I just got a freebie spa (1996 Leisure Bay, 5 pp, (2) 3HP pumps, blower, pneumatic controls. I filled it, provided power and tried to start it up. Pump 1, turned on for a few seconds then it blew the fuse and the internal GFCI tripped. I then noticed the infeed gate valve was closed, so i opened it and replaced the fuse. Powered it up again and the pump sounded like it was laboring or stuck. Few seconds later it blew the fuse again.

I disconnected the pump inlet and outlet hoses to see if there was flow and there was plenty.

Any suggestions?

Also, this spa sat for a longtime. When i powered it up, tons of dirts pumped out of the jets. Any tips on flushing the entire system to clean all the pipes internally? Thanks.

Certainly sounds like you need a pump replacement at this point...