It sounds like your shower valve was installed prior to any of the safety features being required...so, it is unlikely that it has a spool valve.
If you just open up the hot valve, is the water hot? Do you have good flow? If not, it could be multiple things:
- if your supply pipes are galvanized (possible on an older install), they could be all rusted up and restricting the flow
- if there is an input filter to the valve, it could be clogged up with crud
- there could be something blocking the flow - maybe a bad washer or some other stuff
In the long term (maybe short!), you might want to budget to replace the old valve system with a new one that has the anti-scald technology. This can be done by cutting a larger hole from the front, but is easier from the rear if there's access. Then, to cover the larger hole, most companies sell a remodeling or rennovation plate that fits underneath the 'normal' trim to cover the now larger hole. This works for most, unless the existing handles are REALLY far apart, which is not common.