Magura Marta brake strength?

I have had Magura Marta brakes on my Cannondale Rush for some time. They have never been very effective but I rode around it. Now that I have XTRs on my new bike the Marta shortcomings are glaring. I replaced one master cylinder that was obviously bad so the front and rear feel similar. I've replaced pads and bled the brakes myself and at a shop. I'll probably sell the Rush and I want the bike to be right for the new owner.
Am I expecting too much from this setup? I don't think I could lock up the rear brake while leaning forward riding through sugar sand.
Any suggestions?

I've got Magura Marta brakes on my bike and they work perfectly well. The front brake will easily throw me over the handlebars and the rear brake will lock up any and all types of tires on any surface you wish. Other bike I've ridden with Maguras work just as effectivly. Rest assured, your expecations are not overly high. In my experience, they work great.

Disk brakes are about the only thing on a bike I have never actually worked on. So, I don't have much practical experience to base any suggestions on. My first guess, however, is you have a leak in a line or master cylinder. This should be indicated by working better right after bleeding, but quickly going downhill. Also, I'd closely inpect your pads to be sure they are not contaminated with oil.

Hope this helps.