From one '70 owner to another, welcome!

You'll need to do some Dick Tracey work and investigate how your vacuum lines are run and how they are connected. Hard to tell but could be "bubba'd", leaking actuators, cracked vacuum lines, etc. See the diagrams below for how the lines are routed:





Google "vacuum repair":

http://www.superchevy.com/how-to/vem...systems-guide/

And some how to video's:


Good luck!

 

Those diagrams may look intimidating but the vacuum system is actually very simple. The hoses MUST though be connected EXACTLY RIGHT or you'll get goofy or no operation. Even the override switches (under the steering wheel) must be connected properly as while they are synonymous to a simple electrical on/off switches it will only work properly if the vacuum source is connected to the correct side.

Most of the components are simple and extremely long-lasting. Hoses don't last as long. Hoses may test perfect on the bench but leak when installed. Why? Because the ends stretch out slightly over the decades and no longer seal well. If there's enough extra hose this problem can often be solved by simply cutting an inch or so off the end. Clamps are only used on the vacuum motors. Hoses from the local parts store will NOT work. Get ones specifically for the system from the vendors.

The first tool a new C3 owner should buy is a hand vacuum pump like the Mitee Vac! They are cheap and all auto stores have them. Without a vacuum source and gauge (the mitee vac has a built-in gauge) you're working blind. A vacuum pump like used with HVAC is not a particularly good substitute they are too powerful and can actually damage components.

Once you get everything hooked up properly its a good idea to spend some time checking for leaks. Again, the mitee-vac makes this easy. You can test the entire system or individual components. Both the system and individual components should hold vacuum indefinitely (think overnight).

Electricity is only involved in one component in your '70. It's a solenoid operated vacuum switch identified as "A" in the Willcox illustration. It gets power from the wiper switch. Fortunately it's long lasting but unfortunately it's buried behind the dash. All of the rest of the components are purely vacuum devices and are reasonably easy to access, test, remove and replace. The headlamp switch has a vacuum switch attached to it that does not use electricity in any way.

The three large connections on the relays (part "H") are very close together and the center one is intentionally oval shaped. The center hose effective locks in the other two. This is one of the reasons that you must get the exact right hoses as the outside diameter matters as much as the inside. When reconnecting install the top and bottom hoses first, then push on the center connection.

As I said, the components are generally very long lasting. The most common leak is in the two-way vacuum motors used to open and close the door. I say two-way because there is a rubber diaphragm in the middle with a vacuum connection on either side of the diaphragm. Vacuum both moves the operating shaft forwards and backwards--there is no spring return or similar. The seal around the shaft is the most common source of leaks and is, fortunately, easy to replace with kits from the vendors.

While all replacement parts are available the vacuum motors and relays in particular are of highly questionable quality and made in China. In my experience it is common for these to arrive "new" with SERIOUS leaks. ALWAYS check them with a vacuum pump before installing! They also tend to develop leaks VERY quickly--think days or weeks. Since the two-way vacuum motors are constantly exposed to vacuum even the tiniest flaw in the crimping that seals the two halves together with the diaphragm between will rapidly show itself.

 

Just my IMHO, but if they both open with the headlamp switch then it seems as if someone has T'd off the vacuum hoses and bypassed the over-rides... I'd take a hard look at how the hoses are in the car because the only way to make them both open up at the same time is to block the small port vacuum in the system. Knowing this... this would be the small hoses in my schematic shown above. This would include J, B and E. If you follow J to the tack solenoid you'll see it splits off before heading to the headlamp switch. This solenoid has to have vacuum on it to hold the wiper door closed, when powered up from the wiper switch it blocks the vacuum and causes the system to function properly (raise's the wiper door).

After the hose Tee's off it also goes to the headlamp switch which again, is open until you pull the switch. Once you pull the switch it blocks vacuum to the headlamp relay/relay's (depending on the year car) and the headlamps open. But someone has tied the two systems together.

What does the car do when you turn on just the wipers?

The person that did this may have bypassed the tach solenoid as well... You'll just have to follow the hoses and see what is wrong.

Willcox

Website https://willcoxcorvette.com/
Tech Page http://repairs.willcoxcorvette.com/

Please bump our sale threads if you get a chance..

 

FYI..." H" in the upper diagram is in the wrong location in the lower diagram. For 70, the "H" relay is mounted above the brake booster/master cylinder

 

FWIW when working on the vacuum system I found it much easier to block off the wiper system at the tees. This can take a big problem and turn it into a couple smaller jobs. I always fix the headlight system first and get it working correctly and then add in the wiper system and then get it working. YMMV

 

I noticed that too... that lower diagram looks like one from a 1969 instead of a 1970 but the hoses still went to the same location and same ports, they just moved the relay.

Willcox