- Air Chamber
- Guide Sleeves
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).
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:
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.
Preventing Water Ingress
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.
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.
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.
HOW TO CHECK
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.
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.
MAINTAINING YOUR GUIDE SLEEVES
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.|
CHANGING THE GUIDE SLEEVE SYSTEM
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.|
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.
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.
Abutments are often heavily corroded.
Check for steps in the abutment that can snag the pads.
Carefully dress the abutments with a wire brush, emery cloth or file - be careful not to remove the core metal.
If the abutments are cleaned up without steps then the carrier is ok to use.