It depends on the height of each bleeder, it looks like by the service information they need to be done in that order, as for why there was a pressure difference, I am not exactly sure.
A bad coolant pump will not cause a buildup of pressure like what you are describing. If you had a bad coolant pump the vehicle would overheat while it's sitting still idling.
If the coolant has not been changed in a very long time, there could be some build up around or in one of the bleeders. In the coolant system after so long there will be a type of sludge build up from a reaction that's pretty technical. I'm not sure how the old coolant looked when you drained it, but if it has been in there for years sludge will build up. But an over pressuring issue (if the air is properly bled out) points more to a head gasket that might be starting to leak into the coolant system.
There is a chemical test you can buy at an auto parts store that will test the coolant for any combustion gases. It has a funnel type tube that goes into the radiator opening and the testing fluid in it will turn a certain color if there are any gases from the cylinders being pushed into the coolant system. It's the easiest test to do. Without having to spend a lot of money. Here is a link to one, so you can get an idea of what it looks like. Harbor Freight has a cheaper one for $25 and the fluid for $10 is a purple fluid.
Hopefully it's not a head gasket problem. But if the coolant temperature sensor was bad, you may not have known it was over temperature.
https://www.napaonline.com/en/p/BK_7001006?cid=paidsearch_shopping_dcoe_google
Oct 23, 2022 at 1:25 PM