I am at wits' end with bleeding the brake system. I cannot get a solid stream of fluid to come out at any of the caliper bleeders. Just a trickle of brake fluid and a lot of air with the vacuum pump. Vacuum pump setup holds vacuum on the closed screw.

I've removed the new MC and bench bled it again, reinstalled it and passed several cups of fluid through both circuits. The stream is nowhere close to being solid. There are no leaks in the lines nor any blockages since the fluid is cycling through.

I've used the vacuum pump to bleed tons of newer systems, so I'm not sure what I am doing wrong. Any advice will be greatly appreciated.

If you unscrew the bleed valves a little too far they let in a bunch of air. You need to verify that this isn't related to your problem. If you close all the bleed valves, to you have pedal pressure?

It is best to bleed all four brakes starting at the wheel farthest from the master cylinder. Also, as Scott mentioned just crack open the bleeder screws.

I unscrewed the valves just enough to pull fluid with vacuum applied. Maybe a quarter turn at most. No fluid dribbled past the threads.

With valves closed, it's a little weird. There is solid pressure for about half the pedal travel. If I keep pressing, the pedal sinks to the floor, something goes click and it's very slow to return. Might be booster related as it doesn't feel so abrupt with the engine on, but the click is still there. Pushing the MC by hand is smooth all the way.

start with rt rear/ left rear/ rt front/ left front/ if you don't have a preasure bleeder use the next best thing a person in drivers seat pumping brakes keep master cyl full of fluid the whole time keep doing with a rachet and socket on the bleeder until you get a full pedal, if it doesnt pump up it could be leaking some where? even in the the power brake through the seal in the rear of master cyl i have seen them leak into the power pack. takes a lot to bleed these brakes by foot preasure. just keep pumping and cracking the fitting, untill you get all the air out. good luck.ps before you start crack the line at the master cyl and bleed it there first and then continue to rt rear wheel.

Not sure if this is your problem or not, but, a few years ago I replaced all four calipers on my 250C (W114) and spent two days trying to bleed the brakes, unsuccessfully, and had all of your symptoms. Finally discovered that I had the calipers installed upside down. They were interchangeable between left and right EXCEPT the bleed nipple was at the bottom when installed improperly. It HAS to be on the top to get all of the air out. Hope you problem gets solved.

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Charlie.

Thanks guys, I will try as described. If the master leaks to the rear thru its seal, it is defective, correct?

Update: completely bled the system, from the MC down. I have a pedal, but it's soft and grabs at the bottom. How are they supposed to feel when working well?

I'm beginning to suspect the new master cylinder is a lemon in disguise

Elaborate on what you mean when you say that the pedal is "soft" and it "grabs at the bottom". With car not running, if you hold foot on pedal and put on a lot of pressure, does it get to a solid point, or does it continue to move? What do you mean about it grabbing?

Soft means air in lines. Continues to move means something is leaking. Grabbing - clarify what grabbing is.

Look for leaks everywhere, particularly on the master cylinder where I assume it comes through the firewall and into the driver area. If it leaks on the thrust-pin side of the master, the fluid will be dripping down the floor above or below carpet under the brake pedal as high up as you can imagine. (At least that is where I think it is on a 108...)

The pedal feel should be firm and even. I too suspect a faulty master cylinder.

Again, brand new master cylinder installed. With the car stopped, pressing on the brake pedal has consistent initial pressure for about 2/3 of the stroke. At that point there's a "click" feeling in the pedal and it sinks to the bottom of its travel with about half the amount of the original resistance.

When moving the car, the brakes grab and stop from that "click" point onward. It's a very soft pedal with the engine running. I don't know how else to describe it.

There is some fluid leaking down between the MC and the booster if it sits for awhile. I've seen mention on other forums of a relief valve in the MC that causes fluid to spill backwards of the piston if it travels too far in the bore.

Before I send it back, I want to understand as best I can if it sounds faulty or I am doing something wrong.

FYI: motive makes an automatic brake bleeding system (hear its pretty cool/easy/effective):
Motive Products 0090 Motive Products Brake Power Bleeders and Accessories - Free Shipping on All Orders @ JEGS

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Current:
1971 Mercedes Benz 250 (Euro Spec)
1972 Mercedes Benz 250 (US Spec)
Past:
1972 Mercedes Benz 280

Everything you have posted points to a problem with the master cylinder. Rather than wasting any more of your time or effort I would suggest returning the "new" MC for another unit.