The high limit trip was caused by not powering it down, but the GFCI tripping is not- it's simply coincidence.
There are a number of troubleshooting steps that could help if a tripping high limit is your poblem, but in your case, this doesn't apply.
A GFCI tripping is telling you the is a ground fault somehwere, in effect, a component is leaking electricity. You have already done the troubleshooting and isolated the bad part. No amount of cleaning filters or bleeding air is going to magically repair the heater.
I think you may have misunderstood the manual- it's giving you options to repair a tripping high-limit, not a tripping GFCI.
Once the high limit is reset, the heater instantly energizes (assuming the heat is turned up) If the heater is working properly, it begins to get hot. Yours, however, leaks enough electricity to instantly trip the GFCI. It's not a flow problem, it's an electrical problem.
You may still have a flow issue as well, but there is no getting around the fact that a spa component is leaking electrcity, and all signs are pointing right at the heater.
I hope I'm being clear
Good luck!
~Swine
EDIT:
I Just did a search and found an
earlier post about your flashing ready light. You might want to double check your circ pump flow- especially right after powering up the spa. As the pump ages, it starts slower and slower, sometimes taking several minutes to get going. If the heater begins to heat before there is adequate flow, it will also trip the high limit. Just something to note- I hate to be the bearer of bad news, but you might need to replace both items to fix both problems. A new circ pump will alleviate the tripping high-limit, and a new heater to repair the tripping GFCI.
