Hydraulic seals serve a number of purposes on equally as many machines. Like anything mechanical, they will eventually fail, and hopefully, you will be able to avoid major catastrophe when they do. If you are currently experiencing hydraulic seal failure, and it is minor, then here is what happens next.
The Piston Using the Seal to Keep Pressure Under Control Buckles
Hydraulic cylinders are essentially pistons under pressure. The hydraulic seals keep that pressure contained and constant. When the seal fails, the cylinder/piston under pressure buckles. It is not a pretty sight, and a repair technician will need to put it back together and replace the seal. Now when pressure rebuilds inside the cylinder/piston, the new seal will hold and prevent further damage.
The Flanges in the Oil Pipeline Will Begin Spraying Like a Slashed Artery
Hydraulic seals are used to create an airtight connection between flanges, which are most commonly used in oil pipelines. If the seal fails, there is a loosening and widening of space between the pipe flanges that results in an oil leak, nay, an oil spray that looks like a severed alien artery squirting black oil everywhere. If you run for the nearest valve switch, you can stop the loss of oil quickly. Then the repair technician will need to replace that seal as quickly as possible, or the pressure from the oil drawn through the pipe to the refinery will decrease and the oil still in the pipe will leak straight out of the area where the two flanges meet. (All things considered, this is actually a minor event; there are far worse things that can happen on a pipeline when a seal fails.)
Rockets Blow Up
Hydraulic seals are used on rockets to prevent extra moisture from getting into the boosters. If the seals fail, the rockets could explode in the air, or they would burst into flames on the ground just as the boosters are ignited. NASA hopes for the latter, because even though a rocket on fire is bad, another rocket in the air exploding is far more catastrophic. Flames can be put out on the ground and any astronauts have a fighting chance of being rescued, plus the seals that failed can be replaced. In the air, it is completely, and sadly, a tragedy that cannot be stopped. The NASA program learned these lessons years ago, and great care is taken to make sure hydraulic seals are in perfect condition now prior to each launch.