Some manufacturers say that you need to vacuum all the air out of a break before injecting the windscreen repair resin, is this true?
Of course not. Think of an empty pop can. It is full of air, but you do not need to suck the air out of it before you fill it right? In fact, if you did, the pop can would collapse. The same is true for a windscreen repair. As the resin flows into the break the air is forced to the outside edge.
When the vacuum cycle is initiated, the air is pulled up through the resin and into the injector. When the injector plunger is locked in the top position the air is released through the top of the injector. As the air is removed it is replaced by resin.