Gilly, I know you've talked about these things a bit before, but I have a few more detailed questions. What pressure should be used to flush the brake fluid with a power bleeder? I've got one of the jugs (with pressure gauge) you pump up manually and wondered what your power unit at the shop runs at or what you'd recommend if your shop unit doesn't have a gauge.
The other question is rotor specs. My fronts (99' 320) are stamped 23mm and my rears say 13mm. Is that really the min spec or the service spec you mentioned before? If I'm at or above that, can I put on new pads or do I still need more metal? If these are really the minimum usable specs and an independent shop were to replace my pads, how would they know what to use as the service spec to determine if they can just repad or if they should replace the rotors too. Seems like both should be on there unless there is some sort of industry standard or something. Thanks for your help.

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Lewis,

I would guess (could be wrong) that the number stamped on the rotor is the minimum spec. I would expect to see a service spec in a service manual. Common practice in the industry is to stamp minimums.

If you are at the minimum or just slightly above it I would replace the rotors. It is always a good idea to at least resurface the rotors (turn) when fitting new pads. If you do not have a brake vibration before needing the pads you could get away without turning them. Just be aware that your pads will take afew stops before they are at full effectiveness.

Personally, I prefer to change rotors every pad change. Most people do not. I believe BMW reccomends this as a normal practice on their cars. You certainly do not want to run rotors that are under the min spec. I would also not fit new pads to rotors that are at the minimum. The pads will wear the rotors below the minimum and you could loose some braking effectiveness.

My advice is to turn them if they are safely above spec, the shop doing the work is normally qualified to make that call, and fit new pads. If you are on the borderline for thickness, splurge for the new rotors. Unless you have a bunch of miles on the rears are most likely fine. Fronts wear from 2-4 times faster than rears.

Good luck


Oops, almost forgot the bleeder question I have a power bleeder. I pump mine up to 10 psi, that is what the manufacturer of the bleeder reccomends. There are also bleeders that use tire pressure which is obviously more than 10 psi. So I would not worry about doing and damage to the system. I think the lower pressure probably causes a little less aeration of the new fluid, which will cause a soft pedal. Answer is, 10-15 psi. Works great on all of my cars.

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1995 G320
1984 280GE
1971 Unimog 416

We use a power bleeder. It does have a pressure gauge for the air going into the top (air) chamber. We usually go up to about 30 psi or so, but again, this is the air pressure in the top chamber. I am not sure if this is an accurate reflection of the pressure being exerted into the brake fluid reservoir.

For some reason, the 23.0 front and 13.0 listed on the rotors strikes me as being the service spec. Usually the minimum spec is stamped into the rotor, and the service spec is in the manual. Maybe I can look at a new rotor tomorrow. An aftermarket shop could possibly get the rotor specs from an subscription service such as Mitchell On Demand. Or if it an MB specialast aftermarket shop, they can get it off of WIS or they may have the maintenance manual (#4) which lists them.

This is how it's supposed to work:
If the rotor is above the service spec (the "thicker" measurement), then the pads are simply replaced. If they are below the service spec, and the pads are being replaced, then the rotors are replaced. Simple as that, really. The Minimum spec is more of CYA for Mercedes. The use of worn out pads and below Minimum spec rotors can be a hazardous combination as far as overheating the brakes. Don't machine the rotors, it is not needed. In general, I see that a new pair of rotors should last through 2 sets of pads. So you'll have a new set of pads and rotors, wear out a set of pads and replace just the pads, then when those pads are worn out, you'll be ready for the rotors again. That's the usual pattern.

Also in regard to what is stamped in your rotor, it really should have some type of descrption preceeding the measurement. It usually is stamped "Min Thickness XX.Xmm", or "Min. Thick. XX.Xmm". This would be minimum thickness, not service spec. I'll post what I have listed for service spec and Min Thickness for an ML320, and what is stamped on a new rotor.

Gilly

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Gilly and Gman, thanks for the details on the fluid change. Gilly, let us know if you pull out the service spec versus min spec difference, that one's driving me batty. Thanks

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ML320: when new=25mm, wear limit (min spec)=23.0mm, wear limit for maintenance service=23.5mm. That has a floating caliper. 23.0 is what's stamped into the rotor (the min spec).
On a truck with a FIXED caliper, such as an ML430:
new=32.0mm, wear limit=29.4mm, wear limit for maintenance service=30mm, BUT what I found is for some odd reason, the have 30.0mm stamped on the rotor, but as I recall, it did NOT say "min spec" prior to the 30.0mm. Really odd.

For the rear rotors
for models like the ML320 with a floating rear caliper:
new=15mm, wear limit 13mm, service spec 13.5mm ("wear limit for maintenance service" is what is normally caled the service spec)
for models with the fixed rear caliper, like the ML430:
New=14mm, wear limit=12mm, service spec=12.5mm

Gilly

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