I've just installed a rebuilt brake booster, new ATE master cylinder, and resealed all 4 calipers. I have properly bled the MC by filling the reservoir, then depressing the plunger, then sealing off the 3 brake line ports (repeatedly).

After installing the MC to the car, i the put my Motive pressure bleeder onto the reservoir and pumped the pressure up to 15 psi. Why the heck does this not cause the rear brakes to activate???

Even after releasing the Motive bleeder, i stepped on the brake pedal and there is almost no resistance at the wheels. I can eventually develop resistance if i pump the pedal several times. There's not much to this system! What am i missing? If i step on the pedal, even if there's a little air in the lines, the brakes should activate to a noticeable amount. I bled the rears, but stopped because there's no resistance when stepping on the pedal.

Did you bench bleed the master before installing? That was necessary on my SD.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

Agree; you have to bench bleed.

You should be able to bleed the mc on the car by cracking the fittings, though I doubt that is the trouble. I would try bleeding them with the old faithful two person method.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Definitely bench bled

By new master cylinder do you mean brand new or rebuilt?

If rebuilt quite a few have had rebuilt master cylinders with issues. Including me.

I put the re-built MC on when I bought the car and despite many bleedings the pedal what I considered poor braking. About 1.5 years later it failed completely.

Couldn't fined the Paper work to get the lifetime guarantee so got a Brand New Fenco (Taiwan, not they also sell rebuilt) MC and it has been good for about 8 years now. (Please note that Fenco was bought out by some other company.)

I have had good luck using a catch Jar and using a Mighty Vac type hand held vacuum pump to suck the fluid through the bleed screws. Just did that last week on a Chevy Cavalier.
But on my Mercedes once fluid starts coming out of the bleeder fitting it will drip out by gravity.

The vacuum bleeding is quite a bit simpler then the pressure bleeder.

If you bleed it a couple more times and it still is not working one of the parts you replaced is not working properly.

There is a O-ring between the master cylinder and the brake booster.

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84 300D, 82 Volvo 244Gl Diesel

It's a truly brand new ATE master cylinder from FCP. Are there any proportioning valves, pressure regulators, or anything else between the MC and the calipers? There definitely are no leaks.

While i've only bled the rears so far, there is zero resistance in the pedal for the first 4 pumps on the brake pedal. Just goes all the way to the stop. After 4 pumps, you can feel resistance on the pedal and the pedal stops before bottoming out. If i hold down on the pedal, the pedal resistance holds and does not bleed away. That leads me to think that the MC isn't entirely failing.

Perhaps there's a huge air bubble that i didn't bleed out? Or, the MC is somehow bypassing?

Your symptoms point to incomplete bleeding.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

So does this:

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

I've never seen any purpose in bench bleeding. The usual problem with bleeding Merc brakes is that the fronts and rears share the same reservoir. There's a molded-in baffle at the bottom which allows the front and rear supplies to be segregated. Even with the translucent plastic, it's hard to see if both sides are full. The best way to fill the reservoir is to fill it to the very brim, almost to overflowing, wait for it to settle down, and top off. That way you know the level is above the partition and the two sides are both full. I don't know about toy stuff like pressure bleeders, but if you do it this way, the pedal provides sufficient power to bleed the system, even if the master hasn't been bench bled. Just keep the reservoir topped off until the last, then use the last bleeder to bring the level down to max.

There's no proportioning valve in a 4-wheel disc brake system, proportioning is a natural artifact of the piston diameters. Pressure regulators are only found in drum brake systems. There is a T block at the rear which splits the line left/right, but it has no "smarts".

I've tried to bench bleed too but have never noticed it helping.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Well, that was unsuccessful. I pressure bled the rears a second time, then pedal bled all 4. Probably ran 25 oz of fluid through, and kept the reservoir filled to the brim.
Might have picked up a little bit of resistance at the pedal? Pretty indistinguishable from before the bleed. Pedal still just goes to the floor with little resistance, but if pumped 4 times, will hold pressure.

Thoughts?? After 4 pumps and holding the pedal down, the brakes do stop the wheels and pressure does not fall off.

How old are the flex hoses? Could they be swollen internally, enough to not permit a fill bleed?

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95 E300D working out the kinks
77 300D, 227k, station car
83 300CD 370k, gone away
89 190E 2.6- 335k, no more
79 VW FI Bus- 145k miles, summer driver
59 VW Beetle ragtop- 175k miles
12 VW Jetta- 160k miles

Just to be sure you are bleeding it correctly.....pump til you get a firm pedal, then have your partner release the bleeder running a hose to a pan. Repeat until you don't have to pump to get a good pedal at near the top without pumping at all. Start furthest wheel from mc and end with closest.

I don't think I have ever used a power bleeder successfully.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Definitely not swollen hoses. This has to be trapped air, or maybe a leak.