Surge brakes pulsating or "machine gunning"
'08 X15 with stock MC trailer. I had the master cylinder on my surge brakes replaced, and afterwards when I brake, the surge brakes pulse on and off as the trailer slams into the hitch, releases, slams in again, and keeps doing this over and over. I was told this is called "machine gunning", though i dont know if that is a universal term or just the local dealer's internal term. They thought it was a bad shock or dampener so that when I braked, the trailer would slam the master cylinder causing the brakes to engage to hard and/or make the trailer rebound backwards ands start a vicious cycle. They replaced the dampener and it's still doing it, although not as often or severely.
The obvious answer is to take it back and make them fix it but they just as obviously are not sure what's wrong or they would have fixed it already. Any ideas? What else could cause this if I have a new master cylinder and shock/dampener? I will be taking it back to make them fix it, but I'd prefer giving them some suggestions as to what to look for since they've done what they know to do.