The Humble Drum Brake

2 Jul

Introduction

The modern drum brake was first used in a car made by Maybach in 1900, although the principle was only later patented in 1902 by Louis Renault. While drum brakes have mainly been replaced by disk brakes, they are used on all the classic MGs. Even some modern, high performance cars such as BMW and Porsche still use drum brakes for their handbrake.

Drum brakes probably reached their pinnacle in the 1955 Mercedes Le Mans car which had massive inboard front drum brakes, nearly as big as the wheels. Even so, it was still out braked by the Jaguars with their new disk brakes.

1955 Mercedes Le Mans car showing the massive inboard front drum brakes.

My reason for writing this article, however, is not to give the history of drum brakes, it is in response to a problem I suffered with my TC’s brakes.

After 30 years since relining the brake shoes, I decided it was time to replace the now dry and cracked linings. I sent my shoes off to a reputable company where they were quickly refitted with new bonded linings. Unfortunately, when you gently applied the brakes the new shoes grabbed, violently pulling the car to one side, and tended to stick on, especially at slow speeds.

Despite my best efforts and the advice of T-Type experts on Bill and Sally Silcock’s spring tour, it took emergency maintenance in the hotel car park to spot the problem. It is this I want to share with other classic MG owners.

I would also like to thank Barrie Jones for his contributions to this article.

Drum Brakes – How they Work

The following diagram shows the orientation of the brakes on the MG TC and earlier cars; on other cars the pivot may be at the top or side of the brake but they all work the same way.

The operation of drum brakes is very straightforward. There are two shoes covered with friction material inside a drum. Each shoe is pivoted at one end and they are applied by an outwards force at the other end pushing the shoes apart and against the drum face. The pivot carries the whole braking force. When the brake is released a return spring pulls the shoes back.

The outwards force may be applied by hydraulic pistonsor mechanically via cables or rods acting on cams or levers. On the MG TA and later cars, the rear wheels use both a hydraulic piston for the foot brake and a cable-operated mechanical system for the handbrake.

Drum brakes are not perfect. The outward movement of the brake shoe decreases towards the pivot as shown in the diagram. Until the top face of the friction material has worn sufficiently, the lower face will not even contact the drum. As a result, new shoes are more likely to feel spongy or overheat until they have properly bedded-in.

A second problem with drum brakes is heat dissipation. As the friction material covers virtually the whole area of the inside of the drum, the only way for the heat to escape is by conduction through the drum. If used for long periods, e.g. when descending hills, drums can get overly hot causing the brakes to fade. In contrast, the friction material on disk brakes only covers a small area of the disk, the remaining part is open to the air allowing the heat to escape.

The final problem with drum brakes is brake dust produced as the friction material wears. This collects inside the drum and can cause the brakes to squeal and wear to moving parts such as the pivots, adjusters and mechanical linkages.

Self-servo Effect

There is one advantage with drum brakes, they self-servo, i.e. the braking force is increased on the leading shoe by the rotation of the wheel as the friction between drum and shoe “pulls” it on. Unfortunately, the braking force is also reduced on the trailing shoe.

The self-servo effect is the cause of the brakes grabbing or pulling to one side. When Adjustment is by two large bolts on the back plate that set the closed position stops at the top of each shoe. The problem with this arrangement is that it is not possible to set the position of the bottom of the shoes. Hence it is the friction material on the leading shoe touches the drum, the self-servo effect increases the braking pressure and, in extreme cases, it can over-brake that wheel or grab. Any differences between the brakes or adjustments on each wheel can be exaggerated by the self-servo effect and can cause the car to pull to the side as the brakes are applied.

The TC and earlier cars have one leading and one trailing shoe per drum. It takes the same pressure on the brake pedal to stop the car when it is going forwards as backwards.

The TD and later cars with drum brakes had twin leading shoes on the front wheels. On these cars there are two pivots and two hydraulic cylinders generating the outwards force. As a result, it requires less brake pedal pressure to stop when they are moving forwards and by the same token more pressure when reversing. Something pre- MG TD drivers need to be aware of when swapping to more recent models.

The Pivot and Brake Adjustment

The early cars had a pivot pin bolted onto the back plate and a hole in each brake shoe (as represented in two previous diagrams – but not the one above, which is a twin leading shoe diagram).

The shoes were held in place by a spiral spring and horseshoe clip, rigidly locating them. It was problems with this pivot arrangement that causedmy brakes to grab and lock on. More about that later.

Adjustment is by two large bolts on the back plate that set the closed position stops at the top of each shoe.

The problem with this arrangement is that it is not possible to set the position of the bottom of the shoes. Hence it is important the lining is the correct thickness (3/16”). Too thin and it will take a long time for the brake to bed-in, too thick and it will bind.

The TD and later cars had a more satisfactory arrangement. Rather than a pivot, the shoeswere fitted into a slot, so that they could self-centre themselves within the drum. There is a second return spring at the bottom of the shoes to hold them into the slot.



On the TD, TF and the MGA the adjusters are of the “snail cam” variety.

On the MGB an adjuster is built into the pivot. This has an adjustment screw with a pyramid-shaped head and when screwed in, it “clicks” every 90 degrees pushing the bottoms of the brake shoes outwards. An advantage of this system is that it allows the bottoms of the shoes to be adjusted to give a better “fit”.

The main disadvantage of the pyramid adjuster is that the casting is made from aluminium and the adjusting screw, which protrudes out of the back of the brake, is steel. Exposure to water and salt under the car causes the screw to seize in its housing and coupled with its small square head, this makes it virtually impossible to turn. When I owned an MGB GT, my practice was to remove and grease the adjuster screw and after refitting, put a tight-fitting plastic tube filled with grease over the exposed threads to keep out water. This certainly helped kept it free.

