250-500cc WR300 smoking. Any ideas?

Hey, I was riding before winter and had a smoky time after some pretty hard hill climbing. It's white smoke from the exhaust. I figured immediately that it was the head gasket, so I bought them and replaced. There weren't any issues with the old ones upon inspection, but changed them anyway.

The next ride was okay, until I really started working the bike again, about 15 min into the ride. It was smoking the exact same way again. I decided to ride it a bit more, so I could get to an easy trail home, and the smoke disappeared while riding like a grandpa.

I was talking with my riding mates and trying to figure out the problem on the trail, so trying to exhaust all possible options I checked the crankcase oil. When I opened the drip screw, I heard a vacuum release!? Rode a bit further and the smoke was gone again. Then it started smoking again after hard work. This time i opened the oil fill cap (since it was more convenient) and I heard the vacuum release again. Once again the smoke was gone.

Soo... I kept riding, when it started smoking, I relieved the pressure and kept riding. Repeat until I covered 30km of single track. I assume at this point it something to do with the water pump. Are there gaskets in there that could be worn?

Oh, forgot to mention, it's got milky oil now.

Did you check the radiator level?
Sounds like the water pump seal, Joe!!

Minor coolant loss.

Water pump seal.

Make sure your crankcase vent is not clogged up. When a hot engine hits foot peg deep cold water it will rapidly cool the transmission and suck water and mud up the vent hose. If the vent hose is open it should never build pressure or vacuum.

The build up does suggest vent tube blocked, you may be OK with water pump seal, how milky is the oil does it smell of gas?

As the engine heats up oil expands so pressure forces oil into your cylinder causing smoke.
Now if while the seal being forced to ingress oil it may put gas into the bottom end.

Also metal condensates so if the water can't escape through the vent when the engines warm then it may turn oil milky.

Or your water pump seal has gone.
Does the water level need topping up after a ride?

Normally crank oil is a blue horrid smelling smoke.
Coolant is a sweet smell with lots and lots of white smoke.

whenever my bikes start smoking we go to the shed for that talk!
I could also recommend trying the patches, put those on the tank
don't waste you time with the gum,,,, motorcycles can't chew, but they can bite as we all know

I was gonna suggest champex but thought better of it.