Old hands know this, but I am often asked why Teflon tape should NOT be used on tapered-thread hydraulic adaptors. Here, one of our members explains why:
"I had a big handful of trouble on a ship because incompetent yard mechanics (they are abundant!) used Teflon tape to seal connections. The tape got into the check valve of the control block of a spring-return valve actuator, making it creep into the non-tensioned position. Everything can be solved, but this problem cost us large sums because we only had few hours to find the fault, and long periods of impatient waiting for the next boarding opportunity of the supertanker. I'd like to share this so others can avoid making this mistake."
There you go. A $1.00 roll of Teflon tape can tie up a supertanker. Metaphorically speaking of course.
The best thing you can do with tapered-thread connections is get rid of 'em. Engineer them out. The reality is however, in legacy hydraulic systems, this is usually easier said than done. Which means they must be sealed. And resealed. Which is why a hydraulics pro never leaves home without a tube of thread sealing compound in his toolbox. My preferred goop, based on performance in the field, is Loctite 577. It doesn't really matter which one you use, so long as it works.
As for thread tape, it should only ever be used on hydraulic adaptors as a last resort. And if you do have to use it, REMEMBER the old plumber's rule for applying it: one and a half turns; one and a half threads back. In other words, don't apply it anywhere near the end of the adaptor, lest it enter the hydraulic system and end up who knows where. And to discover six other costly mistakes you want to be sure to avoid with your hydraulic equipment, get "Six Costly Mistakes Most Hydraulics Users Make... And How You Can Avoid Them!" available for FREE download here.