84  1.9  Westy,  Just replaced the front heater valve, again. The original was leaking internally. The 1st replacement begin to leak externally in October during our 8k mile west trip, (temporarly bypassed it). 2nd replacement started leaking externally about a month ago. Beginning to believe these plastic POS are junk. Ordered and installed #3 last week. 
This got me to thinking that maybe the coolant tank cap may be faulty, building too much pressure, so I ordered a new one with heater valve #3. In the meantime I put together a contraption to test the tank cap. (existing and original from PO). They both begin to bleed off near 20#'s, (think Bentley says 18 or so). Figured that was close for my home made Harbor Freight gauge. Thought I should test the brand new, still sealed in plastic cap, (prd. of China). Surprise, it went to 40#'s before it started to bleed off!  Left the old cap on. Between the heater valves and cap I'm beginning to think used parts are the best bet. Sad that you can't relay on "new" being good.