300D Master Cylinder Question - PeachParts Mercedes-Benz Forum

yelorsirhc · #1 11-05-2015, 12:24 PM

I need to know which ports on the master cylinder go to which brakes. I have included a numbered picture for ease of identification. Thanks!

leathermang · #2 11-05-2015, 12:29 PM

The picture did not post... try again...

yelorsirhc · #3 11-05-2015, 12:42 PM

Quote:

Originally Posted by leathermang

The picture did not post... try again...

It's there now

leathermang · #4 11-05-2015, 12:56 PM

Quote:

Originally Posted by yelorsirhc

It's there now

Where is ' there' ?

yelorsirhc · #5 11-05-2015, 02:21 PM

Quote:

Originally Posted by leathermang

Where is ' there' ?

Hopefully it is there now, I was having issues with photobucket.

leathermang · #6 11-05-2015, 02:36 PM

Shows fine now...

leathermang · #7 11-05-2015, 02:58 PM

Well, one and two should be the brake lines.... three a ' warning device'.....
but can not tell from the manual which goes to which...
they probably expect with metal lines that it will be obvious which one goes to the correct hole... I can not find any pictures which identify which is front and rear...

yelorsirhc · #8 11-05-2015, 03:26 PM

Quote:

Originally Posted by leathermang

Well, one and two should be the brake lines.... three a ' warning device'.....
but can not tell from the manual which goes to which...
they probably expect with metal lines that it will be obvious which one goes to the correct hole... I can not find any pictures which identify which is front and rear...

Not obvious, they disappear into the body/frame of the car. I was hoping to avoid trying to have to trace them.

leathermang · #9 11-05-2015, 03:28 PM

Someone probably knows offhand..... if you can wait till they accidentally show up....
Is yours all connected up ?
Why do you need to know which is which ?

How about this.... why don't you bleed a brake at the rear...
and see which of the reservoir levels goes down ?

yelorsirhc · #10 11-05-2015, 03:49 PM

In the process of rebuilding brakes, hubs, etc. Need to be able to isolate the rear breaks with a plug at the master cylinder. IE the rear brakes are off the car at the moment. Want to plug the hole at the master cylinder and only use front brakes to be able to move the vehicle until I am done doing the rear.

yelorsirhc · #11 11-05-2015, 03:49 PM

Is a copy of the FSM available on here anywhere? I have an 85 300D

BillGrissom · #12 11-05-2015, 03:50 PM

You should always state the year since "300D" spanned several quite different cars.
Looks just like the MC on my 1984 & 1985 300D.
1 = rear brakes tube
2 = left front tube
3 = right front tube

They might have cross-overed the tubes for 2 & 3, but if going on a car the fittings will only point one way. As you can see the 2 & 3 ports go to the same cavity, i.e. they are a "tee" integral w/ the MC.

You do have two electrical "low level" warning swithes on the MC reservoirs. Those are the black rubber "test" caps you can push down to sink the float and verify your dash warning lamp. Don't let those rubbers get cracked and let moisture in. If I recall, they are in parallel w/ some other switches that also short to ground to light a lamp (pad wear sensors or such).

yelorsirhc · #13 11-05-2015, 03:55 PM

Quote:

Originally Posted by BillGrissom

You should always state the year since "300D" spanned several quite different cars.
Looks just like the MC on my 1984 & 1985 300D.
1 = rear brakes tube
2 = left front tube
3 = right front tube

They might have cross-overed the tubes for 2 & 3, but if going on a car the fittings will only point one way. As you can see the 2 & 3 ports go to the same cavity, i.e. they are a "tee" integral w/ the MC.

You do have two electrical "low level" warning swithes on the MC reservoirs. Those are the black rubber "test" caps you can push down to sink the float and verify your dash warning lamp. Don't let those rubbers get cracked and let moisture in. If I recall, they are in parallel w/ some other switches that also short to ground to light a lamp (pad wear sensors or such).

Thanks, this was the info I needed and I kind of assumed that was the setup. But, you know what they say about assuming. Best to be 100% when dealing with brakes. And, that answers another question about the test caps. Thanks

BillGrissom · #14 11-06-2015, 01:18 PM

Confuses a lot of people. I think in all cars, the MC aft reservoir and port is for the front system and the fwd port is for the rear system. That is why in disk/drum cars the rear reservoir is larger (disks use more volume). The reason for the "back=front" is that the front system is more critical in stopping the car so they put it closer to the pedal. I'm not actually sure how that makes it more robust, but I don't recall ever pulling the piston(s) out of a dual MC (just older 60's single ones). I recall reading that there are 2 pistons in there, which might explain it.

leathermang · #15 11-06-2015, 01:46 PM

Quote:

Originally Posted by BillGrissom

Confuses a lot of people. I think in all cars, the MC aft reservoir and port is for the front system and the fwd port is for the rear system. That is why in disk/drum cars the rear reservoir is larger (disks use more volume). The reason for the "back=front" is that the front system is more critical in stopping the car so they put it closer to the pedal. I'm not actually sure how that makes it more robust, but I don't recall ever pulling the piston(s) out of a dual MC (just older 60's single ones). I recall reading that there are 2 pistons in there, which might explain it.

Several mis visualizations occurring in your statement.

The size of the reservoir has NOTHING to do with the amount of fluid pumped towards the brakes...

the connection to the reservoir is obviously blocked off when the pedal starts to move the piston in the master cylinder... or fluid would travel into the reservoir instead of towards the brakes... the ' fluid follows the path of least resistance rule' ......

I suggest that since regular brakes have a return spring to withdraw them from contact with the brake drum... and disc brakes ride pretty much in contact with the rotors.... that the amount of fluid required to engage them is more for the drum brakes than the disc brakes...

I do think the reason for a larger reservoir for the front brakes is that they are more important to stopping.... and thus if the brake fluid level is being neglected it is hoped that the rear brakes not functioning first would bring attention to the need for maintenance.