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Caliper Brake Assembly Maintenance

Careful installation, routine inspection, and proper maintenance of disc brakes on heavy equipment are essential.
 
A brake is a device for turning kinetic and potential energy into heat. Although heat is a natural product of braking action, it also is one of the enemies of brake life. Other brake enemies include dirt, misalignment, improper installation, and misapplication. 
 
Troubleshooting brake systems is a fairly simple procedure and routine inspection maintenance can solve many brake problems before they become severe. Our brake manuals recommend daily inspection. A quick check of system fluid level and a visual check of the brake itself can tip a mechanic to impending trouble. The daily check should include a look at the linings for severe wear, the brake body for leaks and the master cylinder for fluid. In checking the lining. make certain that the entire lining has the same thickness. A taper or worn spot in one area means the lining should be replaced. The lining opposite the mounting often tends to wear more rapidly and inspection should watch for this condition.
 
A daily check of this nature will eliminate the three basic causes of most premature brake failure. Other items to watch for in a daily visual inspection are the discs themselves and exposed lines. If a walk-around inspection reveals one or two cool discs at the end of a working shift, an investigation should be made to find out why those brakes are inoperative. Rock damage or dirt buildup on the discs should indicate corrective action. Discs with heavy grooves or wear marks should be measured to make sure they have not worn past the manufacturer's specified minimum thickness.
  
Fluid Maintenance
Without regular inspection, a leaking system can run dry and potentially cause a catastrophic failure. Even if no catastrophe occurs, you will have to refill the system just to determine where it is leaking.
 
A system with no visible leaks, a dry master cylinder and no makeup reservoir could be indicating excessive lining wear. In some cases, the makeup fluid required to compensate for lining wear can pull system fluid levels down enough to make it appear that brake failure has occurred. If this is the case, check the linings before tearing into the system.
 
Fluid loss calls for an inspection of all system lines and fittings. Always check the lines before going into the brake itself. If a loose fitting or a punctured line is found, drain the remaining fluid and inspect it for contamination. Dirt can be sucked in through a loose fitting or a puncture and it will cause accelerated wear on seals and surfaces. Water in the system, even a very small amount, drastically reduces the boiling point of the fluid and a little water can cause the fluid to boil off in normal operating temperatures. Foamy fluid, caused by excessive aeration, should be discarded. The tiny bubbles will eventually consolidate into large bubbles which will block the system.
 
Whenever handling fluid which might be returned to the system, the standard procedure calls for a clean container free from any other residue or film which could be dissolved by the fluid and dumped back into the system. After refilling a dry system, a thorough bleeding of the brake system will be required.
 
If a leak is found in the brake, the culprit will be the pistons, piston seals, caps, cap seals, pipe plugs or bleeders. In this case, a major brake repair will be necessary.

Dirt Problems
Mud and wet sand can cause extensive brake problems if left unattended. Equipment exposed to mud should be inspected and the brakes should be cleaned before the mud dries overnight. Dried mud build-up around the boots may cut them and/or disengage them from their mating piston groove.
 
Wet sand or clay, particularly with some binder in it, turns the brake into a self-destructing grinding machine. Most lining wear will occur when the material has been left to dry. Rigs running in wet sand or clay should receive frequent lining inspections along with a daily cleaning.

Noise and Check Valves
Brake squeal in a disc brake system does not necessarily mean anything is wrong. Squealing when the brake is not applied can be reduced or eliminated by removing or opening the check valve. Essentially, the check valve puts pressure into the system to keep the lining and disc in slight contact at all times and helps to minimize brake reaction or response time.
 
There are several methods of overcoming check valve back-pressure, including drilling or punching a small diameter hole into the end of the valve. Another method is simply to remove the check valve, but bleeding will be easier when the punched or drilled check valve is used. This will relieve the pressure on the system and should reduce operating heat and noise.

Component Repair
When the cause of a brake problem has been identified, the manufacturer's manual should be consulted for proper procedures on disassembly, repair, and reassembly. Replacement parts should be bought from the vehicle manufacturer. Another part may fit but it won’t necessarily be designed for all the stresses and tolerances the vehicle manufacturer designed into the brake. And a ''will-fit" part could void warranty on the brake as well as the entire machine.

Linings
In most installations, disc brakes allow quick replacement of linings without requiring removal of the wheel or rim.
 
The first step is inspection, and the mechanic should have at least ¾ of an inch on the opposite side of the brake to accomplish the lining change. After loosening the self-locking cap screw holding the torque pin, the pin must be pulled. An improvised puller can be made from a 3½-inch-long bolt with ½-13 thread, a 1½-inch pipe nipple and a couple of flat washers. The opposite torque pin may have to be removed and can be driven out if necessary.
 
When the pins have been removed, the bleeder hose should be attached to the bleeder valve and the valve opened to relieve pressure behind the pistons. The lining can then be removed by sliding it out from around the disc.
 
Before installing the new lining, insert a piece of bar stock or a flat tool approximately ³/₁₆ inches thick by about ½ inch wide and rotate it against the pistons until both pistons are fully retracted. Close the bleeder valve. 
 
The new lining carriers can now be slipped into place. Reinstall the torque pins, tighten the self-locking cap screw and actuate the brakes several times. Top off the fluid reservoir if necessary and then send the machine back to work.
 
Internal repairs and component replacements require an inter-related procedure that is best described as the total assembly of the brake.

Assembly
The basic disc brake described here contains the essential disc brake components: a disc, torque plate, torque pins, self-locking cap screw, tube and nut assembly, carrier and lining assemblies, dust boot, pistons, bleeder valves, inlet cap, caps, hex head bolts, and preformed packings.

