I've tried measuring the current from the main motor and it was higher than the 10A limit on my multimeter. So I've ordered a new motor a few minutes ago with a new pump seal. (I'm assuming the pump seal comes in two parts, ceramic and spring).

How do I measure the winding resistance and what should it be?

The other two pumps (spa/waterfall) just hum and trips the breaker after 5-10s.
I've tried replacing the starter capacitor from the main motor and didn't help. I took it out and noticed I couldn't move the impeller by hand. Didn't see anything stuck. I've also noticed that there is some corrosion (green) on the brass/copper ring inside the diffuser. I'm thinking water go to this part and caused the motor to seize up providing enough friction on the impeller for it not to start. However, the motor still doesn't start under no load, I think maybe the motor is fried? What do you think?

I might try taking everything (impeller, diffuser, seal) off and trying it again.
If there are some measurements I'd like to fix it.

These two pumps are hardly used since they are for the spa and waterfall which we haven't used in years. Maybe running these hardly used motors once every other day for 5 minutes might be good, not sure...

thanks in advance
Jack