No ABS light but mudluator keeps over flowing

When I start the car the ABS comes on but shuts off as it normally would. A few months ago the fluid overflowed out of the modulator and caused my master cylinder to go dry and I lost my brakes. I got a new master cylinder installed and I pulled an ABS unit from another legend. I had a shop install it all and it worked for the last few months. I noticed my car having a bad brake feel and sure enough it is doing the same thing. There are no vaccum leaks. The car stops fine but if you hold your foot on the pedal it will slowly go to the floor. If I try to test the ABS by driving, it will not engage. The tires just lock up and I leave skid marks. Again, there is no ABS light on. Please help, I just spent more on the car then I paid for it 3 years ago for a timing belt and all new ball joints.

Thanks