Rear brake pressure W124

[rob m]

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