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Reducing Brake Squeal

September 24, 2010 | Troubleshooting & Repair

By Robert Harrington, Sam Cotten, Pat Tobin, Larry Dent & Edward Jedrziewski

Q: My Roadster has developed the world's loudest, squealing brakes you have ever heard. Instructions in the manual suggest cleaning or replacing the brake shoes. As I have done neither, does anyone have any helpful tips, and if they are able to be cleaned, what do I clean them with?

A: First DO NOT work on your brakes without proper masking and ventilation. Asbestos brake pads, especially on older cars, are very toxic. Paradoxically, the bigger "threads" of asbestos are more dangerous (harder to get out of your lungs once in there), but the dust is pretty nasty, too. Remember to mask-up when you're cleaning up after working on the brakes, just stirring up dust off the floor is hazardous.

Often, brake squealing can be taken care of by just blowing them out with air (or removing any glazing)--using proper precautions, but for a persistant squeal, pull the drums and shoes--notice where the back edge of the metal on the brake shoes rubs the backing plate. These spots on the backing plate are likely dry and particularly after a rain or not much use they may develop some rust on them. What happens is that when the brakes are applied with out-of-round drums, the metal edge of the shoe is rubbed back and forth across this spot and develops a squeal. These spots on the backing plate should be smooth and have a very light film of high-temp lubricant applied--the metal edge of the shoe just needs to be clean and smooth.

With this rub spot taken care of, even slightly out-of-round drums can still be used without developing a squeal, so you don't necessarily need to have the drums turned--although you should have them checked. If they do need turning turn only as little as needed--as they may have been turned a number of times over the years and be near the wear limit.

While you are in there, check the wheel cylinders also--they should be tightly mounted and not leaking. Each wheel cylinder has a small aluminum cup that sits on top of the rubber piston. If these cups are corroded, they will not let the springs fully retract the shoes. The cups need to be removed, cleaned, and put back in with no lubricant.

Also check the flexible hoses that connect the lines. These can develop blockage that will not let the fluid back into the master cylinder. Without the release of fluid, the shoes drag, get hot and squeal. The best Porsche guy in the Northern Virginia area says that these hoses are the most overlooked item in most restorations. They look good from the outside, don't leak, but can be severely blocked.

You can use an air nozzle to blow off the dust, but I think the more up-to-date way to clean the brakes is to use a few cans of aerosol made specifically for cleaning brakes ( it keeps the air-borne dust down ).Using a drain pan under each wheel, flush and clean the brake mechanism and backing plate parts, after everything is dry apply a light film of high-temp lube to the rub spots ( if there is a groove worn in a rub spot on the backing plate, it will need to be filed flat - if it is a deep groove - the backing plate will need to be repaired or replaced), make sure the metal edges of the shoes that will contact the backing plate are clean and smooth and re-assemble.

Glazing

Squealing can also be caused by glaze on the shoes. Just sandpaper them with coarse abrasive paper--while properly masked. That will take care of it for a while. But it may come back.Why the glaze? Usually it results from hard lining material and/or using the brakes too lightly.The brakes on the 356 are enormous for the weight of the car. They are adequate for hard use from very high speeds, the kind of use our 356s don't often get in this country, especially at their advanced age. Therefore, the original linings tend to glaze from not being used hard enough.Your best bet in the long run would be to switch to a softer lining material. The lining on the 356 A's was rock hard. I'm under the impression that it was changed about the time the B's came along, but your car is an early B.

Adjusting

Make sure that all the brakes are adjusted so that there is no drag on them, hot or cold.  This means setting a larger gap cold than a lot of people are used to. The brake shoes expand quite a bit when hot; dragging shoes is the main cause of brake fade, and very likely the source of a lot of out-of-round drums.  Setting them up with a "light drag" is not a good idea.

With everything back together, test the brakes before you leave the driveway and then do a careful check-out test drive and make sure you are comfortable with the feedback you get from them.

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