Overflow issues.

[Agu]

Two things come to mind.

If you're using a Durso type standpipe it should be bigger than the bulkhead size. Don't ask me why but it just seems to work better that way.

Second, is the drain line hard plumbed or flex ? If it's flex check to see if it's sagged somewhere. A sag will block the water until there's enough pressure to force all the water through, just like what you describe. The same thing can happen with a long horizontal run of pvc but that's rare.

 

[BigBert96]

I use a flex hose for my drain line. It doesn't sag. It pretty much goes from the bulkhead to the wet/dry. There is maybe a 1 degree "sag", but thats nowhere near enough to cause any blockage or pressure problems.