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You will find a little information at the JD-H restoration site (below) by scrolling all the way down, left side to 39-H. The chord length of the brake facing changed about 3/4 way through the 39 production run, and there is some guidance on handling that one if your tractor came along earlier than around H-7800. You are faced with removal of the rear axle assembly, and breaking it down quite a bit. This is also a great time to renew the rear main oil seals in the main case -- very often the root cause of JD-H brake failure if the failure is by reason of oil saturation. And these seals fail because owners fail to maintain a correct adjustment (bearing loading) of the rear axle assemblies. Many on this board will advise you to do one side at a time in an attempt to avoid opening up the rear end -- i.e., removal of the rear differential cover. And just as many will tell you to bite the bullet, remove the rear cover & go after both axles at the same time. I think this is the way to go. A note of caution here! Be certain to secure the tractor so that it don't fall over during this operation. Securing the tractor: One simple method involves a sturdy timber (say) 4x4 some four feet long, a pair of 6-1/2 or 7-inch long bolts 5/8" diameter and a pound (or more) of flat 5/8" washers. Drop the drawbar, bore a pair of holes in the 4x4 to match the two rear drawbar mounting holes, then bolt the 4x4 in place using the flat washers between 4x4 and main case. Then you have a three-point foot print once again to support the tractor while both axle assemblies are removed. The delicate part of this operation comes with reinstallation because during reinstallation you should also perform bearing load adjustment to prevent a flopping axle from wrecking the new seals and allowing oil to once again pour into the brake region! Much is to be considered here. You should definitely be armed with basic documentation for your tractor. PC304 Parts Catalog is a basic MUST HAVE book, available from Deere, Deere Pubs, and even on eBay. You also should have a service guide or manual. And some sort of an operators manual, present day or early vintage, proves very useful as well. At the site noted below, some literature can be found for sale that would be helpful. Much is contained (in these documents) about the subject area you are reaching into. But that choice, of course, is yours!
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