you said you're using festo valves? If my memory is correct, they have built-in push to connect fittings?
We have some plastic plugs from SMC that are designed to go into a push to connect port and plug it. They're basically a special plastic version of pneumatic tube - specifically; without the hole in the middle.
SMC Push To Connect Port Plug
[ .02 $ ] Fewer push-to-connect[ions] on the robot means less air leakage - they don't leak much, but they all do slowly leak. Connecting a hose, and then a cap, is two connections, versus a plug, which is only one. [ / .02 $ ]