In general, just replacing a worn out or damaged vacuum pump w/o first diagnosing the exact problem that caused the degasser to fail is a bad idea. Often, the replacement pump is installed back into a contaminated or damaged system, resulting in decreased lifetime of the pump and continued contamination throughout the system. Use of hexane can contribute to this. *A damaged degasser can lead to direct contamination of your mobile phase (pervaporation and condensation often occur and this results in the vapors and liquids inside picking up the internal contaminates, then reintroducing them back into the system).
The most common reason for bubbles to appear going out of the multichannel gradient valve is from a loose low pressure fitting upstream. Usually a loose fitting exiting or entering the degasser will cause it.
If you are using a G1379-series of degasser, then it is possible that you may have damaged one of the vacuum chambers (very easy to do on that specific model). The damaged channel may have caused the pump to fail. If the chamber is damaged, it may show a restriction or may introduce air into the system. The G1322A degasser usually can handle hexane w/o any issues, but not the G1379A or G1379B (or any of the newer models).
I should point out that it is also possible the degasser is not the source of the bubbles, though it should be properly checked and serviced. Additionally, you should switch hexane to a different channel (from D to B) to see if the problem re-appears using a different VALVE position and also a different degasser CHANNEL.
If the seal inside the proportioning valve has dried out (and it will if you run hexane), all you need to do is connect that channel to a bottle of pure IPA and run it to waste at 0.5 mL/min for an hour or two. That will re-hydrate the solenoid seal inside the valve. Next, switch it on/off (50/50 cycle time) using two bottles of IPA so the valve switches between two positions (i.e. Channel C and D) for 20 minutes. After that, go ahead and run hexane again and see if the performance improves.
FYI. Related Article: "Reasons For Air Bubbles Exiting The HPLC Pump";
https://hplctips.blogspot.com/2020/02/air-bubbles-exiting-hplc-pump-reason-why.html