I believe we have a good chance at success in step 1 (rubber diaphragm equivalence). You'd be extremely hard pressed to find anything better than the mcmaster carr epdm in regard to thickness and similar flex. This is comparing against my "good" diaphragm in the HAC chamber of the vacuum advance.
8610K61 Weather-Resistant EPDM Rubber Sheet, 12" x 12", 1/64" Thick
Step 2 will be lining everything up and using grey rtv to ensure no leaks with the diaphragms when sandwiched in the hemispheres.
Step 3 will look something like the following.... I had some leftover 2.5" ss muffler pieces which turns out to be almost a perfect fit
for inserting where the crimp previously existed. I'm not going to re-crimp these if I can help it because I don't want to beat on it with a hammer anymore and if this doesn't quite work as desired...or needs to be amended in the future...I can take it apart. You can kinda see where I'm headed with this by looking at these two pieces. I'll use the outside edge of these cut pieces to clamp with some L hook machine screws and a piece of flat bar. Cinch (clamp) everything up using these new edges and test the vacuum.
The refurb of the vacuum advance is complete and its been iteratively undone and re-done about 4 times given mistakes and inadequacies. Having the ability to separate it fairly easily instead of re-crimping as original equipment turns out to be a really good thing.
When you have this correct a few things will occur.
Both ports will start to move the advance pin around 5 in. Hg and will complete full movement at that port around 10 in. Hg.
Both ports will hold the vacuum without dropping
Experimentation shows that you should stick with the Ultra Grey RTV for all the part assembly. I had tried cyanocrylate (super-glue) for the hub portion of the diaphragm with ultra grey elsewhere. While it initially showed good results, after a week the diaphragm where I'd used super-glue started to no longer hold vacuum solidly. I think the super-glue just becomes hard and no longer pliable enough for the vacuum.
I had initially lauded the mcmaster-carr epdm material. However once I received the diaphragm material from the dental supply house, I decided it was an even better match for the original equipment material. The original equipment diaphragm material is a type of cloth with a rubberized coating. As such it really doesn't stretch much. The epdm material is quite good but stretches a good bit more than the original diaphragm. Comparing original to the new diaphragm material in regard to both stretch, thickness, and flexibility shows the dental equipment diaphragm to be more appropriate. Obviously I cannot test years of duty cycle for either material but one would assume something specifically designed for diaphragms should hold up well. Thus I am recommending this for diaphragm material.
DCI Equipment (from american dental associates): Dimensions: 11" X 8.5" X .013" thick sheet
I'm not sure the plasti-coat really does much and I have suspicions over many duty-cycles it may not hold up. My HAC port diaphragm was in good shape but I still gave it two shots of plasti-coat spray. As previously mentioned, I had to take the advance apart multiple times and each time the plasti-dip material seemed to have rubbed off some. I suppose it doesn't hurt to use but if you can completely replace (or amend) your existing diaphragm with new material I think it would be better. Obviously I've mentioned that super-glue doesn't seem to really work great in the vacuum so exactly how someone would partially amend the HAC diaphram with new material is an experiment for some future poor soul. (look back in this thread and you will find that I've pointed out completely replacing the HAC diaphragm is impeded by how it's hub is pressed on instead of screwed on)
Other tips:
Assembling a new diaphragm and putting ultra grey between the hubs and letting it dry should be step 1
Assembling the hemisphere closest to the dizzy (advance pin side) first is easier than assembling the HAC side hemisphere first
Be careful of lining up the clasp on the advance pin with the inside of the dizzy connection during assembly of the hemisphere in step 2
On the 1979 dizzy the vacuum advance sits very close to the dizzy cap and the metal edged top of the dizzy housing. If you are adding a metal ring here instead of re-crimping like original then you may have to notch the metal ring slightly.
Always use ultra grey RTV on the metal assembly portions on each side of the diaphragm. When the diaphragm is secure in the vacuum advance it should be sandwiched tightly between the metal portions of its respective hemisphere with good vacuum supporting vulcanizing silicone on both sides.
On the 1979 dizzy the vacuum advance also sits very close to the oil filter. Watch the distance if you are adding something to support a non re-crimped assembly.
It may be productive to support the assembly with some type of clamping system as well as the RTV + metal rings like I've done it. Again, I can't test this with years of duty cycles. The RTV that I've used seems to give me enough difficulty in disassembly (after a few days) and given the small amount of movement in the chambers and low vacuum, I believe it should hold up for quite some time. If you don't re-crimp then you can test it and amend it in the future if things don't play out well.
Also also. If someone attempts this and finds better ways of accomplishing this task....please just use this thread (or point to your work in this thread) if possible. I'd like to know better ways of doing this in the future.
As another aside, I tried ordering different varieties of vacuum advance from both toyota dealers and yoshi parts. Pretty much everything I attempted to order as "possibly still available" is actually not available. Obviously all the other potential vacuum advances are listed directly as "no longer available". I can't find anything that is not "dead"
19220-61102 - no longer available
19220-61011 - no longer available
I also attempted to check with other vendors here such as @4Cruisers. While I appreciate everyone pointing out solutions they achieved a few years ago....all I get out of it is "envy of you being ahead of me". I don't see any supply for vacuum advances. I don't see much sustainable in regard to replacement parts for toyota dizzy's in general. I know the world market dizzy in the 1977/1978 vintage is available but even the vacuum advance for that dizzy seems to be non-existent as a separate unit. I just find a lot of dead ends in this area.....perhaps there are bright spots i'm just too dim to see. I see a DUI in my future if this goes kaput on me.