Assembly begins with the torque plate. The plate will have two or four piston bores for a corresponding number of pistons. Half the bores will have a blind closure and the remaining half will be closed with caps. Each bore contains two machined grooves, one for the packing, the second for the dust boot.
 
The preformed packings are inserted into the grooves of each bore. The dust boot is placed in the boot groove. The boot has an outside lip and an inside lip. The outside lip is inserted in the housing and the boot will have enough hoop strength to support it in position until the piston is installed.
 
Before installing the piston, it should be lubricated with the fluid to be used in the brake. The piston is then placed inside the boot and pressed into the bore until it contacts the packing. The groove near the face of the piston is for the inside lip of the boot. Using both hands, press the piston with a rolling circular motion until the piston has engaged the seal and then firmly until the piston is bottomed. The dust boot lip must be well seated in the piston groove. Follow the same technique with each piston until all are seated.
 
Check the end of the torque plate to be sure the inverted flare fittings have been installed. If not, install the fittings, then install the tube and nut assembly with a brake line wrench.
 
Next, install both caps over the piston bores. Before placing the caps over the bores, make sure the packings are not spiraled or twisted and that they have been lubricated with the fluid to be used in the brake. If the brake has four or more pistons, check the service manual for location of the inlet cap. Install the caps with hardened flat washers under the bolt heads and torque the bolts to 38 to 42 lb. ft. dry thread.
 
When the cap bolts have been torqued, install the long bleeder valve on the side opposite the caps. This is the side of the brake that will be inside the wheel well and the long bleeder will assure accessibility even with wheels and rims in place. Install the short bleeder valve. Locate the torque pins and check their O.D. against the holes. Remove burrs and corrosion with emery paper. Grease on the pin and in the holes will facilitate installation.
 
Note the circumferential groove around each pin. The self-locking cap screw must contact this groove when it is installed. Put in three of the torque pins but leave the fourth barely started until the lining is in place.
 
Install the lining and push the remaining pin home. The cap screw does not require any torque to activate the self-locking feature. However, a torque of 38 to 42 lb. ft. (dry) will ensure that the pins won't move in service. It is important that the cap screw contacts the circumferential groove in the pin.
 
Installing Brake
When the brake has been completely assembled, it can be installed on the vehicle as a unit except for the disc. Normally, the disc is installed first. Use hardened flat washers and grade eight bolts. The disc is going to take a lot of torque and the bolts must be up to the job. If there are no grade eight bolts in the shop, let the job wait until you get them. Torque the bolts to the manufacturer's recommended specifications before installing the brake head.
 
Keep all mounting surfaces of the brake head clean and dry during installation. Center the brake head over the disc. The brake will accommodate up to .050-inch misalignment in either direction, but it is preferable to have it centered as exactly as possible. Use hardened washers and grade eight bolts for mounting the brake head. Some heads will mount with cap screws. In either case, the washers should be next to the cast iron housing, and they should always be flat washers. Lock washers cannot be used in any part of the brake head or disc mounting assembly.
 
Operational Check
When the head and disc are in place and the bolts are torqued, check the torque pins for positive clearance in relation to the linings. The pins, linings and shoe assemblies should all have positive clearance in relation to the disc. If any torque pin is in contact with the disc, loosen the self-locking cap screw one turn and move the torque pin away from the disc as far as the groove will permit. Retighten the self-locking cap screw.
 
If the lining is not free to slide on the pins, check the pistons for full retraction. Open the bleeder valve and push the pistons back into the head. If this problem persists after corrective action has been taken to no avail, contact the man­ufacturer's field representative or service engineering office for assistance.
 
Be sure the linings are burnished. If the linings or discs were painted or coated, drag the brakes until any paint or coating has been burned off by friction.
 
If the system has an air boost, make sure the limit does not exceed 1500 psi under normal brake application.
 
Assembly techniques call for good housekeeping – and common sense. Keep sharp tools away from rubber seals. Make sure rubber seals and O-rings are the right size and the proper rubber compound for the fluid to be used in the system. Lubricate the seals with system fluid before installation. Keep oily hands, rags, and tools away from exposed components and keep the work area clean.
 
As a final check on the assembly, make sure the long bleeder valve is accessible with the wheel and rim installed. If not, check to see if the correct bleeder valve is being used.
 
Maintenance Beats Repairs
Realizing that disc brakes require attention and service is half the battle. The tremendous torque and heat loads imposed on heavy equipment brake systems require careful, routine attention even with the best system.           
 
Remember that the best place to check when a brake problem is reported is the fluid. Then check the fluid system. Water contamination and excessive lining wear can cause low system fluid levels without any system leaks. Try to give each brake system in the spread a daily visual check for fluid level, obvious leaks and dirt buildup around the disc and brake head. Dirt, even on a self-cleaning brake, can cause accelerated wear and seal damage.
 
Use fluid that meets vehicle manufacturer's specifications and replace the fluid periodically to keep the system free from water and dirt contamination.
 
Keep the service manual handy and refer to it when making any adjustments or overhaul of the system. Replace components, including linings, with components supplied by the vehicle manufacturer. And be especially careful with small items like nuts, washers, and bolts. When the manufacturer specifies a hardness or grade, it's a good bet the specification came about as a result of failure testing. Anything less than the specified hardness or grade is asking for trouble.
 
Finally, don't be afraid to call on the manufacturer of the vehicle for advice or assistance, particularly if a pattern of problems or failures begins showing up despite good, routine maintenance and properly completed repairs.
 
Many times, the vehicle manufacturer will contact brake system suppliers if he does not have a solution to the problem. Operating conditions sometimes cause certain problems to appear, and it is possible that the vehicle manufacturer has encountered a similar problem in laboratory testing or in the field. If he has, he'll be glad to share his knowledge and experience with you.