Martinfan,
On my red 450 I used the X model recovery tank which is custom molded to sit between the frame tube beneath the engine, expensive but very cool. What I noticed that being very low (like yours) it took a number of rides before it would recover. Essentially, the hose has to be completely full so it can siphon back and forth. My other tanks were mounted at the rad level and worked right away. Also, if you have a leak in the hose where it connects to the rad, air enters and breaks the siphon. My cap is a 1.1. Hope this helps. Cam.