Sac001,
I'm not sure of what they are looking for in the tech section. I can comment on the heater valve here and if someone wants to copy and paste it, add to it, correct my information, etc, feel free to do so and post it.
All I can speak to is the 77, though I understand the set up is common to most if not all of the sharks. I am not 100% sure on this stuff as I have pieced it together from visual inspection of my system, examination of the AIM and from various, sometimes slightly contradictory posts here.
The basic concept is simple. Press the temperature slide control all the way to cold and vacuum is released which closes a water valve in the supply side line to the heater core line and prevents hot water from getting to the heater. There are suggestions that this only works when the a/c is on but I didn't see anything in my system that would prevent it from doing this anytime the slider is slid to the left.
The workings are a little more complicated. As I understand the thing, there is a vacuum valve on the top of the ventilation box that contains the heater core. This is up behind the lower passenger side dash pad inside the car. When operated mechanically by the spring loaded wire attached to the temperature slide releases vacuum it releases the vacuum that controls the valve in the heater line.
When you go all the way to cold with the temperature slide on the console, the valve, which appears to be part of the hardware for the door that controls the amount of air flow over the heater core, is activated when the spring loaded wire that snakes out from under the console and around the duct work moves the air distribution door to a position that bypasses the heater core. It appears to be a plunger type of valve that releases vacuum when depressed by the pressure on the wire from the temp slide at the same time that the temperature door prevents hot air from entering the ducting.
It is important to adjust the spring loaded wire using the turnbuckle on it to make sure that this vacuum valve is activated before the slide gets all the way to cold. If not properly adjusted, the slider will hit the left stop on the console before striking this vacuum valve and will not allow the valve to release vacuum.
The vacuum runs from an unknwon source through this valve and out a wire sized hard plastic vacuum line that was white originally (mine was yelllow now due to age). I'm not sure of this but I think it goes into a fitting at the firewall and comes out the other side as another small vacuum line. In any event, it comes out high on the firewall right behind the distributor and snakes its way along the firewall and insided right fender to about 2/3's of the way to the front of the car and then goes down and attachs to the vacuum valve in the supply side heater line. When activated by vacuum, this valve stops the flow of hot water to the heater core.
I replaced the deteriorated vacuum line with a bigger rubber vacuum line by cutting it back as far as I needed to, got the rubber line at a VW dealership, and slipping it over the end of the plastic line. It also slides over the fitting on the heater line valve without the little grommet that is already on ther. I had a new vacuum valve for the heater line and bench tested it before putting it in the car using the line from the car. I had six pounds of vacuum and when I bench tested the valve it appeared to activate the valve and would prevent water from going to the heater core.
I put the unit back in place in the heater line, took the car out for a spin and was disappointed to find out that I was still getting hot air. At this point I had had the dash apart for a month and was extremely frustrated.
I took the valve out of the heater line and went to Ace Hardware. The had a good selection of small ball valves, as I recall, that had threads on each end that would accept barbed ends to put a hose over and I got one for each side that matched my heater hose diameter, either 1/2 inch or 5/8's inch and put it in line with new clamps. No more problem with heat coming out of the system at any position on the temperature slide, though I can stop at any time and open the valve and get hot water to the heater core. (I have never had to do this as everyone knows that getting the cabing of a shark hot is not difficult.) The temperature of the air coming through the ductwork is likely 5 or so degrees higher than the temperature of the ambient air temperature and I attribute this to all of the other heat sources that the duct work is exposed to.
The only exception is when the car sits for a while off and the heat builds up in the ducts from, I suspect, contact with the firewall, floor and drive tunnel. Start the car, rurn the fan on high and it goes away in several minutes.
I only used one valve and elected to remove the vacuum controlled valve and replace it with the manual valve as it was not working and I wanted to reduce clutter. Others have simply added the manual valve in addition to the vaccum valve..
I don't know if two manual valves would do a better job or not, though it would be necessary if you have heater core problems.
I'll get the digitial camera out tomorrow and where visible take pcitures and post them on a web page referenced here for viewing.
I apologize for the rambling and typos but is getting past my bed time.
Bob
as I knock them out.
