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8.617-955.0 and  8.624-922.0 and 8.617-344.0 Prochem and many other brands Truckmount Carpet Cleaning Extraction Machine Chemical Pump Repair Kit.

You will receive 1 each:
8.617-955.0 Chemical Pump Diaphragm
8.624-922.0 Chemical Pump Plastic Disc
8.617-344.0 Chemical Pump O-Ring

This Kit will solve 99%  of the chemical feed problems you're having.

If you have any questions regarding the use or application of this kit, contact me prior to ordering.  I will be happy to answer any questions.