I dont know what string to use in my toro 8" battery trimmer

Be aware that "weed eater" is a registered trade mark of "WeedEater". What you want is the price of a Toro String Trimmer. Here are Home Depot's prices:

https://www.homedepot.com/b/Outdoors-Outdoor-Power-Equipment-Trimmers-String-Trimmers/Toro/N-5yc1vZbx8iZ1i9

You have to hold in the two tabs at top of spool assembly

Look at this tip, an oldie but a goodie:

http://www.fixya.com/support/r2827867-winding_line_trimmers

Should be what you need.

Best regards, --W/D--

Here is a parts and diagram breakdown on your machine. It also includes a list of any and all parts that you can purchase to repair it.

http://www.ereplacementparts.com/toro-51430-500000159999991985-trimmer-parts-c-121776_123781_124021.html

This can and is a frustrating problem. Some things that help are to be sure to use the right size of trimmer line. Be careful when rewinding the reel to not twist and cross the lines. I realize we should not have to give this much attention to it, but until somebody "builds a better mousetrap", we only have so much to work with.

flooding out...change the air filter...

you can order the battery here
http://www.partstree.com/parts/

There is a screw underneath the Toro label.   I learned the hard way.

Check to ensure that the string is the correct size as specified in the owners manual. If ok check age of the string. If old it has a tendancy to be brittle. If new and supple try letting the trimmer get up to full speed and let the tip of the string do the cutting. Avoid taking on too much at once with this trimmer as the electric and battery trimmers are not as powerful as their gas counterparts. I hope this helps.