by Steve Turchet
Many of us in the historic military vehicle (HMV) hobby have probably been in situations when we needed to leave the a vehicle’s engine running while parked on a slope — the parking brake wouldn’t hold. This can happen on any size or type of vehicle from a WWII jeep to a Vietnam-era 5-ton truck. At best, these situations are annoying. At worst, they can mean a runaway vehicle and possibly, a lot of damage.
How much holding power should a parking brake have? The California vehicle code used to say that “a parking brake should be able to hold a vehicle on any slope on which it is operated.” That probably didn’t take into account some off-road situations that a driver may encounter with a jeep, MUTT, M37, HMMWV, or M35.
Still, most parking brakes were designed to hold a vehicle on any slope on which it might be operated. If your vehicle’s brake won’t hold, it usually means there is something wrong with the mechanism that applies the brake and/or the brake lining.
Even though some people call a parking brake an emergency brake, most parking brakes were not designed to stop a moving vehicle. In this article, we will cover the parking brake systems of most common collector vehicles in the HMV hobby and learn how to service and maintain them.
TWO TYPES OF BRAKES
There are two basic types of mechanical parking brake systems used on most vehicles with hydraulic service brakes, as well as on many vintage vehicles equipped with air brakes. The first type uses a drum or disk on the transmission or transfer case output. When a lever or handle is pulled in the driver’s compartment, it actuates a rod or cable linkage and a brake band, brake shoes, or brake pads clamp to the drum or disk, locking the drive shaft so the vehicle’s rear wheels can’t turn. This system is used on many common HMVs, including most jeeps, Dodge WCs and M37s, Kaiser M715s, some early model HMMWVs, most deuce-and-a-halfs, five-ton trucks, and also on many larger vehicles and construction or material-handling equipment.
The second basic type of parking brake system is often called a “U-brake.” This uses a single cable that runs to both of the vehicle’s rear wheels and is attached to a pull point, forming a “U.” When a lever or handle is pulled in the driver’s compartment, the cable mechanically applies the brake shoes or pads at both rear wheels. The U-brake system is used on many CJ civilian jeeps, automobiles, light trucks, and CUCVs.
Some early U-brake systems applied only one brake shoe at each rear wheel. The result was that the parking brake usually kept the vehicle from rolling forward, but was less efficient in keeping the vehicle from rolling backward. Most later and current U-brake systems apply both shoes at each rear wheel so they have the same holding power in either direction. In addition, most later-model HMMWVs use a similar system that mechanically applies the rear wheel disk brakes.
Both systems are independent of a vehicle’s hydraulic service brakes. If in proper working order, they will hold a vehicle stationary and may even bring it to a stop if the service system fails. Likewise, most parking brake systems are also independent of any “park” feature in an automatic transmission.
It might be said that there is a third basic type of parking brake system, often called a Mico Brake (many were manufactured by Mico Corporation). However, few U.S. military vehicles were equipped with Mico Brakes with the exception of some G-749 series trucks. And, since a Mico type brake uses a vehicle’s hydraulic service brake system, it is useless if the service system fails. Mico type brakes are not meant to be left applied for more than an hour or so because this may cause the wheel cylinders to leak.
The two basic parking brake systems can be sub-typed by the components that hold a vehicle in place. The three basic sub-types of the first system are the external contracting band, the internal expanding shoe, and the external contracting pad (or disk type) brake. Similarly, the second basic parking brake (or U-brake) system can be sub-typed into shoe or disk.
HOW PARKING BRAKES FAIL
Since we’re talking basics, it may also be said that there are usually two main reasons why a parking brake won’t hold. The first is that the mechanism that applies the brake — the handle or lever in the driver’s compartment, and/or its connecting linkage — is worn-out, broken, or out of adjustment. The second basic reason a parking brake won’t hold is that the lining, shoes or pads are worn-out and/or not properly adjusted. In older vehicles one may encounter a combination of any of the above.
In theory, the band lining material, shoes, or pads of the first type of parking brake system should never wear out nor require much adjustment for the life of a vehicle. After all, a parking brake is meant to be applied only when a vehicle is stationary and released before the vehicle begins to move. So, unlike a service brake system, the lining material should seldom come in contact with a rotating drum or disk. Therefore, the only problems you should have with the first type of system is normal wear on the pull-handle or lever mechanism and their connecting linkages.
In practice, however, parking brakes are often accidentally left on when a vehicle is being driven, and the driver only realizes this a few miles down the road when puzzled by the vehicle’s lack of power and / or begins to smell the hot parking brake band, shoes, or disk pads. In extreme cases, an overheated parking brake may cause a fire… especially if oily or greasy. In fact, one of the reasons why HMMWV parking brake systems were changed from a disk on the drive shaft to a U-brake configuration was that the heat caused by the drive shaft brake being left on could melt the plastic fuel tank.
