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[kcl]

How does the HBV work? Is it more prone to leak A/C on or off? Had the stat leaking before, sorted that and all was well for weeks until a few days back, I can smell coolant on exhaust and it is leaking, not bad but still. My other option for the leak is the oil cooler but I do not feel confident re-sealing it 

[fiend61]

hmm not sure whether it will be more prone to leaking with ac on or off  :-/  thought it would leak  no matter what ,  i know that vacuum should only be supplied to the diaphragm in the hbv when aircon is turned on and heater controls are set to "lo"

[YZ250]

My HBV leaked much worse when the heater was set to hot. I turned the setting to cold (not aircon though) just to get me home as it seemed to slow the leak down when set to cold. This was the case on my previous Omega as well. Not sure what influence the aircon has.  :-/

You could let the car get to running temperature and then turn the heater right up high to see if coolant floods out. 

[feeutfo]

There is no rule to how they fail. It depends where the leak is on which section.  But playing with heater setting hot and cold with engine running helps if trying to diagnose hbv if it's not obvious. You'll need to get a hand in there and see if it's wet under whatever setting. Careful though. The water will be Hot!

Pressurizing the cooling system slightly will help diagnose other leaks in the v.

[kcl]

Ok, thanks guys. Pressure test: would one bar be enough? Don't want to blow anything up so maybe 8 bar is too much