Fault Finding – Solving Your Air Brake Problems - Part 1
Fault Finding – Solving Your Air Brake Problems - Part 1

Your air brake system can produce a number of issues, and it’s not always easy to find the solution. Here are a few great fault finding tips that will go a long way in finding and fixing the problem with your brakes.


The Problem: Slow Air Pressure Build-up

Here are a few possible causes and suggestions:

  1. Restriction in the compressor inlet pipe.
  2. Compressor cylinder head gasket leaking.
  3. Restriction in the compressor delivery pipe. (Check for carbon build-up).
  4. Unloader piston in the compressor head sticking. (On American compressors).
  5. Unloader valve cut-out pressure set too low.
  6. Blocked air dryer cartridge.
  7. Bad leak somewhere in the brake system. (Couple workshop air supply line directly into the Air Dryer inlet port and listen for air leaks without the engine running.
  8. Couple an air gauge directly onto the end of the compressor delivery pipe to check the performance.


The Problem: Air Dryer Constantly Purges

Here are three possible causes:

  1. Bad leak in the delivery pipe between the air dryer out port and the multi-circuit valve inlet port. 
  2. Faulty check valve at the unloader in the air dryer.                                                                                            
  3. Air leaks in any of the multi-circuit valve delivery pipes.                                                                                   


The Problem: The Multi-circuit Protection Valve

Here are a few possible suggestions:

  1. Circuits 21 and 22 should open up first to allow pressure to supply the main service brake system via the Foot Bake valve. The opening pressure of these two circuits are normally set at about 6 to 6.5 bar.                       
  2. After the service brake pressure has built up to about 6,5 bar, the third and fourth circuits (23&24) open up at about 7 bar to supply pressure to the park brake and trailer control systems, as well as the Auxiliary system.               
  3. If these pressure settings are not set correctly, it is possible to be able to release the Park brake and the Trailer brakes, without having enough air pressure in the Service brake system to apply the vehicles service brakes.


The Problem: Air Pressure Only Builds Up to Around 6.5 Bar

Here’s what you can look for:

1) Usually the problem is due to a bad leak in circuit 23 or 24. As mentioned above, circuit 21 and 22 start opening up at about 6,5 bar and build up until about 7 bar, when circuits 23 and 24 open up, indicating that there must be a bad air leak in one of these circuits, preventing the system to build up to its maximum operating pressure. One of the most common reasons for this is a leaking Park brake actuator (Spring Brake booster).                                                                                                                                                               

2) It is easier to find an air leak if the engine is not running. That is why it is a good idea to couple the workshop air pipe into the beginning of the air system, like into the inlet of the Air Dryer or into the Compressor delivery pipe, for example.             


The Problem: The Quickest Way to Establish the Reason for an Air Brake Valve to Leak

  1. The numbers that are cast into all the valves should be remembered like this. Any port beginning with the number 1, (i.e. 11 or 12 etc.), are air supply ports and are continuously supplied with air pressure when the system is charged. Any port beginning with the number 2, (i.e. 21 or 22 or 23 or 24 etc.), are air delivery ports. Any ports beginning with the number 4, (i.e. 41 or 42 or 43 etc.), are signal, or control, ports. Ports marked with a number 3 indicate that it is the exhaust port.                                                         
  2. If a valve is leaking through its exhaust port, remove all the fittings out of the ports beginning with the number 2. If after you have done this, the valve still leaks, then the valve is faulty. However, if the valve stops leaking then it is a good indication that there is another valve somewhere down the delivery pipe line that is feeding pressure back, causing it to leak through the valves exhaust port. Establish which valve, or unit is causing this feed-back and replace that unit.


With these handy tips, you’ll find it much easier to determine the problem within your air brake system and make the necessary repairs. Need the help of an expert? JMR specialises in brake systems and parts and we uphold the highest standards in workmanship. Contact us today for all your brake system needs.

Wheel Fastening on Trailers: What You Didn’t Know
Wheel Fastening on Trailers: What You Didn’t Know

For many years, the majority of vehicles have originated from Europe; as a result, South African trailer manufacturers have followed the DIN Spherical Stud mounting which has allowed a high level of wheel fastening compatibility between trucks and trailers. This also allows for an interchange of wheels within the fleet and the possibility of carrying only one or two spare wheels suitable for both the truck and trailer in a combination.

However, most of the “new generation” trucks coming into the country are now fitted with spigot centre wheels, while the majority of trailers are still fitted with stud centred wheels. This obviously creates a great problem with spare wheels and combinations of vehicles in which the truck is fitted with spigot mounted wheels while the trailer runs on stud mounted wheels. And because of this, there is the increased potential of wheel mounting failures. It is therefore important to know, that while the stud centred wheel and spigot centred wheel seem to be similar and the stud PCD is the same, the two methods of wheel fastening are not compatible.

Stud Mounted Wheels

In the case of stud mounted wheels, the stud holes in the wheels are countersunk and the face of the nut, or a locating washer, is bevelled to suit the countersink. It is therefore essential to understand that in the case of stud mounted wheels, the concentricity and the securement of the wheel, in relation to the hub, relies entirely on the bevelled interface between the studs, nuts and wheel face.

As per ISO 4107, spherical and conical centring wheels can only be used on stud centring hubs (the wheel is centred and carries the load on the stud of the axle). The main dimensional characteristics of the wheel are the countersunk holes off either a radius of 18 mm or a 40 degree angle. The bore also has an open tolerance.

