Brake bleeding help needed BADLY! - PeachParts Mercedes-Benz Forum

psaboic · #1 02-11-2011, 12:21 PM

Boy did I screw up.

I was going to flush out the old brake fluid on my 83 300D. I was using a Motive pressure bleeder to pressurize the system when the worst happened. As I was doing the first wheel (right rear) I accidently let the master cylinder reservoir go dry. Suddenly as I was letting fluid out of the wheel cylinder, I got a whoosh of air. Now the system has a lot ofair in it. I filled the reservoir again, and pumped up the pressure bleeder several times, but all I get is air, no fluid.

Do I need to bleed the master cylinder now? If so, how.

The brake pedal never gets firm, no matter if the car is running or not.

HELP!

Glenn

vstech · #2 02-11-2011, 12:29 PM

... how did you get air in on your first caliper, if you are using a powerbleeder? you are filling the powerbleeder with fluid aren't you?

gastropodus · #3 02-11-2011, 12:33 PM

This can probably be done with the master cylinder still in the car, but it works best with two people. You might be able to do it with just yourself and the pressure bleeder, but I've always (well, the few times that I've dealt with it) done the master cylinder bleed manually.

Fill the reservoir. Wear nitrile gloves, and put a pie plate under the master cylinder. Disconnect one of the brake lines at the master cylinder. Have your lovely assistant gently pump the brake pedal while you have a gloved finger over the hole mostly covering it until you see fluid coming out past your finger. Have her stop with the pedal held firmly to the floor. Insert brake line back in the hole of the master cylinder, tighten the line. If you're obsessive-compulsive only tighten the line loosely, have her pump once again so that it weeps past the loose connection, hold pedal to floor, and tighten firmly.

Repeat on a second brake line at the master cylinder, preferably not the one on the other side of the round body - do one forward or aft of the one you just did. That should get you bled on the two chambers of the master cylinder, and then you can go forward from there with the pressure bleeder.

Cheers,

Kurt

bustedbenz · #4 02-11-2011, 12:40 PM

You do know that the reservoir is two-chambered and the only way to fill the back half is to overflow the front half into it through a tiny gap near the top?

We replaced a master cylinder on my 300SDL the first time we ever did brakes because we didn't know that.

psaboic · #5 02-11-2011, 12:40 PM

Ok, I was looking at the master cylinder reservoir. Only the front half is dry. the back half has fluid in it. But no matter what I do, I cannot fill the front half of the reservoir. I think if I can get fluid to the front part of the reservoir, that will solve my issue

VStech. No, I was only using the power bleeder to pump air. I usually do it this way, but I never let the reservoir go dry (except for this time)

Glenn

Stretch · #6 02-11-2011, 12:56 PM

Quote:

Originally Posted by psaboic

Ok, I was looking at the master cylinder reservoir. Only the front half is dry. the back half has fluid in it. But no matter what I do, I cannot fill the front half of the reservoir. I think if I can get fluid to the front part of the reservoir, that will solve my issue

VStech. No, I was only using the power bleeder to pump air. I usually do it this way, but I never let the reservoir go dry (except for this time)

Glenn

Glenn,

You should be able to fill the reservoir just by tipping brake fluid into the hole at the top - perhaps you are having trouble seeing the fluid in there? How clean is the exterior of the reservoir?

What sort of pressure bleeder are you using? Are you following the manufacturer's instructions?

With my pressure bleeder it has a separate reservoir that connects to the one in the car - this separate reservoir is the one that has compressed air acting on the top of the fluid...

psaboic · #7 02-11-2011, 01:11 PM

GOT IT FIXED!! I bled the master cylinder and COMPLETELY filled the reservoir, and I know have bubble free fluid at the rear brakes. Many thanks to all. Lesson learned. Do not work a double shift and then try to work on a car after only 3 hours of sleep!

Glenn

Stretch · #8 02-11-2011, 01:32 PM

Quote:

Originally Posted by psaboic

GOT IT FIXED!! I bled the master cylinder and COMPLETELY filled the reservoir, and I know have bubble free fluid at the rear brakes. Many thanks to all. Lesson learned. Do not work a double shift and then try to work on a car after only 3 hours of sleep!

Glenn

Ahh that was it - 'glad it is all sorted.

Just in case you don't know about it I once read here (on this forum) that a trick to get rid of small amounts of air in a brake system is to wedge the brake pedal down for a night or two or three...

...I apologise to the original poster of this great bit of advice - but I found it helps quite a bit!

jonbobshinigin · #9 02-11-2011, 02:11 PM

I am glad you got it resolved but for others in the future, you are supposed to fill reservoir and the Motive Brake Pump with fluid, which when pumped, simply pumps fluid in as you go...that way you never have worry about this. The whole purpose of the system is not to pump air into the system.