Bleeding Master Cylinder w/Syringe (W123)---How?

[iladelf]

So, my '81 300d (w/84 eng/trans) is having issues leaking brake fluid that I have to top off when the "brake" light goes on. It appears to be leaking from the master cylinder, as I see residue on the booster, and the paint is either ate away or getting eaten away on said booster. This past spring, I replaced both the rubber grommets on the bottom of the MC, and the o-ring between the MC and booster. Seemed ok until the problem showed up again recently.

My question is this; I'm replacing the MC and booster, but...how do you perform an MC syringe bleed on one of these reservoirs? I think it's a great idea, but...aren't there 3 different brake lines coming off a W123 MC? Reason I ask this is that I got confused, looking at what to buy online, MC-wise. I clearly see the two fender-side ports, but there's a third on the engine side; where does that go? (See the photo in the link below in reference to the 3 ports I'm referring to. You'll have to scroll down and select the FTE or Meyle to clearly see this.)

1981 Mercedes-Benz 300D Base Sedan - Brake Hydraulics - Page 4

Looking at the video below, it seems the syringe bleeding procedure would be very similar for our W123s, since we only have a single cap covering both the front and rear brake reservoir compartments. Again though, does that mean you'd have to do the syringe procedure for the two fender-side ports AND the single engine-side port?

https://www.youtube.com/watch?v=OUvBHnOh9ao&t=2m22s

For the record, I looked around the forums, used my Google Fu and looked at the CD W123 manual I've downloaded, and really couldn't find an answer to my question.

[Diesel911]

The third port at the rear bottom of the MC is for a Mercedes with a Manual Transmission and is used to supply brake fluid to the clutch cylinder which if you have an automatic transmission you don't need to use that port.

I have never done the syringe MC Bleed so I cannot help with that.

Repair Links
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[iladelf]

Originally Posted by Diesel911

The third port at the rear bottom of the MC is for a Mercedes with a Manual Transmission and is used to supply brake fluid to the clutch cylinder which if you have an automatic transmission you don't need to use that port.

Well, that tells me a lot, Diesel911. I didn't remember seeing that 3rd port when looking at my booster/MC combo previously, and since I have an automatic, that would explain why. Thank you, sir! I kept seeing pics in Google Image searches that showed the third line, and then looking at the new MC choices I had, obviously I got confused.

I think the video I posted looks like a good way to do the bench bleed. Will try it once I get the parts and let folks know.

[Dan Stokes]

I have a large veterinary syringe that I use to back bleed brakes/hydraulic clutches.

1) fill the syringe with brake fluid
2) burp the air out of the syringe (hold it upright and push any air out of it) then using a short section of flexible tubing (I use see-thru fuel line attached to the outlet of the syringe - a twist of wire tightened on the hub for the needle will keep the hose in place.)
3) attach the other end to the bleeder - I just slid mine on and it was OK.
4) crack the bleeder open
5) push the brake fluid up to the master by pushing the plunger of the syringe.
6) CLOSE THE BLEEDER before removing the hose!

Some notes:

You may need more than 1 try at this.

Remove most of the fluid from the master before starting as it'll overflow if you don't.

This was the only way I ever got my hybrid Chevy/Mercedes clutch system to bleed so it CAN work. This is essentially how the high-dollar pressure bleed systems work but I have about $1.75 (IIRC) in mine.

EDIT: I'm not sure if I detailed this procedure in my build thread on here but if not it'll be on my build thread on BangShift - the address is in my build thread on here.

Dan

[iladelf]

Well now I'm more confused than ever. Reason? I looked closely at the brake lines coming out of my reservoir today and...all three port ARE being used (I think). Let me explain; the two fender-side ports have hard lines that go down and in toward the center of the vehicle and disappear, while the third (engine-side) port also has a hard line that snakes around the back of engine compartment on the firewall before disappearing underneath the battery tray. Without getting underneath the car, I suspect that the two-port side goes individually to a front wheel (1 apiece), and the third port heads to the brakes in the back. To me, that would make sense, since the front brakes are considered to be more important than the rear. It appears to me that in order to bleed the MC correctly via syringe, you'd have to do so with all 3 ports. And I still need to find a syringe; no one in my area carries one (that I've found yet).

Dan, I appreciate your input, but not sure I follow your methodology. I tried to find your build thread here---didn't find it, and sorry partner, but I'm not going to go through the 27 pages on BangShift to find what you are referring to. (A search for "master cylinder" and "bleed" came up with zero on BangShift.)

EDIT: After doing a little more searching here on the forums, I think I know what you mean now, Dan. You're referring to the "reverse bleed" technique at the wheels in the video below. I'm thinking that's great for bleeding the brake lines, but I'm talking about bleeding the MC first before installing it. Perhaps your technique, if done in a passenger rear/driver rear/passenger front/driver front order would work INSTEAD OF bleeding the MC first. Look forward to your thoughts.

https://www.youtube.com/watch?v=lBk00597EyE

[Diesel911]

Unfortunately I got it wrong.

The brake fluid supply for the Clutch Cylinder does not come off of the Master Cylinder Body but comes off of the Plastic Reservoir Body.

[Stretch]

I think you might be getting confused with what is commonly called a "bench bleed".

Bench bleeding is some strange ritual that seems to only happen in the US of A along with alien abductions / alien probing and spontaneous combustion. Bench bleeding is an utter waste of time.

Just use a pressure bleeder.

Hash tag cat amongst the pigeons or what? (As the youth might say)