I can envisage the following failure modes:
The outlet valve never opens, but the inlet one opens and closes when it should.
The inlet valve never opens, but the outlet one opens and closes when it should.
The inlet valve opens but never closes, and the outlet one opens and closes when it should.
The inlet valve opens but never closes, and the outlet one never opens.
The outlet valve opens but never closes, and the inlet one opens and closes when it should.
The outlet valve opens but never closes, and the inlet one never opens.
Both valves open but never close.
Neither valve ever opens.
Either or both valves open and close, but not at the right times.
As long as nothing bad happens in any of those scenarios you're good to go.
The outlet valve never opens, but the inlet one opens and closes when it should.
The inlet valve never opens, but the outlet one opens and closes when it should.
The inlet valve opens but never closes, and the outlet one opens and closes when it should.
The inlet valve opens but never closes, and the outlet one never opens.
The outlet valve opens but never closes, and the inlet one opens and closes when it should.
The outlet valve opens but never closes, and the inlet one never opens.
Both valves open but never close.
Neither valve ever opens.
Either or both valves open and close, but not at the right times.
As long as nothing bad happens in any of those scenarios you're good to go.