First of all. Lets clear something up. Back-washing only applies to pools that have sand filters. If you are running a different kind of filter, for example a cartridge filter, then you do not need to backwash (nor can you) your filter.
A leaking backwash line is almost always a result of a worn spider gasket within the head of your sand filter. Depending on the severity of the damage, there are several suitable solutions.
The quick and cheap fix is to put an isolation valve on the backwash line itself. This can then be closed when you are not actively back-washing the filter preventing water from escaping to your drainage system/backwash location, however does not solve the underlying gasket problem. This solution will solve the leak issue, however with the spider gasket remaining worn, all of the water entering the filter will not necessarily be filtered, though the implications of this are likely insignificant.
The more permanent solution is to replace the spider gasket itself. Replacing the gasket will ensure that all water entering the filter is being directed to exactly where it is meant to go and that all the water is in fact being properly filtered. A spider gasket replacement will be several times more expensive than installing an isolation valve on the backwash line and depending on the state of the filter head itself may not be a reasonable fix. Each situation needs to be individually assessed.
The permanent and long term solution is obviously to either replace the entire filter head or filter itself. Sand filters do age, like everything, and eventually will need to be replaced. In most cases I recommend this as a last resort, unless of course my client is upgrading equipment for their own reasons, as it is far more expensive and can trigger costly labor charges depending on the site etc:
Feel free to contact me about your pool issues. I can be available for inspections and am always happy to prevent unnecessary loss of water.