Thanks guys. Still a little confused though, some say replace while others say still good. They seem to still have quite a bit of meat left.
Do you know how old they are? If they are original pads (over 40 years old) and if the lining separates from the shoe under high speed braking and locks up the rear wheel and you go down on asphalt, you just have to ask yourself...
how much meat will be left on your bones when you stop sliding?
Original Honda shoes are less than $40:
http://www.hondapartsnation.com/oemparts/p/honda/06430-393-405/shoe-kit-brakeAftermarket shoes (Vintage CB750 Part #: 29-3121) can be had for less than $25 (scroll down):
http://www.vintagecb750.com/products/2/chassis-brakes/39/brakes-brake-hoseThe age of the shoes would be my only concern. If the previous owner replaced them within the last 10 years or so (provided the bike was not stored outside, exposed to the elements) then you would
probably be fine.
Otherwise, new shoes are cheap insurance. I've got enough to worry about as far as "cagers" trying to run me over without flinging myself down on the asphalt as a result of a failed brake shoe locking up the rear wheel. Sure, you could use them, there is indeed plenty of pad left on the shoe, and they might work well until they are worn to their service limit without any issues, and you could replace them then. I've used old shoes without issue (back when I was younger... and invincible), but I've also had old shoes sitting on the shelf that had the linings fall off. Ultimately it's your hide and your decision to make.
