The SX-280 press is designed to squeeze product mash at a slow and steady pace. Pressing too fast causes the bags to fail and causes the mash to raise up and out of the bag. It is almost always better to press slowly.
The flow control valve meters how much fluid is allowed to pass through it, hence changing the pressing speed. On the SX-280 there are separate flow controls for the forward and reverse directions. The pressing speed difference from a fully open flow control valve to a half open flow control valve is minimal to an observer. This is why it may appear as though you have no speed control. But from half way open to the fully closed position on the flow control valve, there is very noticeable speed difference. This is the zone you will want to operate in.
Troubleshooting the flow control valves (speed controls)
You will want to begin with the press cylinders fully extended and place the directional switch into the neutral position.
Turn the flow control knob for reverse clockwise until it stops. This is the closed position.
Place directional switch into the reverse direction, the platen should not move. If the platen moves the flow control valve has failed and needs to be replaced. If the platen does not move, place the directional switch back into the neutral position. Turn the flow control valve counter clockwise ½ of a revolution and place the directional switch into the reverse position. If the platen is moving very slowly, your flow control valve is functioning normally.