear sign that the pushrod seal/boot has split or is not seating correctly.

The inside of the boot and the pushrod should look like the image below.

Air Chamber 4web ...  


Following the steps above will help preserve the life of calipers on trucks, trailers and buses. The caliper and service chamber interface is often overlooked, but it is always worth taking a few minutes to check the seal carefully to determine if a new air chamber is needed or not.

As a vital part of the braking function worn or seized guide sleeves have severe effects on brake pad and disc wear. This can lead to overheating with severe consequences to wheel end components, which will most likely lead to a vehicle breakdown.

guide sleeve anim web


1. If sliding becomes stiff or seized, the first effect will be the outboard pad showing signs of higher wear than the inboard pad, and the brake will run hotter than normal – heat marks or blue spots may be seen on the disc.

CVW article 2 image 4web ... CVW article 2 image 3web

2. During the vehicle’s regular inspection, try to get a feel for the condition of the caliper guide sleeves. Standing in the inspection pit, and with the park brakes released (make sure the vehicle is chocked and safe!), the mechanic can apply force to move the air chamber/caliper on the guide sleeves. If the caliper can be moved easily the pins are not seized, while if it is stiff then further investigation will be needed.


1. During the regular vehicle inspection, always inspect the guide sleeve boots and tappet boots for mechanical damage or heat damage. They are silicon rubber and are tough, but it is possible for debris to damage the boots. If the guide sleeve boots are heat damaged, this may be a sign that the caliper is not sliding correctly. If there is any damage to the boots, then they must be changed with a new, good quality, guide sleeve repair kit.

2. So long as no water or dirt is entering the guide sleeves, they should last a long time, but they are serviceable items due to the vibrational load that they experience. If they remain free from seizure then they may become worn after time and need to be changed. The MEI maintenance instructions give guidance on how much clearance is allowed before bushes/sleeves need changing.

3. An open guide sleeve with a rubber bush is fitted to some calipers, and these can become clogged with dirt or debris in certain applications. MEI is set to release a fully sealed short guide sleeve on its LB225, LV225, LA225 and LB195 range of calipers (spring 2019) to further improve life and reliability for these applications.

 3. An open guide sleeve with a rubber bush is fitted to some calipers, and these can become clogged with dirt or debris in certain applications. MEI is set to release a fully sealed short guide sleeve on its LB225, LV225, LA225 and LB195 range of calipers (spring 2019) to further improve life and reliability for these applications. closed short guide sleeve 



1. Service issues can be caused by incorrectly fitted guide sleeve protection caps – they can be tricky to fit without the correct tool. If you don’t have a purpose made protective cap fitting tool, ensure that you use a metal drift that fully covers the surface of the cap, and that you can hold it square. If the cap is not parallel, and fitted to the correct depth, there is a risk of water ingress which will cause the guide sleeves to seize.
2. When fitting the guide sleeve protective caps, always ensure that the guide sleeve boot is in the compressed (new pad) condition, otherwise the caliper movement may be limited after fitting the cap.

3. After ensuring the sealing bead of the guide sleeve boot is correctly seated in its groove (on the guide sleeve), it is important to make sure the white plastic washer is correctly fitted onto the boot – this keeps the bead seated in the guide sleeve and without it, water ingress is likely. CVW article 2 image 2web

4. When tightening the guide sleeve bolts, ensure the carrier is securely clamped in a vice at the same side of the carrier being tightened – the carrier can twist or bend (due to the high load applied when tightening the bolt) if this is not followed. If the carrier is still mounted to the axle, there is no risk. Ensure that the guide sleeve nearest the vice (left hand side of the image below) is the one torqued.

Caliper in vice web
5. Only use the original grease supplied with the guide sleeve kit, or fitting kit. Other greases may not be compatible with the guide sleeve boot material.

The pistons and tappet heads have an important function in the brake caliper as they clamp the pads to the disc, and whilst the tappet heads are rarely damaged, the seals behind them have to perform under difficult conditions, particularly heat and dirt.

1. Checking the condition of the tappet seals is important during a regular vehicle inspection, but Inspection of seal is difficult they can be without extending the piston difficult to inspect without removing the pads. However, shining a torch into the area between the inboard pad and the housing can give a good initial view of the seals. The seal in Fig.1 has heat damage, which should be possible to detect without removing the pads. However, a tappet seal in this condition must be replaced immediately in order to prevent further damage to the brake.

Fig 1 tappet 1


2. It is difficult to see the tappet seal if the piston is not extended. Therefore, to carry out a proper inspection, adjust the pistons out so that the seal is fully visible. Take care: Limit the extension to no more than 30mm from the front cover! (Fig.2)

 Fig 2 tappet 2b 


3. If the brake has seen higher temperatures through sticking guide sleeves, it is important to check for other effects and issues caused by this. The main one is the possibility of damaged tappet boots through excessive heat (Fig.3).

 Fig 3 tappet 3 


4. In Fig.4, the primary seals have been removed from the front cover, showing a good example of a damaged tappet boot (right hand) with water/dirt discolouring the secondary seal, putting the brake at risk of water ingress if the primary seal is not changed. The left hand seal is OK and highlights the difference in colour.

Fig 4 tappet 4


5. After removing the damaged seal, the piston threads can be carefully cleaned with a small wire brush (Fig.5), so long as dirt and  corrosion is not excessive. However, take care to assess the condition of the secondary seal and make sure there are clean threads underneath the seal. If corrosion extends under the secondary seal, the caliper should be replaced.

 Fig 5  tappet 5


6. Tappet heads should be carefully removed with the correct service tool as damage to the piston threads will prevent the brake from being adjusted for the fitment of new pads (Fig.6).

 Fig 6 tappet 6 


7. If necessary, remove the secondary seal to make a more detailed assessment and then replace with a new secondary seal from the repair kit (Fig.7).

 Fig 7  tappet 7


8. After removing the tappet seal, check the cover plate – if the sealing interface is damaged or corroded, then the caliper should be  replaced. On re-assembly, lubricate the piston threads with the white grease supplied in the kit (5F1500) (Fig.8).

 Fig 8  tappet 8


9. It is important to lubricate the face of the tappet head that touches the end face of the piston, as the piston must be free to rotate while the tappet head is static. Use black (high pressure) grease supplied in the
kit (5F1505), (Fig.9 and 10).

Fig 9  tappet 9 Fig 10 tappet 10 


10. Lastly, for re-assembly, use of the correct tool makes for accurate refitting of the tappet seal and tappet head (Fig.11).

 Fig 11  Tappet 11


11. Note that the tappet heads and seals can be replaced with the caliper either on or off the vehicle using the MEI tool kit (see the MEI website for full instructions). Tappet Heads and seals are often overlooked and need to be part of the regular vehicle check. Ensure good quality repair kits and the correct service tools are used for the job. It is a straightforward repair, but care must be taken to assess whether corrosion has taken hold or not. The threads need to be able to be cleaned with hand tools so that the pistons wind in and out without excessive force.


Abutments are often heavily corroded.


Abutment 1


Check for steps in the abutment that can snag the pads.


Abutment 2


Carefully dress the abutments with a wire brush, emery cloth or file - be careful not to remove the core metal.


Abutment 4 web


If the abutments are cleaned up without steps then the carrier is ok to use.


Abutment 5