...even if the other one isn't to worn?

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84 300TD 4 speed, green

Yes. Do brake work in pairs, fronts or backs, never one side and not the other.

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'83 240D with 617.952 and 2.88
'01 VW Beetle TDI
'05 Jeep Liberty CRD
'89 Toyota 4x4, needs 2L-T
'78 280Z with L28ET - 12.86@110
Oil Burner Kartel #35

http://i16.photobucket.com/albums/b1...oD/bioclip.jpg

Yes I agree...always in pairs....

__________________
Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

What he said.
Uneven braking could result.

__________________
ASE Master Mechanic
https://whunter.carrd.co/

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D
2003 Volvo V70

https://www.boldegoist.com/

Always in pairs per axle... like they said above..

IGNORE PEH when he comes on and says ' don't fix it until it is broken' ... because you may break more than that with the results of uneven breaking and driving....

I'll toss in my $0.02 worth. It depends. If I replaced the set and 5000 miles later, one broke, I would replace that one, provided I see that the problem is that one only. Now, if I had a bunch of miles, I would replace both.

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01 Ford Excursion Powerstroke
99 E300 Turbodiesel
91 Vette with 383 motor
05 Polaris Sportsman 800 EFI
06 Polaris Sportsman 500 EFI
03 SeaDoo GTX SC Red
03 SeaDoo GTX SC Yellow
04 Tailgator 21 ft Toy Hauler
11 Harley Davidson 883 SuperLow

Why only one is worn.

That is why I said, replace both unless one of a set failed soon after install. Could be bad machining. I replaced both fuel level senders on my 99 C280. Guess what, 2 weeks after that, one of the senders failed. Good thing I bought them from the dealership and they had to go to parts warranty to get it fixed. I was thinking of getting it here but decided the risk wasn't worth it since I would have to drain the tank and it would be a ***** to do in the driveway.

__________________
01 Ford Excursion Powerstroke
99 E300 Turbodiesel
91 Vette with 383 motor
05 Polaris Sportsman 800 EFI
06 Polaris Sportsman 500 EFI
03 SeaDoo GTX SC Red
03 SeaDoo GTX SC Yellow
04 Tailgator 21 ft Toy Hauler
11 Harley Davidson 883 SuperLow

Even on my $500 beater I would never do a single rotor.

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2016 Corvette Stingray 2LT
1969 280SE
2023 Ram 1500
2007 Tiara 3200

Pictures of the bad rotor please.
Some of what you said has me concerned that you have other issues:
#1. Bad flex hose.
#2. Bad caliper.