If you have zero psi in F and R, you have either a shuttle valve that is not shuttling, OR, an internal leak present somewhere.
Just did one of these.
The shuttle clutch isn't fun. They usually get damaged when something goes haywire. There is a new, updated part number for the shuttle clutch-and it is improved. It is expensive but the new ones are less likely to fail unless the wrong fluid is used (too "thick") or something odd like that happens. Rarely, in other words-I am not sure I've even heard of the newer style shuttle clutch failing. A few of the older style ones, though.