My coupled bike is a 1984 Schwinn Letour Luxe, because that was the bike that was still in the states when I ran out of places to leave a bike. I'm trying to work it into the rotation, so it's ready to go when I am, and also because it's a nice ride.
On the rear is a Día Compe long reach centerpull caliper, NOS. The levers are Shimano 600 EX? The rim is Alex ACE19 700C. I initially put Shimano pads on it and had zero braking power. Changed them out with KoolStop Salmon MTB pads, with similar results. Is it possible that the levers and the caliper are incompatible? Brake pads? Other?

About your initial question- yes, Shimano SLR calipers don't play well with non-SLR levers- but it's more that the lever doesn't pull the cable back to start, so you have to flick the lever forward after you brake.

About this- My guess is that's a REALLY long reach on a center pull caliper-

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in photo of rear caliper pads appear to be out of adjustment with rear of pad touching rim first rather than front of pad touching rim first


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I have to admit that contemplating mechanical advantage and straddle cable length always make my brain hurt so here is some advice from Sheldon. I will add that I've always found a cable adjuster somewhere along the track is essential to get the best out of a set of anti brakes.

https://www.sheldonbrown.com/canti-trad.html

https://www.sheldonbrown.com/cantilever-geometry.html

I don't see any SLR (Shimano Leaner Response) calipers or levers so I don't think that is a concern

I think the bike problem is the adjustment of the pads themselves. They need to be adjusted so the front of the pad contacts the rim first by a few micromillimeters and motion of the rim pulls the pad flush to the rim

I'd also consider swapping the hardware around and try and get that fat washer on the inside of the arm, you might be trying to close the brake past it's optimal leverage point

Agree. Would a shorter straddle in addition to the suggestions above give a quicker response?

It might but I haven’t dealt with center pulls in years so I couldn’t tell if that is the correct one for caliper or not

The straddle length is fixed on those Weinmann/DiaCompe 610/750 brakes. I’d try different levers, preferable aero.

Thanks all, especially for pointing out online resources. That aspect doesn't quickly occur to me, I don't know why.
This bike normally gets used 2-3 weeks per year and spends the rest of the time languishing in its case. So whatever adjustments I made for its previous trip are long forgotten when the next trip presents itself. The obvious solution is just to keep in the rotation.

Checking in. I went to the Park Tool video and followed it. And tried every set of pads I could put my hands on. The only ones that showed any improvement are those same KoolStop. I agree they are not the best choice, but also I realize that I simply don't have access to vintage bike parts. The other model suggested is certainly the better option, but I can't lay hands on them. As I was getting ready to move on to something else, I remembered another stash of brake pads, including another set of KoolStops and a couple brand new KS inserts, and will fiddle with them this afternoon or, more likely, tomorrow morning.
I also have the option of drilling the brake bridge, in order to install the Tektro long reach sidepulls I have. But I have not yet made the decision to do that. This bike functioned perfectly in August of 2021, so it's just a matter of figuring out how it was set up then. Memory is a terrible thing to lose.
Edit: I have long been under the impression that the toe in was to be corrected in response to noise, and the Park Tool video , in part, confirmed that. Nonetheless, the adjustment has been made and it's working better. I have hope for the next round of pads.

Did you sand down the pad surfaces? that is a rear brake shown so I wouldn't judge performance too harshly.
I have found that adjusting the pads on center-pulls as close to the rim as possible provides the best performance. Keep in mind that CPs are dual pivot brakes.

As mentioned, cable housing and maybe ferrule choice can impact performance by allowing too much elasticity in the system. Not to mention routing.

^^^^Good points. I did sand the pads. I'm using Sunrace housing. I have a couple extra pairs of ergo levers too, and may give them a try.

just for sharts and fartles, try changing out the levers. I'm wondering if it's not a mechanical advantage thing with the length of those arms and the limited pull of the 600 lever.

this is a 700x38 tire, for size perspective. the brakes are phenomenal with the levers i used

Pull repeatedly on the levers and look for movement in cable housing, hanger, etc. It all adds up and is the bane of centerpulls, which already have plenty of flex in the caliper itself. Sharp bends in the housing introduce friction that also saps applied force. Better cable inner wires are pulled through a die to flatten the outside surface and reduce friction. The biggest improvement, though, will be in switching to aero brake levers. Providing you don't have so much flex that you run out of cable pull, their increased mechanical advantage will make all the difference. If you still require more stopping power, a suitable length dual-pivot caliper is probably your best bet. I've seen some rims on which almost nothing seems to work well. I always figured that there was something about the type of aluminum, just as there are some rims that are much more likely to give up aluminum that embeds into the pads, the Weinmann concave being the first that comes to mind. There can also be a huge difference in the friction provided by new pads and that of old, dried up pads that otherwise look fine.

There is another good point. Really clean your brake surface. I found that to be a huge difference on some rims.

Just as a side note, I found that those levers did not work well with my Mafac centerpulls, regardless of brake pad.

I cleaned the brake surface, and have used the same wheel on other bikes. My first guess was that the arms are sooo long. Ergos it is then. I have had no knowledge, over the years, about length of pull of different levers. That kind of info makes my head spin. There are folks here, who specialize in such, so I just asked.

Have you seen this?

https://www.chapmancycles.com/cantilevers/

The accompanying essay makes my brain hurt a bit, but I find the interactive tool to be very illuminating. For center pull brakes, if I understand what it's telling me, the only thing you really have any control over is the length of the straddle cable/height of the yoke, and once you get beyond a certain point, which I think most centerpull straddle cables exceed, even that isn't a factor.

You are correct. It is a function of the angle between the two sides of the straddle cable. When the two sides of the straddle cable are at 90 degrees to each other, the force on each arm is equal to the force applied at the brake lever. If you have a shorter straddle cable, the force is multiplied and higher than the force from the brake lever. The closer the straddle cable is to being a straight line, the greater the force is multiplied.

I'd also consider using compressionless cable housing, which should also offer a dramatic improvement.

Definitely swapping the Kool Stop pad spacers using the thicker pad spacer on the inside of the caliper will improve braking as Bianchigirl suggested. As the the slotted part of the caliper goes beyond parallel with the rim you are losing leverage.

Brake levers and pads changed. Test ride tomorrow morning.

Ergo levers and more appropriate pads----though still not the perfect pads.
Fail. I swear I can feel those long arms flexing. I might just have to put a 27 incher on back there, but then it might not fit in the case. The good news is that this bike's traveling days might just be over. Air travel anyway. I've set up a loaner for my trip in June.
When I had the couplers added, it was the right thing to do. It has a lot of trips under its belt. But now I ******g hate air travel, with or without luggage, though so much better without. Internationally anyway. When entering the USA, one must go through customs twice. Once in order to get to baggage claim, and again with said baggage. At least that's how it was my last couple trips. And, when coming into the USA, every non-resident is suspect. My Canadian friends won't even change planes there anymore. This is hopefully my last trip north.

Rant off
And get off my lawn!