1995 BMW 525i over heating

Heya. So iv had my bmw for a good month so far. When purchased, I was aware the head gasket was near new, itd had a new oil filter put in as well as a new air filter and a new water pump. The lad I got it from told me when I start the car to wait until the air pump had finished filling up before starting the engine. He also showed me the preassure velve next to radiator and said sometimes the preassure needs to be released and so he showed me how.
now the other day my partner has touched around and released this preassure velv without good reason. The car still seemed to be fine after that.
the next day however after driving around for a time frame I noticed the car over heating. It was at the 3/4 line when I arrived home. From there my partner has further released more preassure from this velv. At the same time he loosend the lid to where coolants put in. This erupted a little but not much as most the preassure had come out from the velve.
my partner had told me not to put water in, coolant only. So later that night we drove into town and the car was over heating rapidly. It was now on the red line but we were pulled over by this point. Water was dribbling out quite fast while it was this hot. It was not coming from a leak. When it cooled down I drove it accross the rd to get some coolant. My partner said we will put it in when we get home. On the way home we pulled over when it got hot again and by then I was adiment it needed water or coolant but im aware u cant put this in until car has coold. My partner undone the preassure velv again and loosend the coolant cap and it erupted a little bigger this time. My partner put the coolant in once it had coold down further. We had to pull over one more time on our journey back home but I had noticed that I hadnt heard the air pump filling up when turning ignition on. Also on the last stop I thought I hear a ticking broken sound once pulled over. Once home, we have pretty much not touched car. Oils fine. No leaks in water pump. Have put water in car litre at a time and waited for it to go down. Have only really put in about 5 litres. And it takes time for water to go down. My mechanic told me on the phone to bleed the water pump hose or something but I wasnt sure what hose to undo. Today we have put a little more water in and as doing so, alot of air bubbled out the top. I was told that trapd air could be causing this. I took the car for a drive, within 5 mins it was at half way and stayed there for a further 5 mins. Its then gone to the 3/4 line but swings from half to 3/4. What are your suggestions? I dont feel as tho its huge as the car is still running smoothly and I cant smell to much from over heating. But the air pump not being heard is a concern. And also how fast it happend. Could taking thermostat out help? Or could air being traped cause this.
yours sincerly jessie