73' Pontiac Ventura leaking coolant

I have a 73’ Pontiac Ventura, v8 350. I took it out for a drive last week for the first time in about 2-3 weeks. It was a short drive and I noticed the coolant was pouring out. At first glance it looked like it was coming out from on the passenger side of the car, behind the front wheel near the fire wall. I got it home and there was no coolant left. The only thing I have found was as I am about to describe. There is a small rubber bulb (for lack of a more descriptive/accurate term. it is maybe an inch or so in diameter, looks to be gumdrop shaped, yet there is a flat piece that comes off of the rounded end. That flat piece is opened almost like a cut piece of hose with a clean edge. When I pinched that end a little remaining coolant leaked out. The “bulb” appears to be attached by some pins that stick out around the base. Now I can’t say with any degree of certainty that this is the source of the leak but it is all I have found thus far. I did have the transmission rebuilt in the last month but I have driven it a lot since and no problems. I am hoping it isn’t an expensive fix. Help!