*PLUMBERSSTOCK EXPERT* responded:
We're sorry to hear that the float of your McDonnell 67G Low Water Cut-Off isn't working fine. Commonly, it is due to clogs or accumulated dirt. If there's a lot of sediments in there, the float has sediments, too. These sediments will make the float stick to the walls even when the water drops. You can try flushing the control with the burner running until the burner shuts off. You may get ideas from this video on how to flush a McDonnell & Miller low water cut-off. Flushing the float chamber (of the low water cut off) should be done weekly during the heating season. Adequate precaution must be taken to prevent burns. If you're not sure about the procedure, it's always best to seek help from someone that can do the job. McDonnell & Miller recommends to replace your control every ten years so if yours is old enough, a replacement is needed. To get further technical assistance, it would be best to contact McDonnell & Miller at 847 966-3700. Thanks for asking help from PlumbersStock.com!