The seals that cover the sensor floats for brake fluid level and the plugs between the master cylinder and the brake fluid reservoir are pretty easy to forget about when they are doing their job. But one day you'll be trying to pressure bleed or flush your brakes and you'll find brake fluid gushing out! Or, you'll find slow leaks of brake fluid when checking your oil or doing other service under the head. Nip this in the bud early on as brake fluid will ruin any paint it touches! The seals are fairly simple to replace, and this guide will help.


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  1. There are a total of four rubber seals that help to keep brake fluid inside of your reservoir during use. When the rubber ages, it can crack, and harden leading to the potential for leaks.
    • There are a total of four rubber seals that help to keep brake fluid inside of your reservoir during use. When the rubber ages, it can crack, and harden leading to the potential for leaks.

    • The first pair of seals cover the brake fluid sensor floats. They can be easily seen on top of the reservoir. These are very visibly cracked.

    • The second set of seals, also called plugs, seal the area where the reservoir enters the master cylinder.

    • The reservoir is not under pressure when using the brakes so these seals can be in pretty rough shape and still work OK. However, if you bleed your brakes using a pressure bleeder you may find fluid shooting out! That will hint at a good time to change them...

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  2. Begin by draining the brake fluid from the system.
    • Begin by draining the brake fluid from the system.

    • You will need to jack up the car, and remove the wheels. See the W123 guide page for help with both of these topics.

    • Now you can open the bleeder on each caliper to let the fluid drain in to an appropriate drip pan. It will take a while as it will drain slowly. Let it drain until it is barely dripping and the reservoir is empty.

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    • Now you can proceed to remove the reservoir. Start by unplugging the two sensor wires from each of the two floats.

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    • With the sensor plugs disconnected you can simply pull the reservoir out of the master cylinder. Pull straight up. It may take some force. You will have a bit of brake fluid drip and splash at this point - have a bit of water ready to rinse it off the painted surfaces underneath.

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    • With the reservoir free you can pull the seals off the sensor floats. They should pull off with just some prying with your fingers.

    • See the second and third picture to get a good view of the condition of these seals once pulled. You can see for yourself why these were leaking!

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    • Lightly coat the new seals with some brake cylinder grease (not brake pad grease!!) or some clean brake fluid.

    • Push the new seals on over the floats until they seat firmly in place.

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    • Now it's time to replace the plugs on the master cylinder. You can get an idea here of their condition - they are pretty rough.

    • These seals should also simply pull out by hand.

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    • In this picture the front plug has been removed. Proceed to carefully clean around this area with a shop towel. Do not allow dirt to enter the master cylinder - clean so that the dirt goes out, not in.

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    • Coat these plugs with some brake cylinder grease, or brake fluid, and push them in. A wide flat blade screw driver used gently can help push the seals in.

    • You can see in this picture both new plugs installed.

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    • Now you can push the brake reservoir in to the front plug opening. Do this with the reservoir at an angle from the master cylinder.

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    • Once the front of the reservoir is inserted you can rotate the rear end in to place and push it in to the plug.

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    • Your brake fluid reservoir is now ready to go, except for the plug connections. Re-install them.

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  3. Is it really ok to just pressure bleed the system after completely emptying the system? No need to 'bench bleed' the master cylinder?

    Andrew Scofield - Antwort

    One quick clarification, doing this job does not completely empty the system. The master cylinder is still full of fluid. Honestly, you could probably do this job without bleeding the system since you are not exposing the enclosed pressurized side of the master cylinder to any air intrusion. However, I suggest bleeding to be safe. That said, yes a pressure bleeder negates the need to bench bleed the master cylinder. You bench bleed a master cylinder so it is full of fluid allowing you to use the brake pedal and master cylinder to pump air out of the rest of the system. With a pressure bleeder, brake fluid is literally forced by pressure into every part of the system pushing air out along the way. One tip is to have the rear of the car lifted just enough on ramps or a jack to make the master cylinder level so you don't get an air bubble at the front of it.

    Nicolas Siemsen -


After your first drive check for leaks.

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