I've had a heater core leak for a couple of months, it never lowered the coolant but it did stink the cab when the heat was turned on, so I finally go around to replacing it. after getting the heater box out I decided to open it up and check the evaporator core, as they are known to plug up with dust/feathers etc. The symptom usually is that there is not alot of air volume from the vents, now that its been cleaned I have x2 more air flow on low than I used to on High!!
I also replaced the R134 with 12a (propane based refergerant "enviro friendly") now I have 0*C /32*F at the vents cool huh!

also note that the entire dash does not have to be removed to get the heater box out, you can just see the strap I used to hold it up and then slid the box out along the floor, its a tight fit but works. now that I've done it I realize I removed far too many things the first time also note there is a heater box bolt hidden behind the air dryer plate which is behind the ecm. in reality there are only a dozen or so screwa/bolts that must be removed to lift the dash enough to get the box out.
the evap can be cleaned without removing the heaterbox if you remove the fan motor, you can see/clean the evap core with brake clean. it will just stink for a bit till it dries off