Air Chamber

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.

Guide Sleeves

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.

Tappet Heads

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

Fully Sealed

Many calipers in the field have short guide sleeves that are not fully sealed. The guide sleeve is resistant to corrosion, but water and debris can still get between the sleeve and rubber bush, causing excessive wear or in some cases, restriction to sliding. As has been discussed in previous articles, a sliding system that is not free to move can cause the outboard pad to drag and increase the wear rate of the friction material. In severe cases, the residual dragging can cause elevated brake temperature.

 Sleeve 1  Sleeve 2
 The extended inboard end of the rubber bush  The relieved casting and extended metal ring for fitment of a sealing boot 


The sealing boot has a stainless steel insert, which comes pre-assembled on the short guide sleeve (below).

Sleeve 3

The first step is to assemble the guide sleeve bolt into the guide sleeve (fitting it later can be tricky, as the black rubber boot restricts the entrance); grease the rubber bush with the grease provided and insert the short guide sleeve from the disc side. With the sleeve inserted into the rubber bush, pinch the grey rubber boot between your finger and thumb, and stretch the bead into the groove of the metal ring on the end of the rubber bush (below).

Sleeves 4

When the boot is fully in place on the metal ring, it should look like the image below.

Sleeves 5

Make sure the bead is seated all the way round the metal ring. Following this, it’s time to fit the carrier. Tighten the bolts to 180Nm +90° and ensure  the carrier is firmly clamped in a vice at the side you are tightening. Be careful not to damage the black rubber boot with the 14mm hexagon key (see below).

Sleeves 6

Make sure the long guide sleeve and end cap are fitted in the normal way and ensure the carrier remains in a position where the inner boots (green and grey) are compressed. Stretch the black boot onto the outer diameter of the short guide sleeve.

Sleeves 7       Sleeves 8


Next, offer up the green plastic cap so that it just locates in the end of the guide sleeve, but don’t push it into place just yet (see below).

Sleeves 9

Slide the boot off the guide sleeve to make the bead drop into the groove in the green plastic cap.

sleeves 10

With the boot fully located all the way round the cap (left hand image), push the cap squarely and firmly, ensuring that it clicks into place in the guide sleeve.

Sleeves 11   Sleeves 12    sealing caliper sleeve 13


Calipers with a fully sealed short guide sleeve will be available from MEI Brakes in spring 2019. The design will enable MEI calipers to fit all applications that previously ran with an open sleeve only. Vehicles operating in all conditions should benefit from the longer service life of the short guide sleeve system with the fully sealed design.

Caliper Replacement

First of all, it is important to be clear on the reason why a caliper needs changing. Failure to recognise the root cause can result in a new caliper having problems soon after instalment, for example, water ingress through the air chamber. Identifying which replacement caliper you need is also a key issue. If the original part number is still visible, then cross reference is easy. If not, you can refer to ‘ADB Identification’ on the MEI website. When changing a caliper, the whole brake assembly is normally removed. On some vehicles, typically trailer applications where access to guide sleeve bolts is not restricted, it may be possible to leave the carrier in place. If the whole caliper assembly has to be removed, it is important to remember that when loosening (and tightening) the guide sleeve bolts, the carrier must be held securely at the same end as the bolt you are releasing.

caliper replacement 6    claiper replacement 1


Failure to hold the carrier can result in a bent or even broken carrier, due to the forces needed to release (and tighten) the bolts. If you have sufficient access to do this on the vehicle, the brake mounting bolts do a good job of protecting the carrier. With the caliper removed the carrier can now be inspected. The pads can make indentations in the carrier abutments and corrosion can also get a hold. This can result in sticking pads which can cause dragging or taper wear. Clean the abutments as shown below (use file or similar) and if indentations remain, it may be necessary to replace the carrier. The carrier shown had sticking pads but cleaned up well. Ensure the guide sleeve counter bores and threads are also clean and free from corrosion/damage.

Abutment 2      Abutment 5 


New calipers come with a complete fitting kit (below). The correct grease is important as alternatives can affect sliding, or may not have the correct thermal properties. The same applies to bolts. Use the new ones supplied and ensure they are tightened to 180Nm, plus 90°. This high torque ensures the bolts stay tight in all conditions. New bolts are needed as it is not recommended to tighten more than once to this level.


Ensure the seal is correctly located in the groove on the guide sleeve.

caliper replacement 2

Ensure the white plastic ring is fitted correctly. This keeps the seal in the groove. See below.