Spigot Mounted Wheels (as per ISO 4107)

Spigot mounted wheels are only used on hub centring axles (the wheel is centred and carries the load on the bore of the wheel/hub of the axle). The main dimensional characteristics of the wheel is the 26mm stud hole with no countersink and the tight tolerance bore of 281 mm ( -0mm+0.2mm). The diameter to support the spigot mounted wheel would be 280.8 mm (-0mm+0.2mm).

Friction at the clamping face is of outmost importance to a spigot mounted wheel. Any loss of clamping force between the rim and drum/hub is very serious on a spigot mounted rim because it relies on it for fixing.

For more details on correct wheel fastening, contact your closest JMR branch.

Did you read the article Hub Failures: What to look for?

Brake Linings on Trailers - Maintenance
Brake Linings on Trailers - Maintenance

Continuing on our first article on brake linings for trailers, we will briefly touch on the importance of quality when it comes to choosing brake linings, provide some interesting facts and give practical advice on maintaining your brakes for optimal use.

Brake Fade and Recovery:

When the brake lining gets in contact with the brake drum heat is generated, this has to dissipate from the outer drum surface. Once the drum gets very hot due to the increase in temperature the lining will start to fade – losing the friction level.

Brake Fade is prevalent in all friction materials though with a good friction material bought from a reputable company the fade process is gradual and within safety standards. Recovery of the brake lining to its original properties is very crucial and this ability to recover is a very important factor between a good and an inferior lining.

Poor recovery of the brake lining will severely damage the brake drum and considering that a brake drum costs by far more than a set of linings it would make in the final end no sense to buy “cheapies”.

That is why JMR is focused on supplying quality brake linings at a competitive price to ensure our clients get the best value for their money. For a free, no-obligation quote, contact your closest JMR branch.

Lining Material:

Resin impregnated cotton fabric was one of the first material specifically developed for friction application, though it has been replaced by brake friction materials made from Chrysotile asbestos compounds because of the excellent heat resistance.

Studies in the 90’s have shown that this material is highly toxic and a high proportion of brake mechanics were afflicted by inhaling asbestos dust. Therefore asbestos brake linings were banned and no longer used. Nowadays brake linings are manufactured from a variety of different material that may be non-asbestos or low metallic. KEVLAR is one of these materials.


When a trailer comes to the workshop for a brake re-line, don’t discard brake linings or swop them before having a close inspection at the linings. Brake lining wear can tell you a lot about the function of the brakes.

Lining wear should be even on a perfect functioning brake assembly. Worn anchor pins or pin holes or S-cam bushes can allow the applied brake force to push the shoe(s) to one side, resulting in tapered lining wear.

If the brake linings are cracked on the shoe there is the possibility that the linings were loose on the shoe, this occurs when the clamping force of the rivets is not sufficient or the rivet holes were drilled too big. Linings must be tight against the brake shoe – no gap must be visible.

Here are a few aspects to remember:

  • If linings are contaminated with grease discard them. Do not attempt to clean them.
  • If the brake linings are in-advisedly mixed, and the incorrect materials have a lower friction than is needed, the output of the brakes concerned will be reduced.
  • If linings are not replaced when worn to the limit recommended, there is the possibility that they may wear right down to the rivets, causing damage to the drum as well as causing extensive heat formation. There will be a great reduction in friction level from the metal-to-metal contact.
  • Badly worn shoe anchor pins and bushes can allow the shoes to move out of place and rub on the drum.
  • Brake shoe return springs which have lost their tension because of previous overheating can allow shoes to rub on the brake drum surface.
  • A faulty quick release valve can cause pressure to be retained in part of an air brake system. This will not cause continuous drag, but will cause the brake to “hang on” for a short time and could cause overheating.
  • Different linings on the axle can cause “brake in-balance” and can be a reason for faster wear on one side.
  • Damage to the brake shoes, torque plates and other components can all affect the relationship between linings and mating surfaces and cause noise.
  • The weight of the vehicle will have an effect on the function of the lining.
  • Type of terrain will influence the life of the lining.
  • Make sure that the brake air pressure is set correctly.
  • Always replace the linings on both sides of the axle.

It is vital and of great importance for any Transport Operator to look at the long-term costs and not only at the purchase price of a set of linings. A poor selection of brake linings will in the end increase the running cost of the fleet dramatically – never mind the safety aspect.

Always remember that brake linings are safety critical components!

Don’t just look at the price – find out where the brake linings were manufactured, who manufactured them, and ask for performance certificates. Always ensure that you make use of a reputable company for servicing your brakes.

Doesn’t it make sense to look after your brake linings?

That is why JMR is dedicated on delivering quality workmanship to ensure that your brake linings are re-aligned and skimmed right the first time. The process that we follow:

  1. The shoes are brought in and inspected for worthiness of relining;
  2. The shoes are then stripped of all old linings and rivets;
  3. The shoes are then sandblasted and cleaned for relining;
  4. The shoes are then sprayed in the renowned JMR colours;
  5. The linings and rivets are placed on the shoes, ready to be pressed;
  6. The rivets are then pressed, holding the linings onto the shoes;
  7. The shoes are then inspected for accuracy and packed for shipping.

To contact your closest JMR Branch, click here.