There is a problem with some replacement adjusters. Barrie Jones reported he had I had a catastrophic failure of a brand-new pyramid adjuster when he rebuilt his V8. The original is made of alloy with two square-headed bolts cast into it to provide the locating studs. In the new adjuster, these were “replaced” by two 6mm lengths of studding tapped into the alloy. The first time the MoT tester stamped on the brakes these ripped out of the alloy. Something that could have happened in a real emergency stop. As a result, the brakes collapsed inside the drum causing the hydraulic system to fail and preventing the handbrake from working. A total brake failure.

Return Springs

It is important these are in good condition and properly fitted. They should not catch on any part of the brake fittings and especially not rub on the hub. They are probably the hardest component of a drum brake to fit and I have never found a satisfactory solution.

On the TC and earlier cars, a shoe has two riveted-in posts, one on each side, the larger is the brake stop (see photograph) and is fitted on the inside. The smaller has two notches around it where the springs fit. Before fitting, ensure the brake stops are set at their minimum position to reduce the amount you have to stretch the springs. I also find a rope pulled tight around the shoes also helps keep them as close together as possible.

My current practice is to use a pair of pointed nose plyers to hold the spring as close to the loop on one end, put the other end of the spring in place then using the fingers of my other hand with my thumb on the brake shoe pull the plyers to until I can get the spring in place. I start by fitting the springs over the outer of the two notches. When fitted, slide them over to the inner notches closest to the shoe. On the rear shoes ensure the middle of the spring goes behind the clip on the handbrake mechanism.

On the later cars the springs fit through holes in the shoes and the shoes are held in place by anti-rattle spring clips that pass through the shoe and back plate. On my MGB I found I could fit the return springs first then leaver the shoes into place with a large screwdriver. The problem here is you need 4 hands, one to hold each of the shoes and the other two to fit the anti-rattle clips. Again, it is useful to ensure the brake stops are set to the minimum position before starting.

Maintenance

Brakes are probably the most critical component of a car, so maintaining them is essential.

Depending on how much you use your brakes it is worth cleaning out the brake dust every 2 years or 5,000 miles. Remove the brake drum from one wheel at a time. It is safer to wash the dust from the drum and around the shoes using a fluid such as Halfords Brake Cleaner rather than an air hose which can create a lot of potentially dangerous dust. The brake adjusters should be freed off before removing the drums, which also checks they are working properly.

It is worth greasing the pivots, adjusters, backing plate where the brake shoes touch, mechanical linkages and/or ends of the hydraulic pistons using copper grease or a high temperature ceramic grease. BE CAREFUL not to get any grease on the friction surfaces. If you do, brake cleaner will remove small traces.

Check the brake return springs are in good condition and properly located. Check any hydraulic hoses and pistons for leakage or weeping. If the pistons have dust covers, they can be carefully pulled back to inspect the pistons and confirm that that they move freely. When a car is left standing, hydraulic fluid can, over time, make these pistons very stiff. A small amount of rubber grease around the piston and under the dust cover helps.

It is also worth checking the shoes have a chamfer on the leading and trailing end of the friction material as this reduces the chance of them grabbing. In practice the chamfer is only needed on the leading end, adding one to both ends makes it easier to reverse the brake shoes at a later date to even out wear.

Finally, before refitting the drum I check that each brake shoe can move freely using a screwdriver against the hub as a lever and check nothing is rubbing on the hub.

Replace the drum and press the foot brake hard to position the shoes before re-adjusting. Screw the adjusters until the drum is binding then backing them off one or two clicks or a fraction of a turn should be sufficient to allow the drum to rotate freely. If a brake still rubs in places as the wheel is rotated, then you need to check the drum is not oval. It may need to be skimmed. The maximum oversize for later cars is .060 inches, beyond which the drum must be replaced.

My Grabbing and Sticking on Problem

Needless to say, when I replaced the shoes on my TC I had followed all the maintenance steps. Everything appeared to be OK, but when the car was moving, a gentle touch on the brake pedal and “bang”, on came the brakes, hard. Even when the brake pedal was released, the brakes on one wheel stayed on. Why?

The problem was due to wear on the pivot pin and the pivot hole in the brake shoes. The shoe with the most wear was able to move up and down by around 30-50 thou, very small, but sufficient to cause the problem. Unfortunately, I had inadvertently fitted this shoe as a leading shoe on a front wheel, which also had the pivot with the most wear.

When I gently applied the brake, the self-servo effect pulled the brake on and moved it down on the pivot, putting it on hard. Because the shoes are firmly located with a spiral spring and horseshoe clip, the return spring was not sufficiently strong to pull the brake off, hence it stuck on.

The solution, I re-manufactured the brake pivots, which can be easily replaced as they are bolted into the back plate and replaced the 4 most worn shoes with new ones. Now the brakes all come on “as one” with no grabbing or uneven braking of the wheels.

The lesson, if you own a TC, YA or earlier car, always check for excess movement of the shoes on the pivots before fitting the horseshoe clip. There should be virtually none. On later cars, after fitting the return springs check the shoes can move up and down relatively freely.

Paul Ireland

Ed’s note: Regarding the brake pivots (original description and part number from TC Parts List “Anchor pin for brake-shoe 66832”) I don’t think they are available from the usual parts suppliers, but as Paul says, they are easily made. The horseshoe clip (“Circlip for anchor pin 66388”) is available from NTG Motor Services https://www.mgbits.com for £3.66 plus VAT (ref K933). The spiral washer which Paul mentions (“Thackeray washer for anchor pin 39382”) is also available from NTG (described as spring washer, ref K934) for £3.40 plus VAT.


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