Strange Coolant Issue

This was the 1st season I didn't summerize my sled and I did the old start the sled once a month thing. I WAS going to pull the motor and go over it and clean the sled but I just ran out of time this summer.
When I went to get her up and running for the season I noticed I had lost some coolant in the bottle. I filled it up and when I went to pull it over I could watch the coolant level go down with every pull???? I could hear the bottle pressurizing because it was hissing so I would turn the cap and the fluid would pop back up.
I ran the sled and thought I got the coolant level to stabalize, I even got the front end up and blead it just incase it was an air bubble....not sure how an air bubble could get in the system just sitting there all summer???

Went for a ride and I could not keep coolant in it. It SEEMED like it was all coming out the overflow but I would not bet on that. Check all hose clamps to make sure one didn't come loose. Still had the same pressurizing issue and would have an empty bottle until I turned the cap and the coolant level would come up a little. I dumped A LOT of water in to get me back and seemd like I was loosing it faster then I could put it in but there was no obvious leakage anywhere.
If I had a hole somewhere would it be able to pressurize like that?
It seems like air lock but I just don't understand how it happened from just sitting in the garage.

Thanks for any help

The motor has been rebuilt recently...two seasons ago.
It ran just fine on the last ride. It must have happened when I started it up last just to get things circulating but it was only for a min. or 2

It is clear and green.
Just tore about half the seld, took off the seat to get to the hoses running to the rear exhanger...no coolant in the hoses, took off the hose that goes into the head...no coolant. Lifted the front up reattached everything good and tight filled with coolant and in seconds came exploding out the bottle.
Took head off....no coolant. ONLY thing I can thing of is the water pump went out and therefore the engine is pumping air through the system and this is why I'm having the pressurizing issue.
What typically happens when a water pump goes out?

I pulled the head off and the o-rings seem ok and so does the outter rubber gasket?
Is it not obvious or something?
It would be nice if it wasn't the water pump since I would probably have to pull the motor to change the impeller.

This is on a 800 Doo motor, thermostate deleted.
My thought was the impellar went out but I guess for it to pressurize like that it has to be some kind of leak in the motor, either o-ring or base gasket. If not either one of those then a crack in the lower case. I HOPE IT'S AN O-RING!!!!!