So today I'm planning on installing a new ATE master cylinder on the 1991 560SEL. Here's the plan:

1. Drain the old brake fluid from the reservoir using my MityVac.

2. Removing the reservoir and old brake master cylinder.

3. Install new ATE brake master cylinder using new rubber grommet/seal that came in the box.

4. Install new rubber grommet between new brake master cylinder and reservoir.

5. Bleed new ATE brake master cylinder on the car using pressure bleeder.

Question; do either of the rubber seals/grommets need any special type of lubrication or do I simply install them dry.

Also, I've read two different schools of thought concerning bleeding the master cylinder. One says to pressure bleed it on the car, the other says to bench bleed it. Comments and assistance are appreciated.....

6 on one hand, half a dozen the other with regards to bleeding the master cylinder. You'll be bleeding the entire system once you reconnect it. The rubber seals are installed dry.

I don't have a pressure bleeder, so I bled the MC on my SL using the bench method. Worked just fine for me. Since you have the pressure bleeder, I'd imagine bleeding would be a lot easier and less messy to do it in the car in a controlled manner.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

You will probably need to coat the 2 grommets with brake paste so that the reservoir seats properly. I have a tube of Permatex Synthetic Brake Lubricant #24127 that I use. You can coat the square oring between the MC and booster as well but it is not required.

Before power bleeding fill the reservoir to the top so you have a minimum of air trapped in the reservoir and the rear chamber gets filled. Make sure that the rubber seal caps on the level sensors are in good shape or you will have brake fluid squirting everywhere. Keep the garden hose handy for accidental spills.

Thank you for the replies; will do on the garden hose though I hope I don't need it. I believe I replaced the top rubber grommet/lids on the reservoir recently but I'll check them again just to be sure. Time to get back to work!

silglyde is best for lubing the internal parts of the master cylinder.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)