Washing machine waste confusion

[Dark Eyes]

When my washing machine pumps out, after about 20 seconds, water starts overflowing out of the top of the downpipe behind the cupboard. I have removed that waste pipe, the ubend and the 90' connector to the off white connection to the down pipe from the sink waste pipework, and there is no sludge or obvious blockage. The sink drains freely, so I uncoupled the ubend, rotated the 90' bend upwards, and put the washing machine's waste hose into it, and ran the washing machine.; as long as I held the waste hose hard into the 90' bend it didn't overflow, if it wasn't pushed hard into the 90' bend, it overflowed.

All I can assume from this, is that the waste hose is sitting in the downpipe, and when it pumps out, the water is hitting the ubend and it can't get around it and through the 90' bend quick enough, and is back-filling up the downpipe - does this seem likely, or is there another issue I have missed?

 

[terryplumb]

The vertical standpipe and trap ,( which you have removed and is clear) joins into a "T" . Check that T is clear going into the vertical pipe that the sink drains into.

 

[Dark Eyes]

My fingers found nothing, it wasn't even that dirty!

 

[terryplumb]

If the vertical standpipe ,the U shaped trap and that T are all TOTALLY clear ,then the water would run away fine, as long as the flexi hose from the washing machine is not kinked ,and goes down into the standpipe about 5 inches.