Not sure I understand what you mean that the decompression valve is not working. You only pull on the decompression lever when you are trying to start your Bravo.
When you squeeze the decompression lever on the handle bar it should depress a spring on the cylinder head that causes a decompression valve in the cylinder head to open. As soon as the moped starts you release the decompression lever so the valve can close.
Don't use the decompression lever to kill the engine. Some people do this and it can it build up an oil and carbon residue outside the decompression valve and cause it to remain open.
You can remove the cylinder head to check the valve.
1 Remove the spark plug
2 Disconnect the decompression cable from the cylinder head.
3 Remove the three nuts and washers and slide the cylinder head off.
4. put the cylinder head on the bench or work area and clean off all the carbon build up from the cylinder dome using WD40 and 0000 Steel wool.
5. once you have cleaned the cylinder dome and can see the decompression valve and with the cylinder head dome up so you are looking at the inside of the head, squirt some WD40 in decompression valve area. It should form a pool. If it doesn't pool up or leaks out, then the valve is not closing properly. If it is leaking try cleaning the underside of the valve and where it seats of any oil and gunk build up.
6. If you can't get it to work properly you need to replace the valve or replace the head with a head that has a working valve.
You don't have to remove the cylinder to perfom this operation.
When you reinstall the cylinder head torque nuts to 7 ft lbs. Don't over tighten or you could snap a head bolt.