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FIC are Japanese cylinders
The high side of the slot slightly pushes out the brake shoe causing it to drag. Then you also have leading edge and trailing edge orientation issues to contend with on the shoes themselves. Lower/angled side of the slot should point towards the longer shoe side. Also, one tension spring must be...
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On factory wheel cylinders the round slotted bolt/screw portion had an angled slot in it (one side of the slot is cut deeper than the other). Your screw might be the wrong way round or, if the replacement cylinders don't have this, it might account for the dragging and lightly filing the slot...
On the subject of the clutch, it might be sticking due to rust or grime impeding free movement (I recall you had to use a bar on the relase/throw out bearing earlier on). Drive it in ideally high gear (one to one), you can put it in low range if brakes are not there, and depress the clutch pedal...
Yep, blow air through the line to clear it. I have had some luck to then, after the line has cleared (fuel will flow freely out of the line without pressure) to then pull the fuel tank drain plug. The nasties are suspended in the fuel still at that time and tend to come out with the fuel.
The dash indicator lights are just two dangly wires which are not taped into the loom. The loom only has the taillights' wiring in it. These two indicated wires are the separate wires going to dash lights from the indicator switch-
This should help you a lot -
And yes, the dash lights which show you are indicating (likely green but maybe red still for you) go into a 2to1 connector, hence why you have less wires in and more out.