The reason is because the ABS unit has a lot of internal microtubes and valves in it, so it may require higher hydraulic pressures to push the new brake fluid through them all.

The motiveproducts' Power Brake Bleeder gauge can go as high as 30 PSI, and I tried that pressure before on my c220 without any problem. However, I believe that 25 PSI is good enough because the fluid already came out forcefully.

Eric