It looks like code 18 usually refers to an accumulator leak. Generally past the o-ring that seals the accumulator to the modulator unit. I recently replaced mine, and eliminated the leak, but I am still pulling the same code.
Prior to the o-ring replacement, the abs motor would run for a very long time, and after being unable to pressurize the system (due to the fluid leak), it would have the light on, and give me the code 18.
Now it runs for the correct amount of time (a few seconds), and than stop. The problem is, the light will not extinguish. If I disconnect the battery for some time, all will run well, but the next time i start the car the abs light will come back on. I think there is a problem with the pressure switch.
I shall post some pictures of the o-ring replacement if anyone is interested. It seems like that fixed the problem for some people. Unfortunately I am not one of them. I'll have to do some more work on it.