Master cylinder question - PeachParts Mercedes-Benz Forum

woodrat · #1 06-16-2011, 04:10 PM

OK, so I got all my new calipers, pads and hoses installed, only to find that I do seem to also have a bad MC. I just can't seem to get any flow of fluid to the calipers now to bleed them, and this MC looks pretty crusty and old. I've pumped on it forever, and no fluid is getting to the calipers.

Here's my question. My MC has a little sensor on it that ties into the same wire that is on the cap where the fluid level sensor is. This silly thing makes this master cylinder cost about $200 more than the one without the sensor. Is there any reason I can't just leave that plug hanging loose and go with the much cheaper MC?

leathermang · #2 06-16-2011, 05:32 PM

Are you using two people for this mc job ?

woodrat · #3 06-16-2011, 05:41 PM

You mean for bleeding the brakes? Yes. The MC is just not pushing any useful volume of fluid to the calipers.

leathermang · #4 06-16-2011, 05:52 PM

Quote:

Originally Posted by woodrat

You mean for bleeding the brakes? Yes. The MC is just not pushing any useful volume of fluid to the calipers.

You might need to treat it as one you are just installing...
take it off and bench bleed it....
unless you just want to replace it... which would be nice to match the rest of the renewed system...
In that case bench bleed the new one before installing...

Yak · #5 06-16-2011, 06:46 PM

It may sound like heresy to some, but using a Mity-Vac on the caliper(s) to get fluid flowing has worked for me in the past. Top off the reservoir, put the vacuum on the bleed screw and pump away. Keep an eye on the reservoir to make sure it never goes dry.

I think if you are using one person pumping the brakes with an open line you are A) likely to damage the MC by stroking the pedal too far, and B) less likely to have success since you may just be moving air back and forth.

Once you get the calipers full and more/less bled of air, then proceed to a 2-man bleed.

The non-heretics will advocate pressure filling/flushing/bleeding.

woodrat · #6 06-16-2011, 07:03 PM

Interesting. In all the 30 years I've been working on cars, this is the first time that I've been unable to get fluid moving and correctly bleed a system by just having one person pump the pedal and the other person work the bleeder valves. I've always managed to get it done that way until now. Now those power bleed thingies are starting to look like something I might need to own...

When I first cracked open the hoses at the calipers, I was surprised at how little fluid dripped out of two or three of them. The one that was the most stuck hardly dripped any fluid out of the metal line at all. I suspect that the MC is probably not as spiffy as it should be.

Yak · #7 06-16-2011, 07:38 PM

Quote:

Originally Posted by woodrat

You mean for bleeding the brakes? Yes. The MC is just not pushing any useful volume of fluid to the calipers.

Oops. I missed this. With two-man, you should have had better success. But, the vacuum recommendation still stands for one-man filling/flushing; but pressure or two-man seems to be required for a good bleed of air.

woodrat · #8 06-16-2011, 08:08 PM

Yeah, I've never had it take this much pumping and yet still have so little in the way of results. Vacuum pump, huh? You mean like to pull the fluid into the calipers by putting vacuum to the bleeder?

Given how crusty the old MC is, it doesn't hurt my feelings much to just replace it; I just don't want to have to spend nearly $300 for that one part, if I can help it. So far, it doesn't seem like anyone knows if I can use the non-sensor one interchangeably, though.

jt20 · #9 06-16-2011, 09:33 PM

I am with Leatherman on this one. Except that you can bleed the MC in the car with the right setup (old, cut brake lines bent into the reservoir) and that you should replace your bleeder valves before making the assumption that no fluid is makings its may to the brakes.

Yak · #10 06-16-2011, 09:49 PM

Quote:

Originally Posted by woodrat

Yeah, I've never had it take this much pumping and yet still have so little in the way of results. Vacuum pump, huh? You mean like to pull the fluid into the calipers by putting vacuum to the bleeder?

Given how crusty the old MC is, it doesn't hurt my feelings much to just replace it; I just don't want to have to spend nearly $300 for that one part, if I can help it. So far, it doesn't seem like anyone knows if I can use the non-sensor one interchangeably, though.

Yes, I meant use the Mity-Vac to produce a vacuum on the caliper via the bleed screw and pull the fluid downstream.

I don't know about the pressure sensor versions. I presume that functionally it wouldn't matter since it's a warning sensor and not some ABS-style pump thing. That's a guess, though.

If you've got access to the manual, there are cutaways of the two MCs in 42-315.

jt20 · #11 06-16-2011, 09:53 PM

Quote:

Originally Posted by Yak

Yes, I meant use the Mity-Vac to produce a vacuum on the caliper via the bleed screw and pull the fluid downstream.

May I just add that the bleeder valves often let air in through the threads when you do this.

And if you are not paying close attention... this method could cause trouble than help.

Yak · #12 06-16-2011, 10:03 PM

Quote:

Originally Posted by jt20

May I just add that the bleeder valves often let air in through the threads when you do this.

And if you are not paying close attention... this method could cause trouble than help.

See #5 above. I'm not suggesting vacuum as a final bleed of air, but as an aid to get fluid into the calipers so he can do a proper bleed via his preferred method

He's tried two-man, one on the screws and one one the pedal.

If he's got a Mity-Vac, I'd try that next.

If not, then maybe the bench bleed of the MC. Pressure would work, too, if he's got the pump.

Skippy · #13 06-16-2011, 10:33 PM

I recently did a successful one man bleed using a mityvac on the bleed screws. I worked it so that I was already pulling vacuum when I opened the bleeder and was still pulling vacuum when I closed it.

woodrat · #14 06-16-2011, 11:18 PM

Quote:

Originally Posted by jt20

you should replace your bleeder valves before making the assumption that no fluid is makings its may to the brakes.

Brand new bleeder valves on the brand new calipers. Even removing them completely did not make any difference. Almost no fluid is making it to the calipers. The pistons haven't moved at all.

jt20 · #15 06-17-2011, 12:24 AM

Quote:

Originally Posted by woodrat

Brand new bleeder valves on the brand new calipers. Even removing them completely did not make any difference. Almost no fluid is making it to the calipers. The pistons haven't moved at all.

Ten you definitely need to bleed your MC.

Get a set of old brake lines, cut them long enough to rout them into the reservoir and pump gently until there are no bubbles (do not advance the MC until it stops!).

this can be done in the car... unless you need an new MC.