Mechanical Brake Locking Problems
By Stephen Kassis
Early Chevrolet cars & trucks from 1930-1933 have very similar brake systems. One of the common problems with these vehicles is that the brakes can lock at the wheel. Sometimes the problem can be temporarily “fixed” by simply driving the car backwards. Other times the locking can be so severe that the vehicle cannot be moved in any direction.
There is a long list of reasons why the brakes will lock. We will attempt to cover the common problems. Probably the most common reason is moisture on the brake shoes or rust on the drums. When our antique cars are not driven for long periods, moisture can build up in the brake linings or rust will form on the drum surfaces. This can cause the brake shoes to grab or stick momentarily. The easiest solution to this problem is to “ride” the brake pedal while driving, until the shoes have warmed up, eliminating the moisture. If moisture or rust was the problem, the sticking should go away as soon as the shoes are warm. NOTE: If you are planning to store the vehicle for a long period, it is NOT advisable to set the parking brake. Moisture and rust can cause the brake lining to seize to the brake drum and cause a huge problem to get them to release.
Bent brake shoes or brake shoe anchor plates can cause a sticking problem. A bent shoe or anchor plate will cause the shoe to contact the brake drum unevenly. This can cause sticking or severe brake drag on one side. A similar problem can be caused if the brake shoe anchor pins are not properly attached to the brake backing plates. This can cause the brake shoes to be stressed and possibly bent.
Loose, broken or improperly riveted brake linings can cause sticking brakes. Any foreign substance on the drum surface can cause sticking. This would include dried grease or glazing that can show up on brake shoes and drum surfaces. Sanding the brake drum will usually take care of this problem unless the contamination has gone into the brake lining.
Another obvious problem that will cause the brakes to stick is a broken brake return spring. If there is no return pressure on the brake shoes after they are applied, they will stick or drag. Many times there will be a dragging or scraping noise associated with this problem. As the brake drum is rotated, broken pieces of the return spring will be caught between the shoes and the drum. This will cause a distinctive and relatively constant noise to come from the brake drum.
Centralizing the brake system is vital to proper operation. Out of adjustment brake rods and cables can cause sticking. If these are not adjusted properly, the front brake cables can come too far out of the housing. This will allow the inner part of the cable to hang up on the outer housing and not release. Of course, the solution here is a complete brake adjustment. There is a very detailed procedure that should be closely followed to adjust mechanical brakes.
The most common and worst brake locking can be caused by the brake shoe operating cam. On a week-long VCCA tour, we had 25 early six cylinder cars. Of those cars, three had mechanical brakes that were sticking. We observed that all three of these cars had about 80,000 miles on the odometer. That seems to be the approximate time when the cam bushings exhibit severe wear. We drove our old cars over 600 miles during that week. One car had such a bad brake problem that the car simply would not move.
Each wheel has a cam that pivots every time the brakes are operated. This cam is held in a cast housing and pivots in a brass bushing. This bushing is not lubricated in normal service and, after 80,000 miles, the bushing will most likely be worn out. When it wears out, the bushing will be oblong in shape. This will allow the operating cam to turn in a slightly tilted manner. These bushings should be replaced if loose against the cam. Often during restoration these bushings are overlooked and not replaced. If your car is experiencing serious brake locking problems, this is very likely the reason.
Badly worn cam bushings can cause the cam to lock in the open or “ON” position, effectively locking that wheel. If you are on the road when the brakes lock up, a temporary solution is to back off the wheel brake adjuster, tapping it back and forth, until the wheel turns freely. The problem with this is that you will lose substantial braking performance. This should only be done ONLY as a temporary solution to an emergency situation. At the same time, the opposite side brake adjuster should be backed off equally to prevent uneven brake pull. Use EXTREME CAUTION when driving a car in this situation as braking performance is seriously diminished. Only drive as far as absolutely necessary â slower and with more caution than usual and only as an emergency measure to get to a safe place. As soon as possible, make proper repairs and completely readjust the brake system.
To replace the brake cam bushings, the cam housing must be removed from the brake system. To do this, remove the brake adjuster which is attached to the brake cam. Be sure to remove the small woodruff key(FS-3877)bolt with a nut that holds the cast housing in place. This bolt also has a bushing. This bushing will not experience the same wear problem as the cam bushings as the movement in this area is limited. It will most likely require extra effort to remove this bolt because it will be tight. Part of the tightness is due to the spring plate on the back of the cam housing. This has a very tight hole that must be aligned when the pivot bolt is installed. Remove the bolt carefully to avoid damage to the threads. One method is to put a box wrench on the head of the bolt and pivot it back and forth while pulling it outward. A little spray of penetrating fluid like PB Blaster on the bushing will help.
Brake & clutch pedal bushings are the same bushing used on the brake cam. Press in the new bushing (SA-126) into the cast brake housing. Fit the cam back into the bushing. If it is too tight, it may require a light honing or sanding with emery cloth to fit the cam to the bushing. Once installed, the cam should pivot freely and not stick. Use caution when honing, as it will be important to have a good fit without it being too loose. Lubricate the operating surfaces with a light coating of high temp wheel bearing grease or anti-seize compound. This is also a good time to check the brake centralizers to ensure that they are free and operational. The centralizers are often rusted and frozen in place. They must be free for proper brake adjustment.
Once the new bushings are installed and centralizers are free, reassemble the brake system and do a complete brake adjustment.