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Thread: Master Cylinder MGM 5.0...I know I am anal!

  1. #1
    I post a lot... Toploader's Avatar
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    Default Master Cylinder MGM 5.0...I know I am anal!

    Ok I know you all will roll your eyes, but..............I checked the fluid today in the master. The front chamber was full (up to about 1/4 from top lip) I assume the front chamber controls the rear drums. Its the large rear chamber that has me concerned. The level is indeed up to a 1/4 from the rear lip.....but the front of the chamber looks to be down at least maybe an inch and a half. I realize the master sits at a significant angle on the brake booster, which would tend to have the fluid more to the front of the chamber, but is this situation normal, where the rear of the chamber fluid is up to almost the top lip, and yet the front fluid level is as I said down to be what looks like an inch and a half.

    Your thoughts are appreciated!

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    I am guessing this is the older type of cast iron master with integral fluid champers without a fluid bottle attached to the top? The type with a metal clip on lid?
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    Quote Originally Posted by jaywish View Post
    I am guessing this is the older type of cast iron master with integral fluid champers without a fluid bottle attached to the top? The type with a metal clip on lid?
    Indeed yes, cast iron master with top bail on top of the cover.

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    In my opinion, it sounds fine. The longer, larger rear reservoir will naturally have larger fluid level difference, front to rear, because of the additional length. Brake fluid will tend to gather towards the front of the reservoirs during actual braking anyway.
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  5. #5
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    If I am understanding your situation correctly, it sounds like your situation is normal for a car with at least somewhat worn brakes that has not had fluid added.

    The master cylinder does mount at an angle that is not level (with gravity), so the fluid is always higher at the rear of each compartment. That is normal.

    There is probably a technically correct fill level, but it isn't marked and I don't know what it is, so I just fill em almost full. We don't know to what level yours was filled when everything was new.

    As the linings on the pads and shoes, and as the metal of the rotors and drums wear down, it is normal that some of the volume of fluid that used to reside in the master is now elsewhere in the system, because there is more available space in the calipers and wheel cylinders. This is particularly prominent on the front disc brakes (rear compartment of master) because disc brakes do not retract as much as drum brakes do when you release the pedal. Drum brakes, even when worn, will still retract a pretty significant distance as they have springs to return them (which will keep the fluid level in that compartment more consistent over time). Disc brakes just pull back enough to stop squeezing the rotor and that's it.

    All of that being said: if you recently added fluid and the level is now down from where it was after you topped it up, and you have not put thousands of miles on the car since that point, your situation is not normal and you need to determine where the fluid has gone.
    Last edited by kishy; 11-24-2021 at 10:29 PM.

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    Thanks to all of you.........Kish, I put on front pads maybe three years ago and at that time I bled all the brakes to bring new DOT 3 into the system. I have this past June used a turkey baster to suck out all the DOT 3 and replace with new fluid. But I forgot how the level would look. To answer your question, I do not see any reduction of fluid due to brake ware. I am planning on doing new rear lining and whl cyls because I have never done that in over 30 years. But that will most likely be this spring. I am just used to seeing master cyls sit at a level and not slanted...which is why I became concerned!

    Again thanks to all. Do not eat too much dark meat tomorrow!

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