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help brake bleeding

9506 Views 5 Replies 5 Participants Last post by  jay-h
This should be simple. I had spongy pedal and it would drop under pressure so I replaced the master cylinder. (this is something I've done plenty of times over the years)

I bench bled the master. Worked as expected.

Installed MC, tried releasing air through fittings. Still spongy, ok needs to be bled properly

Since I already had one front wheel off, I bled that (assistant working pedal). Worked as expected, got bubbles out.

Went to rear (yes I know I should start there. No success at all. Could hear a little air coming out each time, but no fluid other than light spray. Repeated over and over.

Checked threads, saw that it was recommended that key be on (because of ABS). Turned on key. Still no significant flow to wheel. Tried taking off bleeder valve... still nothing coming out. Had assistand press pedal with finger over bleeder hole... nothing.

pumped up pedal (pedal will pump up), then released bleeder valve. Pedal remained unchanged, did not drop down as it should.

What could be happening here? Is there a reset on the ABS unit? Is there a proportioning valve that needs reset? I'm going nuts.
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Be sure the entire master cylinder reservoir is completely full, including the half closest to the firewall. That's often tripped up many an SL owner doing this job.

Are you using a pressure bleeder? If so, go with the high end of the recommended pressure and pump it back up as needed to maintain that pressure. Be sure to fully depress the pedal at least once to get the air out of the master cylinder as you begin each separate circuit.

Remember, too, that there's a lot of fluid to replace and that the pressure bleeder only provides a tiny fraction of the pressure the system operates under. The new fluid has a long way to go and a lot of resistance getting there.

Patience. Best of luck, bud.

Good road,
I concur that pressure bleeding works best for the 560SL. I did mine and it was easy and effective. There are pump-up pressure bleeders that you fill with brake fluid and it uses a adapter cap on the master cylinder. Some additional info is here:
Someone please help 1989 560sl abs brake bleeding - PeachParts Mercedes ShopForum

Observe that the front calipers have two beeders for the cylinders, both should be bled.

See this Youtube on pressure bleeding:
I thought I bled my master cylinder enough and had the same results as you. loosened the lines on the master cylinder, rebled the master cylinder, hooked the lines back up and the lines to all four wheels bled as they should. I would try to re-bleed the master cylinder. doesn't take long and I did it while it was hooked up to the car. Good luck!
I know GlueckAuf already mentioned this, but be sure that the back half of the reservoir is full also. I was replacing my rear calipers yesterday and I let the back half run empty.

After figuring that out and getting it full (I had to fill the reservoir to the top and wait for it to slowly fill the back half, did this multiple times until it was full) I used a pressure bleeder to bleed the lines.

I did it with the battery connected, disconnected and the car on (but engine not started.) Noticed no difference....
Ok. I finally did some more work, had to build a pressure bleeder (wife has a sore back and was quite unwilling to do heavy duty pedal pumping. Granddaughter can't quite reach).

Anyhow, attached is my homebrew pressure bleeder:

Hand pump garden sprayer (Home Depot) $10
0-15 psi fuel pressure gage (Pep Boys) $17
GM 44mm brake reservoir cap (Pep Boys) $7
Vinyl tubing, fittings, fender washers and neoprene washers (Home Depot).

Like everyone else here, my black rubber caps leaked. I temporarily removed the caps, took a short length of 3/8 ID vinyl hose and pressed one end over each of the 'barbs' that the rubber caps go on. Held pressure plenty well for this purpose.


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