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  #1  
Old 12-03-2003, 07:30 PM
chazola's Avatar
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Join Date: Nov 2003
Location: Dorset, United Kingdom
Posts: 1,238
124 Rear Brakes Question

this might sound like a stupid question, but when i jacked up the rear wheels today to check underneath the car I noticed the rear wheels weren't turning freely. With the parking brake off and transmission in Neutral the wheels turned, but with some effort.
I took the wheels off to have a look at the brakes and noticed the pads seemed to be hard against the rotors- i.e. there was no gap. Is this normal with the transmission in N and the parking brake off? (btw, i had the parking brake on before i raised the car, and then released it while the car was still raised).
I haven't noticed any braking problems apart from some very low speed groaning, although the back end of the car sometimes feels a bit 'snakey' at high speeds. There doesn't seem to be any excess heat after driving as one would expect from a stuck caliper.
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1993 320TE M104
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past:

1983 230E W123 M102
1994 E300D S124 OM606 (x2)
1967 250SE W108 M129
1972 280se 3.5 W108 M116
1980 280SE W116 M110
1980 350SE W116 M116
1992 300E W124 M103
1994 E280 W124 M104
----------------------------------------------
"music and women I cannot but give way to, whatever my business" -Pepys
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  #2  
Old 12-03-2003, 11:05 PM
Strider's Avatar
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Join Date: Sep 2003
Location: Washington
Posts: 83
well....

I just replaced my rear brakes last weekend, disks and pads. The pads had a bit of meat left on them, and the disks were within tolerance according to my micrometer, but they pulsated pretty badly when applied at speed....warped disc.

Anyway, sounds like your parking brake is adjusted too tight.

In answer to your question, there will be zero gap between the pads and the disc, which is desirable for minimum brake pedal travel.

How many notches does your parking brake travel when you apply it?

If the disk brake was binding, you'd feel it get tighter through part of the wheel's revolution as you turn it by hand, which would indicate a warped disc, which you'd feel through the brake pedal as you applied them when driving.

Really easy job replacing the rear brakes. The hardest part was having to whack the old disc off the hub, as it was rusted on some. Also, the center of the hub on the disc forms a drum for the parking brakes.

The maintenance procedure actually reccommends you carefully apply the parking brake at speed, say 25 mph, on occassion to clean the parking brake shoes. Could be you have brake dust built up on yours (from the disc brakes), binding up the drum brake.

Honestly, if you still have plenty of brake pad left, and the discs are within tolerance for thickness and runout, then I wouldn't worry much about it, depending on how hard they are dragging.
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  #3  
Old 12-03-2003, 11:19 PM
chazola's Avatar
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Join Date: Nov 2003
Location: Dorset, United Kingdom
Posts: 1,238
thanks for the reply...

I don't have any of the symptoms of warped discs, so hopefully thats a good sign.

The pads look like they could do with replacing soon though.

Parking brake takes about 5 clicks to feel engaged securely witha tiny bit of roll after it's engaged.
I'm used to a handbrake (as we Brits call it) so this footbrake takes a bit of getting used to.
__________________
1993 320TE M104
---------------------------------------------------
past:

1983 230E W123 M102
1994 E300D S124 OM606 (x2)
1967 250SE W108 M129
1972 280se 3.5 W108 M116
1980 280SE W116 M110
1980 350SE W116 M116
1992 300E W124 M103
1994 E280 W124 M104
----------------------------------------------
"music and women I cannot but give way to, whatever my business" -Pepys
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  #4  
Old 12-04-2003, 02:22 AM
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Join Date: Mar 2003
Location: Southern California
Posts: 2,036
All modern disk brake systems are "constant contact" - i.e. the pads don't actually retract any distance from the rotor. Various tests of power absorbtion or increased fuel consumption show that constant contact pads have no meaningful effect, but the constant contact quickens brake response to pedal movement, and simplifies the design.

In the case of 124s and 201s, of which most models share the same rear brake hardware, the parking brake is completely separate from the service disk brake. The parking brake is a shoe type with shoes inside the disk hat section that extend when the parking brake lever is pulled. This design is also shared with the '65 to '82 Corvette disk brake system, but most modern cars have a mechanical piston actuator that pushes the piston on the floating caliper to act as a parking brake. The 201/124 (and vintage Corvettes) use a fixed caliper design with floating pistons on each side and mechanical actuation is not practical. The front 124/210 brakes are of the floating caliper design, which is the most common type on modern cars.

I've found that over time the parking brake shoes glaze and loose effectiveness and the cure is to drag the parking brake a few times to slow or stop the car. This will break the glaze. Keep the button depressed so you can release it instantly if necessary.

The front wheels turn will little effort, but when you turn a rear wheel you are also turning the differential and transmission guts, so the rears will exhibit a lot more drag torque.

Duke
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