Then there are some folks who pull the parking brake handle or lever while a vehicle is still moving — sometimes just to childishly skid the rear wheels like a kid with a BMX bike — and a few people, myself included, have had to operate vehicles with non-functioning service brakes. Of course there are also emergency situations in which you might have to use a parking brake to stop or slow a vehicle if its service brake system fails. So, again in practice, it’s common for parking brake lining material to wear out and/or need adjustment; though the best way to prevent this is to use a parking brake as it was meant to be used... FOR PARKING ONLY. Apply it only when a vehicle is stationary and release it before the vehicle begins to move.
On the other hand, since the second basic type of parking brake system — the U-brake — uses the service brake shoes or pads, these items wear out during the normal operation of a vehicle.
BANDS, SHOES, OR PADS
The two main problems commonly encountered with parking brake systems are wear in the handle, lever, and / or linkage mechanism that applies the brake and wear of the lining material of the band, shoes, or disks. Since there are many different types of handles, levers, and linkages used on common HMVs, it’s impossible to cover all their adjustments and repairs in the space of this article. So, as I always recommend, buy the service manual for your particular vehicle.
The most common problem with any of these mechanisms is wear in the components of the handle or lever, especially in the ratchet or locking parts. When this happens,the handle or lever won’t stay locked after being pulled and / or slips when left unattended. The best fix is to replace worn or broken parts; though if you’re fairly skilled at welding and fabricating, you may be able to build up worn ratchets and other parts in your home shop. Most such components for HMVs are offered NOS, repro, or as take-outs by many of the advertisers in this magazine.
The second main problem usually encountered with parking brakes is worn or out-of-adjustment lining material. Assuming your vehicle’s parking brake handle or lever and linkage components are in good shape and working properly, about the only other reason why a parking brake won’t hold is because of the band, shoes or pads.
Let’s begin with the first basic type of parking brake system that uses a drum or disk. These are usually mounted on the rear of the transmission or transfer case.
We know there are three basic sub-types: External contracting band, internal expanding shoe, and the external contracting pad. Some early WWII jeeps used external contracting band parking brakes, though most later U.S. military jeeps were fitted with internal expanding shoe systems. External contracting band parking brakes were used on many WWII vehicles, such as Dodge WCs, the Chevrolet G-506, and the GMC CCKW and DUKW, as well as on many other trucks, fighting vehicles, and equipment. Although they look very simple, many people don’t know how to properly adjust them.
Adjusting External Contracting Bands
Adjustment of external contracting band brakes should always begin with a thorough cleaning and inspection. If the band material looks worn out, it probably is. The only way to be sure is to remove the band from the drum. This is usually a simple procedure and the process should be obvious.
It’s a good idea to line up all the parts — especially the little springs and other small items — in a row on your floor or workbench in the order in which they came off and the direction in which they were facing. Most parking brakes are simple mechanisms, but they need all their parts to work right, and all of those parts must be installed correctly. On older vehicles it’s quite common for some of these parts to be missing or incorrectly installed. Buy a manual.
An important thing to check is if the drum is loose on the drive shaft universal joint yoke. If so, it will wobble and wear out the band lining.
Another thing to check is if the yoke is loose on the transmission or transfer case output shaft. If so, not only will it make the drum wobble, but it will also wear out the transmission’s oil seal and oil will leak onto the brake. This is especially common on jeeps.
If, as is often the case with older vehicles, the band lining material is worn down to its rivets, burned, charred, cracked, brittle, or soaked with oil or grease, it should be replaced. It used to be common practice to soak oily brake bands in gasoline and then burn out the oil by lighting them on fire — that probably isn’t politically-correct today! The usual options are to buy an NOS, repro, or good take-out band assembly; or you can have your band relined by a brake company or a firm that specializes in friction materials (such as those used on friction cranes).
If the drum is badly worn, deeply corroded, or scored by the lining rivets, it should also be replaced. If not badly damaged, you may be able to smooth it with sandpaper. It’s generally not a good idea to have such drums turned on a lathe because this may increase their inside diameter to the point where a stock band can’t be adjusted properly, and the brake won’t hold. In the old days, most good brake shops could compensate for this by shimming or using thicker lining material.
Assuming that your band is still in good shape (or you got a replacement or had it relined), it’s time to reinstall it. Be sure that all the parts are back in their correct locations. Then, block the vehicle’s front wheels and jack up the rear so the back wheels are off the ground. Put the transmission and transfer case in neutral, and completely release the parking brake mechanism so the rear drive shaft can be turned by hand.
The first step in adjusting an external contracting band brake is to set the band-to-drum clearance at the anchor point. While the illustrations in this article are of a CCKW, the procedure is generally the same for all such type brakes. The clearances shown will usually work on most common HMVs, such as early WWII jeeps, Dodge WCs and M37s, M715s, etc.
