Sorry Check valve Secure 4 Model 350 not longer available. 

Life span 5 years but if your ones is broken you have had it on their for > 10 years

Please call the number below for advice on these new units or a 'LOGOI Valve 15mm 350kpa  plvcv15"

Coast to Coast RV Services

48 - 56 Meakin Road, Meadowbrook QLD 4131

Phone: 07 3805 7117

Fax:   07 3200 5400

Email: salesqld@coastrv.com.au

How this valve worked, was it contained a pressure limiting valve and a non-reversal valve. So to manually replace it you instead need two parts. You could source these parts from your local plumbing store like Reece plumbing.