My EGR vacuum amplifier doesn't hold vacuum, so I ordered a "new" replacement from eBay, $16 including shipping. Price too good to be true? Yeah, this one doesn't hold vacuum either. The exact symptoms (same for original and "new" amplifier):

1. manifold vacuum input to amplifier won't hold vacuum that is manually applied
2. venturi input to amplifier also won't hold vacuum that is manually applied
3. amplifier never sends signal to open EGR valve
4. manual vacuum applied directly to EGR valve opens the valve (so the valve itself works)
5. manual vacuum applied to the line going to the coolant control sensor (on radiator) opens the valve (so the thermostat control works)

Another symptom I just discovered:

6. there's no vacuum signal from the venturi port on the carburetor, regardless of RPM. I don't know what it should be at idle or above idle, but it should be above 0 at higher RPM, right?

1-5 I attribute to two bad vacuum amplifiers. 6 is more of a concern unless I'm missing something.

Ideas?

Manifold vacuum input to amplifier won't hold vacuum. So hidden vacuum leak into number 6 manifold runner. Thats good diagnostic work. The egr Venturi vacuum port restriction in carburetor is quite small .031 in the one measured. Reverse Flushing, vacuum tube over red straw and Brass port, well either clean them out or verify that they are indeed blocked. The tube can be removed they are threaded ,place a small drill bit 5/64 or number 47 bit .078, into tube to keep small pliers from crushing tube. Screws out really easy.

Green dot is location of restriction.

I did a bit more troubleshooting this morning. It would have been easier with 2 vacuum pumps, but I only had one plus a separate gauge. I connected the gauge to the amplifier's output port (that sends full-strength vacuum to the EGR valve via the thermostat) to indicate when the valve-open signal is actually being sent. I used my hand pump to apply vacuum to the amplifier's control input (that is normally connected to the carb venturi), then I pinched that off with vise grips so I could connect the hand pump to the amplifier's manifold vacuum input. With the residual vacuum still in the venturi line, applying vacuum to the manifold line did in fact send the correct output signal (matching the manifold input).

I repeated that with various venturi inputs and found that as low as 5 inches it still triggered the signal. However, with zero venturi input, there's no signal. So the vacuum amplifier works as it should. Next step will be to troubleshoot the carburetor, which I believe is not sending the correct (or any) venturi signal.

So I can use that new replacement vacuum amplifier as an emergency hockey puck as Dan suggested. Around here lacrosse is bigger than hockey but you never know when a pickup game might spontaneously erupt.