What kind of vehicle? 3-brush generators went out of use back in the late 30's on cars. They were used on farm tractors, etc., for a little longer. The voltage must be adjusted against a fully-charged battery, with very low current flow on a 3-brush system, so it's possible the voltage adjustment is still too high. Put a small resistance, like both filaments of a 6-volt sealed-beam headlamp between the generator and the battery before adjusting the voltage. The oldtime Sun volt-ampere testers had a 1/4 ohm resistor to simulate a fully-charged battery, which was inserted into the charging circuit before making the final voltage adjustment. Current flow into the battery should be below 10 amps to get a good adjustment.
Jerry
