Having just fitted a DLS brake to my '74 CR125, I can tell you that making sure the backing plate/shoes fit the drum (I'd recommend following redrocket190's advice) will likely be just one of your concerns. The backing plate slot has to engage the stub on the left fork slider, the shoes must ride roughly centered in the drum, the axles have to be the same diameter, the combined width of the hub and backing plate must be less than the width between the fork slider axle bosses, the brake cable adjuster should roughly line up with the fork tube to allow a smooth cable run, the linkage must clear the fork slider, etc. etc. The easiest way (provided the steering stem cooperates) is to swap out the entire front end - forks, wheel/brake, and triple clamps - as I'm doing on my RM. I realize it's not cheap.
If your '79 CR125R drum is 130mm (every Japanese drum I've seen has this dimension cast into the interior of the hub), the early-80's YZ/IT DLS brake setup I used may be an easier retrofit for you. It has the advantage of a forward linkage that clears offset fork sliders, and was made with two different positions of engagement-stub slot.
My DLS brake is incredible - I'm actually a little worried about bending the fork tubes.
It doesn't have the ultimate power or fade performance of my YZ's disc, but it initially comes on stronger, believe it or not. A novice rider would definitely think the DLS was a more powerful brake.Even if some machine shop time is needed, in my opinion the end result will be worth it.
Ray
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