'96 skylark brakes help
djl56251
08-31-2007, 04:49 PM
I had new brakes installed 2 yrs ago by a "reputable" garage.Since then, I've had nothing but probs with the rear brakes.Right rear wheel locks up on slow speed.Over say 30 mph & up, braking is fine.Just when driving at slow speed.No ABS light comes on.
I've had the car to 2 dealerships & 5 other garages and noone has helped.
So far I've had all hardware replaced, 2 wheel cylinders, brake shoes changed 3X, Drums replaced 2X. Self adjusters changed.
Only solution I can do is adjust brakes in which only helps for a few days till they adjust back out again.Car is a great running car & good shape so I hate to junk it over a brake prob.Can someone offer any suggestions?
Buick Skylark Limited 3.1 162,000 miles.
I've had the car to 2 dealerships & 5 other garages and noone has helped.
So far I've had all hardware replaced, 2 wheel cylinders, brake shoes changed 3X, Drums replaced 2X. Self adjusters changed.
Only solution I can do is adjust brakes in which only helps for a few days till they adjust back out again.Car is a great running car & good shape so I hate to junk it over a brake prob.Can someone offer any suggestions?
Buick Skylark Limited 3.1 162,000 miles.
Blue Bowtie
09-01-2007, 01:03 PM
Welcome Aboard!
Since the problem was not apparent until the brakes were serviced, there is a likelihood that something done during the service has changed the brake function. Not knowing what might have been done during the service, all the potential problems involved should be investigated.
Beginning with the overall braking system, bias between the front and rear wheels is usually designed by the caliper and wheel cylinder sizes, master cylinder size, and proportioning valve settings. Since the problem is evidently a bias issue between the two rear brakes, and possibly not between the front and rear, we can presume (initially) that the master cylinder and proportioning valve/residual pressure check valve are not part of the problem. There is also a possibility that the ABS system is not allowing brake fluid to flow to the left rear brake as much as the right rear brake, but that is a very remote possibility, and should be accompanied by an ABS error. For now, let's examine the more common possibilities.
SHOES
First, the brake shoes themselves should be checked.
Both rear brakes need to be checked for fluid and other contamination. One would assume that of the many shops which have checked the brakes since this began, at least one would have mentioned any possible contamination.
While the shoes are mirror images from the left to the right side, they are NOT interchangeable in their positions relative front-to-back within any brake assembly. Almost all drum brakes are considered to be "self-actuating" and as a result have a primary and secondary shoe. The primary shoe typically has a shorter length of friction material bonded to it, and the secondary shoe has a longer length of material.
http://72.19.213.157/files/DrumBrakeOperation.jpg
Once hydraulic pressure is applied, the primary shoe contacts the drum and the rotational force of the drum causes the primary shoe to impart force against the secondary shoe. The secondary shoe is held in place by the brake anchor pin, so it can only move outward to create more friction (stopping force) against the drum. Even though several shops have checked the problem, verify that the shoes are in their proper positions. Having the secondary shoe in the primary position can lead to grabbing braking action as you describe. Even though several shops have checked it, if the shoes were installed incorrectly two years ago, and the subsequent shops simply assembled them back in the same order, the problem could still exist.
http://72.19.213.157/files/MorraineDrumBrake.jpg
HARDWARE
Problems can result from improper brake hardware installation or assembly. Verify that the park brake link is not bound, the park lever is not sticking, and the return springs are intact and correctly installed. Any astute mechanic should have checked this in the numerous inspections performed since the problem began, but problems may also have been overlooked.
Another potential problem is the self-adjuster mechanism. Both drum brakes need to be checked to verify that the adjusters are installed on the correct sides, since they are different from right to left. One has a right-hand thread, while the opposite has a left-hand thread. Since the adjuster mechanism operates the same on both sides, the left brake typically has a right-hand threaded adjuster set, while the right brake usually has a left-hand adjuster set. If all the brake hardware were replaced as part of the initial service two years ago, it is possible that the shop installed two rights or two lefts. that would make one brake loosen itself when self-adjusting, and the other maintain a normal adjustment position. Obviously, that would affect the bias of the brakes, but should also be noticed in a lower than normal brake pedal height. Still it is worth checking in order to be thorough.
HYDRAULICS
Since there is an apparent bias problem between the right and left sides, it may be due to a brake hydraulic problem. Since the vehicle may have a four-channel ABS system, each rear wheel could have it's own hydraulic system. If that is the case, a brake hose which is failing internally can create a blockage condition which acts as a check valve, not allowing a brake to fully release when the pedal is released. If that occurs, any subsequent brake application will create a pull or grabbing in the wheel/brake with the failing hose. This can be difficult to diagnose without a hydraulic pressure test set, but may be a possible cause. Conversely, damage to the opposite side hose or steel tubing could create a total loss of braking on that side, causing the "suspect" brake to perform all the rear stopping and grab as you describe.