CVW article 2 image 2web

Secure the carrier in the vice and tighten the guide sleeve bolts to 180Nm, plus 90°. The guide sleeve seal must be compressed prior to cap fitment, which happens naturally if held as shown below.

caliper replacement 5

If it is not compressed, the seal can balloon when the caliper is pushed back for pad fitment and become trapped. The end cap must go in exactly square – using the MEI tool kit makes this job a lot easier – if not there is a chance of damaging the cap, or even water ingress .

caliper replacement 4

Ensure that the long bolt (with lock patch) is used for the long guide sleeve on all calipers (see below).

caliper replacement 3

Before refitting the caliper, check the pads and rotor for damage. Now is the time to change them if there is any doubt. After refitting the caliper, check it is sliding smoothly on the guide sleeves before pad fitment. If the old caliper had seized due to water ingress, there is a chance that the water came from damaged air chamber seals. Therefore, it is crucial to check for tell-tale signs of water ingress. If it all checks out, then tighten the air chamber nuts to a snug torque, fully tightening to 180-210Nm. Set the running clearance (0.6-1.2mm) as follows: Turn the adjuster shaft clockwise until pads touch the disc and resistance is felt (do not exceed 25Nm torque). Next, turn the adjuster shaft anti-clockwise by three clicks (note that each click corresponds to 0.6mm running clearance) to give 1.8mm clearance. This is more than required but allows the adjuster to settle to its natural position. Pump the brake pedal >5 times and the adjuster shaft will be seen to index clockwise, bringing the clearance into specified limits. Finally, check for free running of the disc, fit the adjuster cap and the job is done.

Wear Sensor

Air disc brakes for trucks and buses often have pad wear sensing devices installed in the caliper. There are generally two different types, one with a simple on/off switch that warns when the pads need changing, and the other with a continuous wear sensor providing feedback to the vehicle ECU throughout the pad life. Continuous sensors give an indication of when to change pads, but also allow the ECU to make small adjustments on brake set-up to equalise wear and give diagnostic input. The MEI range has two types of continuous wear sensor:

• Gold Pin Sensor (Stepped), which indicates suitability for a MAN truck
• Silver Pin Sensor (Linear), used on all others with continuous sensing

Old versions of the MEI On/Off switch have silver pins with a white dot in the middle. Newer versions have a red sensor

oldonoff on off v linear stepped
 On / Off (old) On / Off (new)  Linear  Stepped


Function of wear sensors can be checked using a Testing Unit. The tester applies 5V to the sensor and then measures the output according to which of the three types is selected. Checking the output is helpful for predictive maintenance and avoiding downtime through advanced warning of pad condition or other issues.
Method for using the tester:
• Adjust the brake such that the gap between the tappet heads and the outboard pad abutment is 105mm. Note: the use of a pad strap (or any straight edge) to help measure distance to the outboard pad abutment

sensor 2

• Next, plug the tester cable into the wear sensor and note the reading – the image below shows a Stepped Continuous Sensor (Gold Pin) at 0.7V, which is correct for a 105mm gap as shown

sensor 4

• As pads wear, voltage increases and at around 2.8V, the tester will show an orange light to indicate that the pads have worn around 15.5mm per pad (74mm gap), with approximately 5.5mm of friction remaining on each pad (see below)

sensor 5

• When the gap is around 71mm (4mm friction remaining on each pad), the tester will show a red light (around 3.3V) to indicate that the pads are close to the end of their lives. The sensor reads up to 3.56V, equating to a 70mm gap and 3.5mm friction material on each pad (see below)

sensor 6

• The Silver Pin Linear Sensor should read 1V at the 105mm gap (see below)

sensor 7
• Max voltage on a Silver Pin Linear sensor is around 3.5V, which is a 68mm gap and 2.5mm friction on each pad

• The On/Off Wear Switch operates in a different way as the voltage does not change until maximum pad wear is reached. The starting voltage is 0.15V and remains at this level until a gap of 68mm, when it jumps up to 5V
• When the tester reads 5V, this means that there has been 37mm of total wear, which is 18.5mm per pad. The thickness of the new friction material is 21mm. 21 - 18.5 = 2.5mm per pad remaining; it’s time to fit new pads (see below)

 Sensor 8

It’s worth noting that wear sensors on all ADB’s can only measure the distance that the pistons have extended, they cannot account for disc wear. Therefore, if new pads are fitted to a caliper running on a 40mm thick disc (original thickness is 45mm), then the wear sensor or wear switch will give a signal to change pads 5mm earlier (2.5mm per pad). This means the brake pads would have 5mm of friction material remaining rather than 2.5mm when running on a new disc. It is important to assess disc wear when checking the pads, as the minimum disc thickness must never be exceeded (37mm minimum thickness for vehicles fitted with MEI calipers). If the disc thickness is ≤39mm at the time
of pad change, then a new disc should be fitted. If the wear sensor or chain cover becomes damaged, MEI calipers are able to be fitted with a new chain cover and pre-assembled sensor.

Distance P web chain cover table web