I recently replaced rear brake pads and rotors on my 91 300 e. I also replaced the solid brake lines from the union at the drivers side front wheel well back and the flexible lines to the calipers. Bled system. Brake system seems fine. Recently I discovered that the rear brakes engage and release but do not have much holding force. As I was backing out of the garage onto an icy driveway the front brakes were holding and the back wheels were spinning on the ice... If I jack up the back of the car and start it and put It in gear the brakes will stop the wheels but just barely. Pedal feel is normal. I can move both pistons in the rear calipers by hand so they are not frozen. There are no leaks in the brake system. Fluid level in reservoir is not going down If it were not for the ice incident I would not have known anything was wrong. I have bled the brakes using a vacuum pump and just tried the old fashioned pump the brake pedal and crack open and close the bleeder. Those both worked fine.
Any ideas what I have going on

Somewhat normal, it all depends on the condition of the brake calipers, master cylinder, and the amount of idle-torque the engine/trans is putting on the wheels, which can vary depending on engine idle speed and Bowden cable adjustment. Does it stop the wheels if you press the brake pedal harder? Ideally it should stop the wheels with medium pressure on the brake pedal. 300E usually idle pretty low, so it could be Bowden cable adjustment causing more transmission engagement at idle than there should be... Does the car have strong creeping movement/pressure when idling and in reverse or drive?

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If you've verified the pistons are moving in the calipers, then the calipers are not getting pressure.

Note: vacuum bleeding is ineffective on ABS equipped cars.

While you have a "volunteer" holding pressure on the master cylinder, go under the hood and crack the lines at the master cylinder and the
ABS behind the left front headlight.

It's not unheard of for the ABS unit to clog and prevent pressure from reaching a brake caliper. If the unit is clogged, it will be obvious when you crack the lines. If the pressure is weak in both the gozinta and gozouta lines, then the problem is probably the master cylinder.

Jim

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