What "check valve" are you referring to, Edro?
I wasn't aware there was one in the MS
Dobs, when changing from pre-spray to protectant, I do as Mike mentioned and just clear the lines a bit.
A few second spray is all it takes.
When going the other way from protectant to pre-sprays, I don't worry about it so much.
Length of hose depends on the model.
Mine has a coiled hose on the spray handle.
Guess it to be about 8 to 10ft in length.
I've seen knock off MSers that have different hoses.
The "real" MS will have a coiled hose and tiny QDs for easy quick change of spray tips.
You should also get a small 4" tip for uph and a 18"(? or so) carpet spray tip
They're the cat's azz, and you'll wonder how you ever got along with out one.
Put an 06 tip in the carpet spray wand.
Less atomized spray in the breathing zone and you can put down product quicker
..L.T.A.