One of my customers has a Blodgett oven that would not fire. I found the pressure differential switch was not made. I've worked on other Blodgetts, but have not seen one with a pressure switch. The rubber tubes going to the ports behind the oven were cracked and came off. I cut the tubes back a bit and pushed them onto the ports and secured them with zip ties. The customer calls 2 days later and said they cracked and came off again. What could be causing this? The convection fan works and pushes plenty of air.