Another possibility is a mismatch of wheel cylinders. Many shops will replace wheel cylinders and calipers as part of a normal, complete brake service. That's usually not necessary, but will assure that there are no problems over the expected life of the brake linings. However, if the shop or shop's supplier inadvertently provided a wheel cylinder with a different bore diameter on one wheel, that could easily explain the bias problem. While the brake used on your vehicle is common on many vehicles, and the wheel cylinders all mount the same, the wheel cylinder bore sizes are different depending upon the application. If the manufacturer inadvertently mismarked a cylinder or box, or parts were somehow exchanged somewhere along the supply chain, it is possible that you may have as small as a 3/4" bore cylinder on one side of the vehicle and a 1-1/8" bore cylinder on the opposite side. That would produce an effective difference of 2-1/4 times the stopping force being applied on one wheel as compared to the other. This is not something that most shops would normally check, but can be checked easily, and without complete disassembly of the wheel cylinder. Simply removing the shoes and hardware, actuator pins, and outer dust seal on one side of a wheel cylinder will allow the bore size to be checked with a dial caliper. If there is a mismatch between the two sides of even 1/8", you've found the problem.
Good luck in your hunt.
Since the problem was not apparent until the brakes were serviced, there is a likelihood that something done during the service has changed the brake function. Not knowing what might have been done during the service, all the potential problems involved should be investigated.
Beginning with the overall braking system, bias between the front and rear wheels is usually designed by the caliper and wheel cylinder sizes, master cylinder size, and proportioning valve settings. Since the problem is evidently a bias issue between the two rear brakes, and possibly not between the front and rear, we can presume (initially) that the master cylinder and proportioning valve/residual pressure check valve are not part of the problem. There is also a possibility that the ABS system is not allowing brake fluid to flow to the left rear brake as much as the right rear brake, but that is a very remote possibility, and should be accompanied by an ABS error. For now, let's examine the more common possibilities.
SHOES
First, the brake shoes themselves should be checked.
Both rear brakes need to be checked for fluid and other contamination. One would assume that of the many shops which have checked the brakes since this began, at least one would have mentioned any possible contamination.
While the shoes are mirror images from the left to the right side, they are NOT interchangeable in their positions relative front-to-back within any brake assembly. Almost all drum brakes are considered to be "self-actuating" and as a result have a primary and secondary shoe. The primary shoe typically has a shorter length of friction material bonded to it, and the secondary shoe has a longer length of material.
http://72.19.213.157/files/DrumBrakeOperation.jpg
Once hydraulic pressure is applied, the primary shoe contacts the drum and the rotational force of the drum causes the primary shoe to impart force against the secondary shoe. The secondary shoe is held in place by the brake anchor pin, so it can only move outward to create more friction (stopping force) against the drum. Even though several shops have checked the problem, verify that the shoes are in their proper positions. Having the secondary shoe in the primary position can lead to grabbing braking action as you describe. Even though several shops have checked it, if the shoes were installed incorrectly two years ago, and the subsequent shops simply assembled them back in the same order, the problem could still exist.
http://72.19.213.157/files/MorraineDrumBrake.jpg
HARDWARE
Problems can result from improper brake hardware installation or assembly. Verify that the park brake link is not bound, the park lever is not sticking, and the return springs are intact and correctly installed. Any astute mechanic should have checked this in the numerous inspections performed since the problem began, but problems may also have been overlooked.
Another potential problem is the self-adjuster mechanism. Both drum brakes need to be checked to verify that the adjusters are installed on the correct sides, since they are different from right to left. One has a right-hand thread, while the opposite has a left-hand thread. Since the adjuster mechanism operates the same on both sides, the left brake typically has a right-hand threaded adjuster set, while the right brake usually has a left-hand adjuster set. If all the brake hardware were replaced as part of the initial service two years ago, it is possible that the shop installed two rights or two lefts. that would make one brake loosen itself when self-adjusting, and the other maintain a normal adjustment position. Obviously, that would affect the bias of the brakes, but should also be noticed in a lower than normal brake pedal height. Still it is worth checking in order to be thorough.
HYDRAULICS
Since there is an apparent bias problem between the right and left sides, it may be due to a brake hydraulic problem. Since the vehicle may have a four-channel ABS system, each rear wheel could have it's own hydraulic system. If that is the case, a brake hose which is failing internally can create a blockage condition which acts as a check valve, not allowing a brake to fully release when the pedal is released. If that occurs, any subsequent brake application will create a pull or grabbing in the wheel/brake with the failing hose. This can be difficult to diagnose without a hydraulic pressure test set, but may be a possible cause. Conversely, damage to the opposite side hose or steel tubing could create a total loss of braking on that side, causing the "suspect" brake to perform all the rear stopping and grab as you describe.
Another possibility is a mismatch of wheel cylinders. Many shops will replace wheel cylinders and calipers as part of a normal, complete brake service. That's usually not necessary, but will assure that there are no problems over the expected life of the brake linings. However, if the shop or shop's supplier inadvertently provided a wheel cylinder with a different bore diameter on one wheel, that could easily explain the bias problem. While the brake used on your vehicle is common on many vehicles, and the wheel cylinders all mount the same, the wheel cylinder bore sizes are different depending upon the application. If the manufacturer inadvertently mismarked a cylinder or box, or parts were somehow exchanged somewhere along the supply chain, it is possible that you may have as small as a 3/4" bore cylinder on one side of the vehicle and a 1-1/8" bore cylinder on the opposite side. That would produce an effective difference of 2-1/4 times the stopping force being applied on one wheel as compared to the other. This is not something that most shops would normally check, but can be checked easily, and without complete disassembly of the wheel cylinder. Simply removing the shoes and hardware, actuator pins, and outer dust seal on one side of a wheel cylinder will allow the bore size to be checked with a dial caliper. If there is a mismatch between the two sides of even 1/8", you've found the problem.
Good luck in your hunt.
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