You're right Pete and thanks for the help. I'll post an update tomorrow after driving it and doing a final bleed on the system. I guess it got air locked after I changed out the water pump. I didn't drain the radiator before taking out the water pump and just drained via the hoses one by one since I was in my parking lot and it was hard to get underneath the car. Lesson learned.

 

So far so good. I drove the car today and no leaks from the cap. I still never saw air pressure build up enough to make coolant leak from the cap before, but I guess there's a first time for everything!

 

It was more like steam pressure than air pressure. lol Because the air was interfering with the coolant from reaching certain parts of the water jacket, those parts became heated above the boiling point of the coolant, and when the coolant eventually got there it immediately turned into high pressure steam.

 

Just a quick update. Periodically I would get some dripping from the radiator cap. Today I managed to get the front wheels up so the nose is higher than the rest of the car. Here's what I did:


Before I started the car:
1. I put 2 AC Delco sealant tabs in the radiator (crush them first). They cost $5.00 at the dealer: https://www.acdelco.com/parts/vehicl...tem-seal-tabs/
2. Fill radiator with coolant up to the neck with both bleeder valves open.
3. check the coolant level in the overflow bottle.

Bleeding procedure:
1. Turn on car with heat on full
2. Opened both bleeder valves (upper rad hose near intake hose) and smaller heater core hose.
3. Filled coolant in radiator to neck until coolant flowed out of the upper rad hose bleeder valve.
4. Closed the valve
5. Poured more coolant in the rad until coolant flowed out of the heater hose bleed valve.
6. Closed the valve.
7. Put on the rad cap
8. Let car run until thermostat opened.
9. Opened upper rad hose bleeder valve and let coolant bleed out until there was a steady stream. (In my case there was no air from this valve so coolant stream was constant).
10. Closed upper rad hose bleeder valve
11. Opened heater hose bleed valve. I had coolant come out in spurts, then nothing, then coolant again.
12. Closed the heater hose bleed valve.
13. After a few minutes I opened the heater hose bleed valve again. Again some sputtering then coolant flow.
14. Closed the heater hose bleeder valve
15. A few minutes later I opened the heater hose bleed valve again. This time there was no air and a steady stream of coolant.


My observations:
You MUST have the car parked uphill or with the nose higher than the rest of the car (ramps).
You MUST fill the radiator with coolant with both bleeder valves open.
When coolant comes out of the lower valve, close it and fill coolant in the radiator until coolant comes out of the second (heater hose) valve).
You MUST run the car with the heat on full and fan on high.
You MUST wait until the car is at operating temperature (halfway on the gauge).
You MUST bleed the lower rad hose valve first (big hose coming out of the water pump). When coolant has a constant flow, close the valve.
You MUST bleed the heater core AFTER the lower rad hose is bled and there's no air in the water pump.
You MUST open and close the heater hose bleeder valve several times (allowing it to build pressure) to make sure you get all the air out. After 3 or 4 times you should have a constant coolant flow from the heater hose valve.


NOTES:
1. Constantly monitor the temperature gauge. A second person to watch the gauge is helpful. Overheating isn't good at all for these engines.
2. Put rags under the intake hose to prevent coolant from dripping on the optispark.
3. Monitor the coolant level in the overflow bottle by checking the dipstick.


These cars are known for pockets of air trapped in the heater core and this procedure is the only way to get the air out because the radiator sits lower than the engine.