Won’t burp cold

When you say cold, do you mean the engine hasn’t run, or the OAT is unpleasant.

Can you feel the compression when rotating the prop? You need to be able to build pressure in the cylinders so that this then pressurises the crankcase and returns the oil to the tank. If your valves only seal when the engine is warm that that would explain your problem. You may need to regrind the valves if that's the case.