Soft brake pedal likely faulty master cylinder or ABS unit

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This article was published 30/10/2015 (3974 days ago), so information in it may no longer be current.

Question: I have a brake problem with my 2001 Sunfire GT with the 2.4-litre engine. Some days the brake pedal is firm and the brakes work excellent. Other days they are a little down and not so firm. A sudden stop and the ABS chirps occasionally.

Recently when the tires were rotated and both brakes were checked, the front pads have 50 per cent life left. The rear shoes are new recently and the rear brake cylinders were replaced. They adjusted the rear brakes when the drums were off. The brake fluid is always up and I’ve never had to add any. Would this be a problem with the master brake cylinder or the ABS side? No trouble lights on the dash come on. I would appreciate any help. Thanks, Don

Answer: The problem could be either in the master cylinder or in the ABS unit. I suspect the ABS unit, but it is the more expensive of the two to replace, so you should do a couple of checks first.

The master cylinder is divided into two chambers on all vehicles since about 1967. Each chamber will provide brake pressure to two of the wheels. This design is used so if there is a failure in one system, the other part can still stop the vehicle. It is possible one of the seals in the master cylinder for one chamber is leaking internally. This would allow the brake pedal to travel further during a stop and not feel as firm.

Unfortunately, the hydraulic unit on the ABS system can provide exactly the same symptom. The ABS unit has two pistons, one for each half of the brake system, just like the master cylinder does. The pistons in the ABS unit rest at the top of their stroke and during an ABS stop, they are moved up and down by electric motors to vary the pressure at the wheel brakes, very similar to what we used to do when we pumped the brake pedal on older cars.

A small electric brake is applied inside the ABS unit so the hydraulic pressure from the master cylinder can’t force the ABS pistons down unless an ABS stop is required. If one of the electric brakes isn’t holding, the piston can move down when you step on the brake pedal and feel like you have poor brakes.

Most shops would replace the master cylinder as a first step. It comes as a complete assembly and this may still be the best way to start. I would check the movement of the ABS pistons first when the brake pedal is pushed and this can be done by removing the black plastic cover on the bottom (screwed on) of the ABS hydraulic unit. Beneath the cover you will see two gear sets, one for each ABS piston, and these are driven by the electric motors. When the brake pedal is pushed, these gears should not move. If one is slowly rotating, then the electric brake is not holding and the ABS electrical module needs replacing. If the gears hold stationary, the problem is likely in the master cylinder. Note, do not touch or have anything near the ABS gears when the ignition is on. The computer will run a test cycle on the system, usually when the key is turned. Anything touching the gears could damage them.

 

Question: I have a Mini Cooper Classic with all-season tires. The tire specifications for the car are P175/65R15 and I have a set of winter tires with 195/65R15 size. Only the first three digits are different and I don’t know what they mean. Can I use these on my Mini Cooper?

Answer: The first three numbers in the tire size indicate the tread width in millimetres. The 195/65R15 tires are 20 mm wider than the P175/65R15 tires. They are also a little taller, because the next two numbers indicate the tire sidewall height as a percentage of tread width, in this case 65 per cent of the tread width.

The 195 width tires should work, as Mini has used even wider tires on some models, but your speedometer will read about four per cent low. As a double check, because you are using different rims, be sure to check the tire clearance on the front with the wheels turned all the way left and right before driving.

 

james.kerr@sasktel.net