Ok guys....need some help here. I have a solo backpack sprayer with the adjustable pressure knob. While using it a couple of times this past week, spraying roundup, the right side of the unit seems to be collapsing like the unit is drawing an internal vaccuum or something. Still sprays fine but it is collapsing. After using it I can unscrew the top and hear the pressure being drawn inward. Is there an air vent that could be clogged not allowing the unit to draw air from the outside to be pressurized inside? If so.....where is it. Please help guys, this sprayer is to good to replace I just want to fix it. Thanks for the help.