My Max-D 5000 (the broken one) has some problem with the firing mechanism. Whenever I shoot, it shhots a small stream, and i can take my finger off the trigger and it still fires
If I pump like crazy after pulling the trigger I get the full Max-D spray
Is there any way that I can possibly repair my Max-D?

There are is a very simple repair for this under different names. Just a few are "Silent max-d trigger mod", "Max-D trigger fix", "rubberband repair", and more. The problem in your gun is likely to be that the spring that closes the valve is worn out (extremely common). All you need to do is open the gun up and adding rubber bands in addition to the springs to get it to close. There are a few guides on various forums, just look around if you need more help.

I finished that, and I CVF modded my Max-D, making it a Max-P. When i gave it 22 pumps, like it said in the article on this website, it did not fire. When I opened it up, it exploded! Part of the case broke, but the rest of the innards are OK. I am currently thinking of using duct tape to repair that little crack or 2 in the case. Would that work??

All guns that are pressurized have theri pc(s) fly off when you open them (except for some WWs). Was it totally filled when you began pumping, because that might explain why the pressure got so high. Also where is the crack in the gun? Is it on casing (because duct tape would work then), resiviour, specifics would help.

Hmmm.....you might want to just remove the old trigger valve if it doesn't want to open when you apply too many pumps. Try a stronger pvc ball valve, or something of the like. That will also give you the option of multiple nozzles.

to answer your question, it is on the case, and the gun was like 1/3 full (i realised after it exploded that I forgot to secure the pc to the rest of the guns)
please note: I tried to do this mod off of sscentral forums, jsut said some minor anesthetic mods or something and said to cvf the gun.
see this link to the page in sscentral forums