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check the voltages at the CRT, all the pins and the HV as well. Generally brightness will be controlled by bias of the CRT and anode voltage.
a lot of times low brightness is caused by a weak video out tube. This happens on sets with directly coupled cathode (common on RCA color sets of that vintage). As the 12BH7 (or what ever the video out tube is) gets weak it conducts less and the plate voltage goes up. This in turn results in a higher voltage on the cathode of the CRT, which effectively cuts off the current flow in the CRT resulting in less brightness.
The grid and screen voltages all come into play as well so a voltage check at the CRT is the best place to start. HV will of course effect brightness as well, so that should be setup early on.
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