i have a 71 ss454 convertible that has air conditioning.. i have gone completely through the whole under dash and cowl area, redoing everything acording to the asemble manual.. they show 2 different set ups when it comes too the under hood hose routing, 1 has 2 vacuum lines coming out of the fire wall 1 goes too the main vacuum supply, the other goes too the heater bypass valve.. well my car only has 1 hose coming out of the fire wall, im asuming it is for the main vacuum supply? where is the line for the bypass valve ? did some cars come with and with out this valve? i have already bought a nos valve and am ready too asemble my intake with it...let me know who can help me please

20round, this past summer I did the same thing you have done to your A/C system. I have a '71 El Camino with a 350 and factory A/C. My car only has the single vacuum hose firewall grommet which, as you noted, should be routed to manifold vacuum. That would suggest that there is no factory heater bypass valve. I was also a little confused by the factory assembly manual which appears to be a little vague as to whether there is supposed to be a bypass in the heater hose.
Regardless of what the factory used I wanted to use a bypass valve to reduce the coolant flow through the heater core when the A/C was on. This greatly improves the cooling capability of the A/C when it's 100+ here in the summer. I switched to R-134A refrigerant, a '91 C-K pickup single serpentine belt front accessory drive with a cycling clutch R4 compressor A/C system, and a CS-130 series 105 amp alternator. The higher alternator output at idle really helps the blower motor air output stability and the reliability of the A/C compressor engagement. The original POAST valve and expansion valve are difficult to service now and almost impossible to find new. The cycling clutch and orifice tube make it much easier to control the A/C system pressures and thus, A/C system performance.
I added a generic in-line apply-vacuum-to-close heater control valve to the hose going into the heater core. It's almost hidden next to the hood hinge and gets the control valve out of the intake manifold where they often corrode themselves permanantly into place. For vacuum control I T'eed into the cowl vacuum pod hose that sits in the cowl opening on the passenger side. This ensures that the heater control valve gets vacuum and closes only when the A/C system is in the MAX, NORMAL, or BI-LEVEL positions.
I'm really happy with this set-up. It has performed flawlessly since it was finished in August. It only costs about 25 bucks to fill the system with R-134A and it blows 49 degree air out of the ducting when it's 106 outside sitting at a stoplight.
It's not an exact correct factory spec resto, but then, it started life as a plain-jane El Camino and I'm more concerned with function than originality.

Hope this may help,

Louie

If I remember right there were two ways of getting A/C on these cars, from the factory or as a dealer-installed add-on. Is it possible that factory air had the bypass while dealer-installed did not (or vice-versa)?



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Cecil Hawkins
San Diego, CA
1971 GMC Sprint

Before (http://www.chevelles.com/showroom/Cecil/FrontDriverSideView.jpg)
After (http://www.chevelles.com/showroom/Cecil/NewBlueFrontDriver.JPG)

Some time ago, it was stated here (by Fred Aldrich?) that the heater valve went out of use in mid 1971. That would tend to make things a bit confusing for you. If you have the single vacuum hose, it probably hooks up just like the '72 models. If you want to add the valve, then add the missing hose.

i went though this last year i could not find the hose.the book said it had an orange strip and came out of the fire wall around or near the heater hose fittings still couldnt find it and mine had the vavle from new took it years ago and now cant find the hose.the only time the vavle works is on max a/c it dosnt not work under normal or bi level.a/c work good with out it {A134}even on max


chris