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  #1  
Old 02-22-2007, 06:50 PM
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Question Replacing brake master cylinder. Any tips/tricks?

Hey, folks:

So I'm gonna swap out the master cylinder on my '68 220. Are there any tricks to this? Anything specific I should keep in mind?

Other than the bolts holding it onto the vacuum booster, how is it connected mechanically, and how do I disconnect such? Will it be obvious when I unbolt it?

Thanks in advance. I've swapped out MCs before, but never on a car with a booster...and it's been a while, anyway.
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Old 02-22-2007, 10:32 PM
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not difficult. just undo the lines and push aside enough to get the mc out. unbolt the two bolts holding it to the booster and it should lift out. if there is a linkage after it is loose you should be able to unfasten it. i dont think there is a link on that one.

all very obvious i believe you will find.

tom w
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Old 02-23-2007, 12:17 AM
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Thanks, Tom. That's kind of what I figured, but I just wondered if there was anything weird. German engineering and all.
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Old 02-23-2007, 12:41 AM
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on these era of benzes they are very mechanic friendly.

tom w
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Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins& six speed; I have had about 35 benzes. I am finishing a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual....I also have a Lotus 7 replica autocrosser with a modified K20 Acura engine.
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Old 02-23-2007, 02:33 AM
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There is one very important thing you must do, and that is be very careful when you screw your old brake lines into the new master cylinder. They're soft and you can easily strip the threads if you crossthread.

Im just lucky I didn't ruin my new $100 master cylinder and got the 12 dollar brake line instead.

Ugh
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  #6  
Old 02-23-2007, 10:56 AM
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I've read that it is very important to replace "o" ring in order to keep everything vacuum tight.
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  #7  
Old 02-23-2007, 02:35 PM
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aircap:

If you order the parts from Phil I suspect he can quickly verify whether the o-ring is included. But, the parts photo seems to indicate it is. Also, the cylinder is held with nuts; there are studs fitted into the booster.

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  #8  
Old 02-23-2007, 02:57 PM
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The ATE cylinders come with the o-ring installed ..is it a good idea to mount it with the fluid reservior istalled beforehand and bench blead??
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  #9  
Old 02-23-2007, 03:32 PM
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The rubber plugs for brake line openings are useful for bench bleeding the MC. Get an eye dropper to fill the chambers with DOT4 through the openings and then cap em.
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  #10  
Old 02-23-2007, 04:30 PM
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aircap:

Yes. The usual method is to install with the reservoir attached. Bench bleed by installing fittings in the outlets with a rubber hose from them back into the reservoir. Your master cylinder may come with plastic bleed fittings designed for a push on rubber hose. Put fluid in the reservoir and pump it through cylinder; it will expel all the air back into the reservoir via the hoses and when you make a second stroke of the cylinder the hoses in the fluid reservoir prevent drawing more air back into the cylinder; it's just like bleeding your calipers except then you usually discharge the fluid from the caliper into another bottle/reservoir as you attend to each wheel.

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  #11  
Old 02-24-2007, 12:16 PM
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Hey, everybody...thanks for the advice.

Replacing the MC on this car was simple as pie. The new part came with not only a brand-new O ring, but also little plastic fittings and hoses to screw into the ports for bench bleeding! It made that process a snap, even without a bench vise. (I placed the MC flat on top of a big coffee can and bled it by hand.) Bolted it on the car, hooked up the brake lines and -- even before bleeding at the wheels -- the pedal was firm as hell. Unbelievable! Now it stops like an ace.

Oh, and funny story about the vacuum booster: You may remember I wrote that I suspected fluid was seeping into the booster, then getting sucked into my engine during hard left turns, causing a massive smoke-screen effect? Well, my suspicions were confirmed. The vacuum booster was filled with brake fluid all the way up to the height of the MC. There was enough to fill a 3-lb. coffee can over halfway to the brim. I pumped it out with a hand siphon.

So now my car stops way better, and the resultant tightening up of the vacuum system actually made it run a little better. Though I still haven't got my hands on a timing light (it's been a weird week), I experimented a little with turning the distributor, and discovered that turning it about a half-inch counter-clockwise (looking down on it), it ran remarkably smoother. I had to turn down the idle a bit. Does that mean it was too far advanced before? At any rate, soon I shall lock it down official-style, but for now, it's much improved.

You guys are awesome, seriously. Thanks so very, very much for taking time to help out a clueless n00b out here on the vast prairies of America.
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  #12  
Old 02-24-2007, 01:05 PM
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See what I meant about the booster being full..

Anyway, if the timing is too far advanced , the engine will ping.
A good test if one does not have a timing laght is to go up a slight hill in top gear at 20 MPH and apply the gas without having the car downshift.. ..........listen for engine ping..if there is some , back off the advance until there is none...that will give you approx timing... it actually is a better way to timing the car than a light b/c you are timing the engine to max power for that engines compression. and gas being used. The engine will tell you what it likes better than a chart.
My 190sl actually has a cable/knob on the dash that turns the distributor and lets the driver time the car for different octane ratings/conditions in the same way...

Last edited by Arthur Dalton; 02-24-2007 at 01:58 PM.
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  #13  
Old 03-03-2007, 01:00 AM
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Thumbs up Sterling Advice !

Arthur's right on here , it's called a ' Throttle Ping Test ' and it used to be a regular part of tune up service back when we actually did " service " .

You can also use a vacuum guage to set ignition timing , connect it to unrestricted intake manifold vacuum and then advancd the timing untill the needle begins to jump ~ retard it to the highest _steady_ vacuum reading and you're good to go .

Did you use a pressure bleeder to bleed the brakes or an assistant pressing the brake pedal ? .
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  #14  
Old 03-03-2007, 01:04 AM
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i always sought a trace of ping that would go away after a second.

tom w
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Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins& six speed; I have had about 35 benzes. I am finishing a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual....I also have a Lotus 7 replica autocrosser with a modified K20 Acura engine.
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  #15  
Old 03-03-2007, 09:52 AM
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Exclamation

That's really pushing the envelope and I hope you never loan out the car .

Or drive to Death Valley (I go there a lot) .
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-Nate
1982 240D creampuff 367,000 miles (SWMBO's)
1978 300CD back from the dead SOLD
1980 300CD 90K Miles ICE COLD AC ~ SOLD
1984 300CD KEEPER ! 420,XXX miles
1984 Euro 300TD Fully optioned
1974 350SLC 4 speed stickshift SOLD & missed
Krazy Kommie Ural Motos (3)
BMW Motos /5's SOLD
Vintage Air Cooled VW's all gone =8-(
Junk
Rust
Arthritis
Crushed Spine,Broken Neck&Back
Memories
Peace Of Mind
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