Draining waste

thepipeman

I have just recently bought a 1990 Ford Transit Frontier when i turn the tap on to drain the waste water about half comes out the tank and then stops it has done the twice i have tried the tap is at the bottom so is very confusing why it stops.If it was
blocked then it would not flow freely till half empty i have had to empty  it by a cup then teatowel any ideas if i am doing something wrong

 Thank you

Roy