Check your disconnect fittings (ball/pin lock. Disassemble them and make sure they're working correctly. It sounds like the poppet valves are fine, but you can disassemble them too.
http://www.bluemaize.net/im/appliances/cornelius-keg-parts-0.jpg...
BYO and morebeer have some good articles related to this, for anyone interested in reading further:
Decoction Mashing: Techniques
https://byo.com/bock/item/537-decoction-mashing-techniques
Kettle Reactions - The Science of The Boil
https://www.morebeer.com/articles/how_to_boil_wort