Durso standpipe issues with addition of new pump

[rudy]

I have ran a mag 1200 on my system with a 1.5 inch fitted bulhead as a drain. I have a durso standpipe on the drain to minimize the noise. I have just added a mag 500 for extra flow and to run a u.v. sterilizer and upon doing this my durso is not working any more. The water will fill up in the overflow and then drain rapidly into the sump. Any ideas on what is going on and what I can do to correct it?