Have a 92 deville that the brakes just go to the floor with almost no pressure. There are not leaks anywhere Lines, pistions, master cyc; and I am not losing brake fluid. I unpluged the ABS pump or what ever you want to call it its the thing that the brake line from the master cyc run to the is mounted on the crossmember on the driverside of the motor. when unpluging the power supply to that it got better. The brakes would still some times go to the floor but pump it once or twice and they would be fine. the problem seem to be getting worse agin almost like it was before I unpluged the ABS. I work on alot of cars and were are no codes showing anywhere and nothing mechaical wrong with the brakes when there is pressure then stop fine and quite, pads seem fine. I need some ideas before the brakes go totally out. I thought about replacing that ABS thing that I unpluged the power to but If that expensive I don't wnat to replace it unless I can figure out alittle better that is the sole problem. Let me know what you think. Thank you

 

You need to isolate the brake system components and see when the pressure drop off goes away.
Either start at the rear wheels and work toward the master cylinder or vise versa.

It takes a little creativity to isolate sections with only steel lines, but if you are up for the task im sure you can figure it out.

Hope this helps.

 

I put my money on the M/C seals leaking internally.