How to troubleshoot the following hydraulic brakes problems
I recently got a mountain bike with hydraulic breaks. Rear breaks were performing a bit strage from the start but I didn't really pay attention to that because overall they were working. However later when I held bike upside-down for some time and then put it back into normal position, rear breaks stopped working - the handle was pressing all the way down without any resistance. However when pressed repeatedly, some pressure would build up and they would work fine for a short period of time.
Well, I assumed that the problem was caused by air in the system, then I contacted LBS where I got bike, they confirmed my assumption and agreed to bleed breaks for me. I got to the place, the mechanic mentioned that he was going to do this procedure for the first time and got to work. Everything went relatively fine, there in fact was air in the system and after some time he managed to get the thing working. I got on bike and started cycling home but very shortly discovered that even though the actual breaks handle works fine now, the stopping doesn't happen when I press it. In other words, the breaks do get pressed, they come in contact with the disk (to some extent at least) but the stopping is so sloooooooooooow. When I am rolling down the hill and press breaks, instead of the wheel immediately stopping (as it used to work and as front breaks still work), the bike starts to gradually slow down and after I don't know how many seconds comes to a stop.
So when the mechanic was doing the bleeding, I though I had seen him spill some of the oil on the disk and when I touched the disk it was a bit oily. I returned to the shop and described the situation. He took out the wheel, very thoroughly cleaned the disk with soap and then with sand paper, then disasembled the breaks and cleaned break pads, then put everything together, tested it and said that everything works as it should. Well, I am not sure if the situation improved but even if it did, it was by a very slight margin. And basically I am out of ideas where the problem might be. I can try to return to the shop and describe the problem again however I've been thinking that may be it's something I could troubleshoot with your help
One other thing I've noticed when cycling recently is that when breaks are pressed really hard, the disk starts making that noice that is usually expected in humid weather. Except that now it makes it even in dry weather and when it is wet outside, the noise gets extremely loud. I initially didn't pay attention to that thinking that the thing might be slightly wet after cleaning however this noise didn't go away over time.
Do you have any idea of what it might be or what I should test to get more insight into the problem?