I am planning of changing the master cylinder tomorrow on my 1980 300D.
What can I expect as far as difficulties in bleeding the system?
Is there any thing that I can do to make the bleeding easier, such as pre-bleeding the MC before bolting on? Are there any tricks to bleeding, or has anyone tried it with the MityVac ? Any help will greatly be appreciated.
Robert

Rob,

I did mine, 1977 300D, about a month ago. It is a pretty straightforward job. If you have the Haynes Manual, the directions there are pretty good.

The key for me was to have some good penetrating oil and hit all of the fittings, nuts and bleeder valves and let them soak for a while prior to starting the job. My rear bleeder valves were especially hard to open, but penetrating oil and patience was the key. I also got a 9MM, I think, deep socket, which made opening up the bleeder valves much easier. Also, make sure that you have towels, covers and dropcloth, etc to ensure that you don't get any of the fluid on any paint as brake fluid will really mess up any painted surfaces. . Also, ensure that you replace any of the rubber grommets and gaskets, especially the one that seals the MC to the brake booster. Finally, get plenty of fresh DOT 4 and just replace the fluid while you're at it.

I also bench bled the master cylinder prior to installation. It was pretty simple and I didn't have a vice as the haynes manual called for. I just filled the reservior (sp), held the MC in my hand, had my son remove one of the plugs, i pushed in the plunger in and held it in, and had my son replace the plug, the let the plunger out, I repeated this about 2-3x per plug. I held the MC over a 5 gallon plastic pail with some plastic and newspapers under it to catch the BF as it came out.

What I would do the next time, is replace the reservior. The one in the car had a bunch of crud in it that was a pain to clean out (I just rinsed it out with new brake fluid). Doing it over, I would just replace it and start fresh.

Let me know if you want some more comprehensive directions, or send me a private message with your phone number if you wish to discuss.

Good luck,

JCD

That was a great description of the process.
I have an 83 300D that I am about to switch the MC for.
If you can give or point me to some more detailed instructions, I would really appreciate it.
Thanks.

I haven't done it in a while, but if you check the DIY pages, there should be some good instructions.

I also remember that I did mine based on instructions in the Haynes manual as well as a thread by Jim Smith. Do a search on the Jim Smith thread and I'm sure you will get outstanding instructions.

I have just forgotten, now that I have a 1991 300D and my old one is in the boneyard.

The reason you have to "bench" bleed the new master cylinder is because on the bench you can keep it level. If you notice in the car, the master cylinder tilts upwards toward the front. This tilt makes it impossible to bleed properly in the car.

Unless, of course, you point the car down a grade sufficient to level the master cylinder. Doing this saves a lot of mess on the bench.



Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles