1991 Dodge Dakota no cool

hello,, me again---another problem---the air has always worked great in my truck till just recently---it started blowing just cool air---then its got to blowing hot air in the a/c mode..--i felt of the heater hoses and they were both very hot-- whitch my weak mind tells me that the heater valve is stuck in the open position---or --no vacum to close it---in either case--it looks like a nightmare to get to under the dash..--my question is--can i bypass the old valve and install a new one on the firewall---then put a T in the vacum line that runs the old valve and hook to the new one. heater hoses are easy to get to. if this is feesable --should i use the heater hose comming from the water pump?.. thanks---don-- PS gauge shows a/c to be fully charged..