Note: Many anchor bolts or screws are secured with lock wires. Don’t replace the lock wire until all the adjustments have been made, and you have road-tested the vehicle — you may have to make more adjustments.)
The second step in adjusting this type of brake is to set the band-to-drum clearance at the bottom, and the final adjustment is setting the band-to-drum clearance at the top. After these three adjustments have been made, rotate the drum or drive shaft by hand to make sure the band isn’t dragging on the drum. Then make any adjustments in the linkage to ensure that the band clamps tightly around the drum when the handle or lever in the driver’s compartment is pulled. If the band is new or relined, the handle or lever probably can’t be pulled out (or back) as far as before you replaced the lining material.
The important thing to check now is that the band fully releases when the handle or lever is disengaged so it doesn’t drag on the drum. (Of course you will lubricate all the linkage points so everything works smoothly.)
After making sure that the band fully releases and checking again that it doesn’t drag on the drum, take the vehicle for a drive. After a few miles, stop to see if the drum and band are getting hot (a new or relined band may make the drum feel warm until the band runs-in. After it does, you may have to re-adjust the band). Then, park the vehicle on a steep slope. Check to see if the parking brake holds. If all is okay, drive home. If a lock wire had been used, re-install the lock wire on the band’s anchor point.
Adjusting Internal Expanding Shoes
Adjustment of the internal expanding shoe parking brake, as used on many jeeps, also begins with a thorough cleaning and a careful inspection. As with the external contracting band, a manual will be handy to make sure your brake has all its parts — and that they are installed correctly.
It’s fairly common on jeeps for the drum to be loose on the drive shaft yoke, and / or the yoke to be loose on the transfer case output shaft. It’s also common for the transfer case oil seal to be leaking. Now would be a good time to replace that seal.
Check the drum for excessive wear, deep corrosion, scoring or cracks. If badly damaged, replace it.
Likewise, examine the brake shoes and replace them or have them relined if necessary. When everything is back together, adjust the shoes so they don’t drag on the drum when rotating it by hand but are tight enough that they clamp firmly against the drum when the lever or handle is pulled.
Although the illustration shows 3/32 of an inch clearance between the actuating lever and the back of the brake mounting plate when the brake is fully released, the mechanism is usually so worn on older jeeps that you will probably have figure out the best adjustment on your own.
It’s common for the actuating lever bushing and its pivot bolt to be badly worn on jeeps, so you should consider replacing them. Also make sure the spring that holds the bell crank in the released position is strong enough to do its job.
After making all the adjustments, check that the shoes fully release when the handle or lever is released. Then take the vehicle for a drive. Stop after a few miles to see if the drum is getting hot. If all seems okay, park on a steep slope and set the brake to see if it holds.
Adjusting External Contracting Pads (or Disks)
Adjustment and inspection of an external contracting pad (or disk) type parking brake is no different from the two other types. Do a thorough cleaning, check the pads for excessive wear or damage, and check the disk for deep corrosion, scoring and cracks. Also check that the disk isn’t warped. (On older HMMWVs with this type of parking brake, corrosion is often present and the disk is often warped.) Final adjustment is made so the pads fully release and don’t drag on the disk. Then take the vehicle for a drive as in the two previous examples.
On most vehicles with U-type parking brakes, the shoes, or disk pads are adjusted — or self-adjusted — during service brake maintenance. The main adjustments are only in the handle or lever and linkage mechanisms so that the parking brake mode is fully applied when the handle or lever is pulled and releases all the way when the handle or lever is released.
If the brake shoes or pads are badly worn or out of adjustment, this should already be apparent by poor service brake operation. Again, if the shoes or pads are badly worn or out of adjustment, the parking brake may not hold, nor will the service brakes work properly.
For HMMWVs with U-brake systems, the important things to check are that the pads fully release and don’t drag on the disk (or rotor). Also inspect the brake cables because they may be crushed or damaged by the rear control arms. There was an official fix for this problem that required new cable brackets, but the military also recommended wrapping the cables with rubber tape at the points of wear. Many surplus HMMWVs are heavily corroded, so it’s important to be sure that everything that moves underneath is free and well-lubricated, including the parking brake components.
No smart driver ever completely trusts a parking brake. You should never leave a vehicle parked on a slope with only the parking brake to hold it. The vehicle should be left in either first if facing uphill or reverse gear if facing downhill. In additional, always turn the front wheels in the appropriate direction if parked on a street with a curb. If the vehicle has an automatic transmission with a park position, “PARK” should be selected.
If the vehicle is to be parked on a slope for an extended time, or if the engine has to be left running, block the wheels securely using actual chocks if possible. Bricks may crumble, and stones or wood blocks can pop out. Steel chocks can be found at many auto parts stores — though be sure they’re adequate for your vehicle’s size and weight. Wooden chocks can be easily made. Chocks are also handy when using your vehicle’s winch.
Another smart habit is to always be sure the parking brake is fully released before driving away!