How do i by pass the rear heater lines? id like to do this as close to the engine as possible

You may have a broken vacuum line that opens the heater case door or if it functions from a electronic blend door actuator it has failed or the spring/arm attached to it has broken this holds the door closed when the heater is not in use and hot air is entering through the heater ducts on the bottom by your feet and depending on what country your in € right hand /left hand drive) the heater case is on the passenger side , also you must check the engines running temperature because even if not overheating a hotter running engine from a cooling system getting clogged up somewhere will cause a higher passenger compartment temperature from the heater core boiling in the heater case check your heater hoses ,heater by pass valve if applies and any vacuum lines for breaks that would allow vacuum to escape and cause the valve or blend door to not stay closed

Bad heater control, plugged heater core between heater core and a/c condensor, broken vacuum line to control. Can you hear the gate open and close under the pass side dashboard.

There is a vacuum operated control valve in the heater core lines at right rear of engine-the passenger side of engine. If it is closed, no warm coolant will circulate through the heater core. The vacuum signal for it comes from the heater controls at the dash. If the valve is stuck in the closed position, it may need taken off and cleaned or replaced. If the vacuum signal is not working, you may have to pull out the heater control assembly and check that vacuum lines are still plugged into the back of it.

The core may be plugged, or it may have an air lock, or the core may be fine and the doors in the heater case may not be opening properly to pass heat.

With the engine at normal operating temperature, feel both the inlet and outlet core hoses at the firewall. If the coolant is circulating both hoses should be hot. If only one is hot, try cracking open the outlet line, with engine running, to get flow started from an air block. (The outlet line will run from the firewall to the front of engine near or at the water pump housing. The inlet for the heater comes off the top of engine from a head fitting and goes to the firewall.) If no flow results, the core may be plugged. To flush the core, take both hoses off at the firewall or off at where they fit on the engine, and put a garden hose into the outlet-flush it backwards-run until clear-let the water run to ground or use a catch can if able to. Some people use compressed air into the core instead of water flushing. Either way, you should see some sediment come out if it had any blockage. Once you get the heater core circulating, any further heater problems need to be addressed at the heater case. Make sure the temperature blend door can open and close-this door opens to let blower fan air pass across the heater core.
Good luck.

It is possible it is coming from the engine block drain that is not closed tightly or is faulty and leaking. These block drains are hard to spot so you will need a very strong light and an inspection mirror to help you locate it. If the block drain looks secure, look for other coolant system hoses/heater hoses and coolant bypass hoses near the rear of the engine to be leaking at pipe unions.

The plumbing to the rear heater is metal lines under the body and then connect with rubber to the core in the heater, with the age of the van the leak is most likey the metal lines corroded. If you don't need the rear heater operational, the lines can be by-passed in the front of the van [ it will not affect the cooling system].

just keep bleeding,or maybe your master is weak?id bleed on a few times see what happens start from the farthest tire from master so rear pass,rear driver,front pass,front driver.

Have you thought of sandpapering the lines clean, then using JB Weld?

I've done this many times
You will need a tubing cutter
Screwdriver
I think a 7 or 8mm socket
hose clamps
the appropriate amount of heater hose
zip ties

cut the steel heater line at the front in a place you can get access to,leave yourself enough steel hose on the rear to allow you to put the heater hose on cut off the steel hose and discard, and use 2 hose clamps on either end do the same for the other heater hose,this way is cheap and easy and permanant.Be sure that when you run the heater hose keep it away from hot or moving parts,use the zip ties to fasten it along the way,this should take about a half hour or so depending on your skills and working conditions,remember to buy some antifreeze to top up the system because you are going to spill some,remember to clean any spills animals love it and it is deadly.

It actually sounds to me like you're low on coolant. My Jeep leaks coolant and whenever it gets low the temp gauge will go real high and then drop, and the heater doesn't work well. This is because the cooling system of your car is supposed to be a closed system, full of coolant and no air. When coolant leaks out, the space it used to occupy is now occupied by air, which does not transfer heat well. When 'air' is passing through your cooling system, no heat can be transferred from your engine to the heater and radiator, resulting in a hot engine and no heat at the heater. Then when a pocket of water passes through the system, the temperature gauge quickly falls as the water absorbs the heat from the engine. The hot water that cools the engine is where the heater gets it's heat from as well, so when water passes through the heater core, the heater works, but when it's filled with air, it doesn't.

Hot water runs through the heater core regardless of whether or not your thermostat is open or closed. That's why your heater works in the winter even before the engine is at normal operating temperature. The fact that the heater stops working is a good sign that the thermostat is probably not the culprit.

As for the water pump. If the water pump was bad, your temp gauge would go into the red and stay there. Water pumps generally don't work intermittently. Either it's good and ot works, or it's bad and it doesn't.

However, if you are in fact low on water, as I suspect, it means you probably have a leak somewhere. The leak could be in the water pump housing gasket, so depending on where you take it for repairs, they may try to sell you a new water pump anyhow. So just beware of that.