Most likely the seals are damaged or dried out in your MCGV. Make sure you use ALL of the channels in the pump on a regular basis. Do not cap any off or just leave them sitting in unused solutions. After removing all of the lines (4) from the bottles, try flush each one (100% A, etc), one-at-a-time. If you use the system with ANY buffers, start with a 100% pure water flush, then with pure IPA at a normal flow rate for 10-20 minutes EACH to remove deposits and clean the seals.
For more info, please refer to this unbiased article on how to diagnose, troubleshoot and solve the issue.
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HPLC Gradient Valve / Proportioning Valve / MCGV Leaks. How to Identify Them";
https://hplctips.blogspot.com/2021/08/h ... valve.html