it is most probable that the "air piston" that should lift up the syphone is not extracted to its maximum range.
What should you do:
1. remove the syphon from the cistern:
close the faucet in side the cistern, turn the syphon anti clockwise, turn it upside down and get it out of the cistern (you may need to move the float aside)
2. disassemble the "air piston" housing and verify it is clean and run freely.
3. if you see the piston can work its full range, fine. if not, try to buy "only" this little piece, rather than the whole syphone with its outrages price.
Get it all back inside...
good luck.
Shimon
February 2010
howtomendit.com for free repair help, information and advice.
