Servo type or self energizing

Non servo or non self enewrgizing

They are very easy to identify. A servo type drum brake will have a common anchor for both brake shoes. The non servo type will have separate anchor for each shoe.

Also non servo type drum brakes will have shoes of the same length, while a servo type drum brake will have 2 different lengths of shoes. We will discuss servo type first, as they are far more prevalent.

For this article, I will be talking about the left rear of the vehicle facing the drum brake. The shortest shoe or primary shoe (the metal frame of the shoe is the same, but the friction material is shorter) ALWAYS goes toward the front of the vehicle.

When the brakes are applied, the short shoe grabs hold of the rotating surface of the drum and then mechanically multiplies the pressure, through the adjusting screw, to the rear shoe. (That is why the secondary shoe or the long shoe will always wear fastest) It takes about 1/2 as much pressure to operate a drum brake as it does a disc brake! (350 to 500 psi)!

On the non servo type of drum brake, we do not have the capability of multiplying the pressure to the secondary shoe because we have separate anchor pins. So the front shoe will ALWAYS wear the fastest. The only time the secondary shoe does anything on a non servo type is when you are backing up.

Brake drums should ALWAYS be turned prior to overhauling the brakes! Most drums have a minimum thickness cast into the outer rim of the drum. The reason for the mini-mum thickness is for heat dissipa-tion and correct shoe to drum contact. HEAT is a huge factor when talking about any type of braking system. It is always a good idea to check shoe to drum con-tact by placing the shoe inside of the drum and checking the shoe to drum contact. We want about 80% shoe to drum contact. Re arced shoes used to be available for drums that have been turned out to maximum specs, but they are no longer available, so this is something to watch for.

Overhauling wheel cyl-inders is almost a thing of the past, but if you insist on overhauling them instead of replacing them, try to get "H" kits from NAPA. They include new sintered iron pistons that sweat a silicone lubricant when subjected to heat.

I ALWAYS replace one side at a time so I can use the other side for reference purposes. (This coming from a brake expert is probably good advice!)

Please use the correct tools when working on drum brakes. Using needle nose pliers to remove return springs is not only dangerous ... they don't work very well either. There are correct tools available at any parts house and some chain stores will loan them to you.

I hope this helps some of you who are using drum brakes. I should add that there is absolutely nothing wrong with drum brakes below 100 mph. The ONLY advantage of a disc brake below that speed is the fact that disc brakes have a foolproof self adjuster and drum brakes self adjusters are sometimes a little "balky"

If you have any questions or I have forgotten to cover some thing you can find me on the BBS almost every evening.

Fat Pat

Editors note: Fat Pat "Brakeman" Anderson is a brake expert who has produced brake videos and taught courses on brake repair and service. Pat has a long history with automobiles, parts and T-buckets (though with the sale of the flaming chicken Bucket, he is currently without the best ride in the world!) Thanks Pat for this article, keep 'em coming!
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