The 20mm bolt is probably to long. Are you able to tighten down the original factory bolt? If you can then the thread is not messed up. If you can't you messed up the threads with that other mystery bolt you used. You have a couple options at this point.
1. If you are lucky, you can clean the threads out with the correct tap that matches the proper thread count and type. M8
2. If there is too much material gone because some other bolt was forced in there then you can try and run a tap through the hole to the proper depth using the next size up, still using the same thread count. That way you are just making it a little bigger.
3. You have to drill out the hole and put a Helicoil in it that is of the proper size and thread count as was in there. I am guessing it is M8 x 1.25 x (whatever depth the hole is). 1.25 is the thread count. You have to confirm that though.
4. If you have to drill out the hole because the threads are to far gone, you can for go the Helicoil and just use a tap to make it whatever size you want and then get the appropriate bolt to match.
Keep in mind. I am going off what you are telling me, so it is difficult to be certain what you have going on. From what you are describing it sounds like you have installed the improper size bolt and stripped the threads in the PS pump housing. That would explain why the bolts will not hold any longer.