2004 GMC Yukon battery and censor

So I bought a 2004 GMC Yukon for a winter vehicle
here in RI. ( private seller) it has 188,000 miles I
bought it for 3,000... about two months after I
bought it I had a dead battery..got a new one..Then
I said let me get the brakes done because I was
aware they weren't done recently..so I get it back
from the brakes and I notice some pulsating in the
pedal every so often ...Now at this point my regular
mechanic is in Portugal..but one of my old friends
just opened their own shop/ after working for a big
dealership in the area for 25 years so I figured he
must know what he is doing...so I say it just
started doing this since the brakes were done....he
tells me it doesn't have anything to do with it...so
now I'm kinda nervous about taking it back to
him...So my regular mechanic is back he replaced
the sensor but it is the main module...so I pulled
the fuse for the abs for now cuz I can't even drive it
it pulsating so much...so a week later ...dead
battery...I bring it back to the one I got the battery
from...he said battery totally dead( after I charged
it to get it there) he puts a new battery in said
something must have been defective..ok so now
today battery dead again what the hell is going
on???