[Richard]

Bob

The top and bottom both screw off.
Turn off both water and air pressure before you touch it!
Make a diagram or take pictures of all the connections. You have to remove all connections to get the valve out of the unit. It's a real pain. It helps to loosen the two wing nuts that hold the two black cylinders in place. Once you have the sequencing valve out, use the exploded view diagram in the manual to help with disassembly. Use very light silicone grease when putting it back together.

Check carefully where the vacuum breaker attaches to the toilet. You don't want a water leak there when you flush. I have to fiddle with tha connection almost every time I touch the sequencing valve.

The toilet works by cycling the sequencing valve through a full stroke of a spill inside the valve. As the spool moves it sends air or water to the appropriate line to operate flapper, flush, and water refill. A stuck or malfunctioning sequence valve could explain everything you have described.

You can pay a couple of hundred for a new valve or you can put new o rings available at lowes and silicone grease such as light bulb grease from the auto parts store and have the same result. I find I need to disassemble
And lube the valves about once a year