It seems like a few MM/D50 owners are interested in finding an easy solution for bigger front disc brakes, and from what I gather no one has found one yet. I found one.
I spent a good part of the day at the J/Y looking for options for bigger front brakes for my 87 D50 4x4. I found a great fit with parts from a Montero Sport. It comes stock with dual piston calipers, and 12-1/4" ventilated rotors. I had a 4x4 D50 and Montero Sport right next to each other, making it very easy to compare/test fit parts. The good news is it works very well with only 2 issues, both solved in an hour with minimal use of a grinder. The other solution is to switch to 16" wheels, which is expensive and requires larger tires, too.
The hub and spindle are the same dimensions as my MM, even down to the bearings. I don't know if the Sport shares a lot of other components with the trucks, would be interesting to find out, but these parts work.
These are the parts I took from the Montero Sport-
3. caliper bracket
4. rubber brake line from caliper to union with metal line
5. dust shield and bolts
I'm doing this swap with my original, stock master cylinder, but if it doesn't work well I'll swap in the Sport master cylinder also. I think dual piston calipers have a greater volume of brake fluid, so it may be necessary to swap master cylinders, as I'll find out.
The first issue is the dust shield, and there are 2 solutions-the four bolts that hold it to the knuckle are indexed, or rotated, about 3/4" compared to the Sport, and it interferes with the lower caliper bracket, so it either needs to be cut with tin snips or the four holes redrilled to line it up correctly. Easy fix. The second issue is more time consuming-with the stock 15" wheels the caliper body makes contact with the inside of the wheel, preventing it from seating flush with the hub. You can either switch to 16" wheels, or do some grinding, as I did. I have pictures to illustrate, but it is a minimal amount of grinding, and it all fits great now.
Anytime you alter or modify brake components it should be done with care and common sense-mucking about with brakes can cause injury or death. So, with that in mind, let me hasten to state that the grinding involved is minimal, and no where near the piston bores or other sensitive areas. I took off less than 1/8" on the outer corners of the bracket and caliper itself, and am confident nothing has been compromised or made unsafe-the calipers bodies are pretty stout, as you can see in the accompanying photos.
So, with that said, this looks like a practical, affordable solution to weak stock brakes, made worse by installing larger tires.
Here are the photos-