@Bruh44
There is a great variety of threads used in hydraulic systems.
Japan uses metric types that usually have a JIS (Japanese Industrial Standard) but they also use a style that is from other countries like the BSP (British).
America uses JIC (Joint Industry Council) and SAE (Society Automotive Engineers) and also NPT (National Pipe Thread). And there are others.
The NPT and BSP both have straight thread and tapered thread versions to make it even more difficult.
The JIC is used to seal hydraulic hoses in the USA since they are easy to get swivel fittings for so the hoses don’t have to be rotated to tighten up the thread. This system is also confusing since there are 37 or 45 degree angles on the ends that seal in different system.
The naked eye is not a reliable way to see what systems are involved. Knowing the tractor was made in Japan but the loader may have been made in the USA also does not make it any easier.
The only reliable method is to get a hydraulic thread measuring system and then learn to use it. Like this
From hydraulic system adjustment and maintenance to making factory-quality assemblies on the field, Gates manufactures hydraulic equipment and hose crimpers to improve your operational capacity and system service life with ease. Cut through tough hoses with portable Power Cutter and Gates hose...
www.gates.com
Or you can make educated guesses using the diameters only and fittings that you know were in the female thread before.
In my limited experience too many of the sizes are too close together and the threads on various fittings will start into the female thread and seem to be working but do NOT really seal. For instance if you get straight NPT fitting it will screw into a tapered NPT hole but will never seal at hydraulic pressure. NPT is used in many places but really needs to be avoided as much as possible. Most common couplers unfurl come with NPT tapered threads though.
I suggest that you probably have some fitting with NPT threaded into a hole with a different type of thread and it will never stop leaking. It may have been delivered to you with the wrong fitting installed.
I also suggest that you know what threads are in what part before you install anything again. You may be able to take the hoses and the couplers to a hydraulic supply shop and have them identify those threads. They can then sell you the proper fitting and seal for the thread if needed. They may be able to assemble and test it for leaks.
Alteratively get the thread gauge and identify the fittings that leak and get new parts which match.
That is a much longer description of what
@TheOldHokie said already.
I personally just installed several hydraulic hoses and fittings (some were tapered NPT) and none are leaking. I knew what the threads were in each part and got adapters as needed. Nothing had to be tightened up exceptionally tight. I did use Teflon tape (higher quality brand) and snugged the 1/4 NPT up pretty well using normal hand tool wrenches.
Sort of interesting while trying to positively identify the thread into the coupler I was pretty sure it was 1/4 inch NPT so used a street elbow to check. That elbow would start but not make a turn or two before it was too tight. That made me think it was NOT what I had assumed. I then got a better quality brass fitting and it screwed in perfectly with 1/4 inch NPT thread. The street elbow had damaged threads and was trying to steer me wrong but the measurements on the coupler threads were still telling me that it was 1/4 NPT and a better fitting proved it.
Good luck on this!