1999 1500 5.2 L with glaval conversion. The rear heater is blowing cold air only. Under the hood there is a vacuum line going to some sort of plastic unit that is located at the hose that goes to the rear of the van. I feel that the hose is hot on the engine side of it and cold on the side that goes back toward the heater. I feel very little vacuum in the line. The same vacuum line also goes to some sort of vacuum unit connected to a lever inline to the front heater core. I am not sure what the function of these vacuum unts are. Please help. I know the line is clear going back to the rear heater core as I just changed the radiator and had the hose to the rear off so I could use air to blow the old coolant out.

 

The rear heater and front heater units use the vacuum to operate in-line water-valves. If there is any grim in the cooling loop, these valves will not operate right. Also, if the vacuum lines have a leak they will not operate well either.

For my rear unit, I have disconnected the vaccum line and twice a year adjust the vavles manually to flow the coolant. In the summer, the rear valve is off and in the winter, it's wide open all the time.

Of course, I have to remove the bench seat, but twice a year isn't too bad. The van does get clean well inside afterward too.

 

the one that is at the place in the hose directly before it goes back to the rear of the van is just a plastic unit with no lever on it. Can it be cleaned? I didn't see any type of way to adjust it. I will trace the vacuum tomorrow. I know the front heater works normally.

 

Since you blew out the rear hoses it's possible they are air bound and no coolant is circulating through them. Usually a quick trip around the neighborhood with the heat on will kick start the coolant circulation.

 

The line to the back could be a Tee or a Y connection in the cooling loop. I'm not certain what it really looks like under the hood section. I've only taken the rear unit apart and know what's in there.

 

Thanks everyone for the help.
I ran it around for around 15 miles last night and it still blew out cold. This morning I took the inline plastic peice that was at the beginning of the feed hose to the rear heating coil. It is a valve and is open without vacuum. I bypassed that with a connector and ran it to operating temp and the heat worked. I then put the valve back in. I still can't get either of the two valves to work. I turned on ac and turned everything off, turned on heat turned it off, but still nothing. I didn't check to see if the ac works in the back yet as it was very cold in my area and I probably would not be able to tell. At least I have some hot air in the back now

 

Did you check for vacuum at these water valves?
If there isn't any vacuum, any chance you knocked off or pinched the vacuum hose that feeds the heater controls?
Could the valve be stuck shut? When you hadthe valveoff, did you look through or run water through it?