German Brewing

I've had that happen before, probably a slow leak somewhere

I've had leaks in my gas in post, the lid, and now the valve itself (after dropping it) which I'm trying to sort. A keg can leak one batch and not the other depending on the seal, lube, and how everything was cleaned and reassembled.

I guess as a last resort one could submerge the whole keg equipped with the spunding valve in water. Depending on the gauge though I might keep that above the liquid level.

I had this happen too. I've done 3 LODO batches now with increasing attention to detail. First batch I force-carbed because I didn't have spunding valves. Tasted fantastic. Drank it fast before any O2 in the CO2 started staling the beer. Second batch I used my new spunding valves, but experienced pressure loss and had to hit it with the gas. Third batch, I tried 2 different things. I fermented to completion, and then quickly added boiled sugar solution to the fermenter. Left it for a few hours until pressure started building up in the fermenter. (I ferment in an SS conical under a little pressure ~ 1psi or so.) Then transferred to kegs under pressure. This time, I used some keg lube on the kegs, and happily I could see the kegs increasing in pressure up to about 20 psi. I should say that I brew ales - never tried a lager yet - so I was carbing the kegs at room temp, 20C/68F. The second batch I had in a fridge at 5C. Poor old ale yeast don't stand a chance that cold. Taking them back up to room temps didn't really help. I think keg lube is a must, when you have low initial pressure, to help seal the cornelius. After that, you need the right temp for your yeast. Even with ales, there is a vast improvement in the malty flavour. Loving it!