Is it leaking in the core, or at the heater valve on the firewall?
Either way, here is what I did.
My 85 -- years ago, blew the valve portion. I realized you had to replace the whole thing because of that...terrible design.
I found a replacement valve at the auto parts store that fits a front wheel drive 2.2 mid/late 80's chrysler product. I cut the valve off the line, and put the plastic replacment valve in line..and hooked the vacuum back up. Fixed for $10 and an hour or two messing around.
Now....consider this. If your core is ruptured, go ahead and yank it out, cut the valve, and save time removing it that way.
Have your core repaired at a local radiator shop. It might cost $25 or so. Get the valve from the store, and reassemble. Wrap the whole job up for under $50 and two weekends (one to tear it down and get the core out...over the week to get it repaired, and the next Saturday to slap it back together).
I did a core replacment in my 90 Vette. Let me promise you that the wrap around dash is 1,000 times more difficult to mess with than the 85.