1.8 petrol (shudder) throwing coolant out

I recently rebuilt my 1.8 1997 Petrol engine after the cam belt tensioner broke and the cam belt slipped. Replaced 8 valves, replaced the head gasket but I didn't get the head skimmed as the gasket was done 20k earlier (don't know if the head was skimmed them as I didn't own the car).

Since the rebuild when the engine warms up it starts throwing all the water out of the expansion tank cap. (The rubber seal Is warped). There is a little oil in the expansion tank, but there was before the rebuild and I tried flushing the system through, but couldn't get it all out. I don't know if this oil is new or not. As soon as. The engine is turned off the expansion tank empties and I have to top it up again back to the level.

Could this be as simple as the expansion tank cap not being air tight?

If not what else could it be? It's an old car and I have spent more than I should have (wife would kill me if she knew lol)

Are you sure you have bled coolant system properly? Heater full on and open bleed screws till good solid flow of coolant evident, also could be faulty or dirty pressure cap.
Failing that what heights were the liners above the block when head was off, and do you think it could have been topped up with cold coolant and cracked the head ?

I've tried bleeding the system, I get a flow from the bottom bleed screw, but not from the top one. (Water comes out of the top one if I squeeze the top rad pipe). I'm using water as the moment as coolant is expensive and I'll replac the water with coolant when I fix the problem. Liners were correct when the head was off and I don't think the head is cracked. Pressure cap is suspect, so could that cause the problem?