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Old 06-13-2013, 12:01 AM
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MC reservoir craziness

A few days ago I replaced the dragging LR brake caliper, but I had a heck of a time bleeding it. I took the cap off the reservoir and saw fluid at both the front and rear compartments.
I had my 5 yo pump the pedal, but I could not get any fluid out! Even when I disconnected the hardline from the caliper no fluid came out. Hmmm....

Thinking the reservoir's outlet is blocked I removed the rear level sensor to find no fluid! Apparently each compartment (front and rear) inside the reservoir is further divided into 2 compartments, and the one close to the fill neck can be full, while the one that connects to the MC is empty. What a weird design...

Csaba
72 280SEL 4.5

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Old 06-13-2013, 10:59 AM
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Nope. Standard dual circuit design. It's been used since the mid '60's. Prevents you from having total brake loss in the event of a single circuit failure. Having the rear dry is usually an indication (with the exception of a leak) of master cylinder failure. The fluid is leaked into the vacuum booster where it is then passed into the intake manifold and burnt. Check for seepage beneath the M.C. at the booster mounting.
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Old 06-13-2013, 11:11 AM
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Mike, I'm assuming he lost his fluid when changing the caliper, not due to another cause

Csaba, when I bled my brakes on my 4.5 I took the sensors out and filled from there. Otherwise you need to really top off the reservoir to get it into the back circuit.
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Old 06-13-2013, 11:58 AM
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Yup and that is why I said " (with the exception of a leak) " and removing the calipers would qualify.

By the way, if the rear system was completely dry then you're gonna have a heck of a time trying to bleed the air out of the system by the "pump" method. Your best bet will be to either use a power bleeder or gravity bleed the cylinder. The M.C. hydraulic system doesn't have enough of a throw to force the air due to its travel being restricted by the front circuit. That's why when you replace a M.C. it is recommended to "bench bleed" the cylinder before installation.

Gravity bleeding is easy. Fill the reservoir, loosen the rear line on the M.C., go have a sandwich and wait for a clear flow of fluid. Tighten the line, go to the SHORTEST line on the circuit (usually the LR), open the bleeder, get another sandwich, something to drink and wait until you have clean fluid dripping, tighten bleeder, go to the other caliper on the circuit and repeat. This can take a considerable amount of time but if you're alone and don't have access to a power bleeder whaddya gonna do?

When you have clean fluid then it's wise to bleed the entire system by either pressure or the pump method.

You'll note this is the OPPOSITE of how you pressure/pump bleed the system regarding the progression of bleeding. Pressure/pump bleeding you do the LONGEST line first.

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Last edited by Mike D; 06-13-2013 at 12:29 PM.
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