Hose coupler leaking

[waytoobusy]

Good morning. I've looked around here and learned alot in the last year.
I have hydraulic hoses routed from my rear remotes to the loader on my tractor for using a grapple. The initial set of hoses I started with was too short to route properly under the tractor. I purchased a second set of hoses, then coupled the older hoses to the new with 3/8" female to female couplers. One set of hoses works fine. The other set (return line) leaks under pressure.
I changed the coupler, however it is still leaking under pressure.
Threads on the hoses look okay. The hose ends are 3/8" pipe and the coupler is 3/8" pipe.
Could I just have two bad couplers? Is there something I should be putting on the connections? From what I've read pipe tape should not be used in hydraulic systems.
Thanks in advance for any help.

 

[J_J]

If the threads are NPT, you need some kind of sealant.

Pope dope, Loctite 542. Some people still use Teflon tape if done carefully.

Pipe threads are designed to be used only a few times, then the threads are too deformed for a good seal.