Just got this machine used with 20,000 pages. Worked great for 150 pages. Now the black is almost all gone. So, I primed the developer unit manually. Black came back almost perfect. Now the density senor read below normal before I primed it (-4.35) now that I have filled it up it reads (1.9). My question is why doesnt the machine replenish the toner, when it clearly knew it was below spec? And why doesn't it error out and say add toner?