Silicone sealants are a great invention that has changed our lives for the better. They can seal and bond virtually anything, they come in all sorts of colors and packages, and they are a must-have in any workshop.
Since these sealants are so cheap and universal, they are often used by mechanics to "economically repair" oil leaks in hydraulic components - an innocent practice, that (unfortunately) sometimes evolves into what I like to refer to as "silicone fever" - a mental condition under which an "overhauler", instead of replacing old and leaking elastomeric seals (like O-rings, X-rings, D-rings and such) with new ones, fixes the oil leaks by applying silicone sealant (often in ridiculous quantities) under, over, and even instead of the said O-rings. In some rare and extremely acute cases, such "handymen" apply silicone sealant on new O-rings as well. "Just in case" - they say...
Does this solve the oil leak problem? Yes it does, most of the time.
Is it cheap? Yes, it is, as instead of buying new O-rings or (which can be even more expensive) complete seal kits you buy a relatively inexpensive tube of sealant, which will be "good for a whole lot of them pumps!"
Is it fast? Yes, it is - the time saved is the time you'd have wasted looking for those new O-rings.
Is it a good practice? No, most of the time it is still a CRAPPY PRACTICE!!!
Why? Because it can cause more problems than it can solve!
The main reason why silicone sealants and O-rings shouldn't mix can be seen on the pictures to the left. Where you have O-rings you have oil passages, and where you have silicone - you have excess sealant, which after hardening can detach itself from its "birthplace" and easily block a control orifice or two. This is exactly what happened to this closed-loop Rexroth pump, which, after the "fast silicone re-sealing", stopped giving flow in one direction when a small piece of hardened sealant blocked the servo-cylinder control orifice.
Another good reason to avoid the above-mentioned technique is the fact that people logically associate "crappy technique" with "crappy mechanic/workshop", so to an experienced eye such a "vandalized" pump/motor/valve can say a lot of bad things about the "artist behind"!
The use of a silicone sealant to solve the "leaking O-ring" problem in a hydraulic component should be restricted to "absolutely-no-other-solution-available" situations! And even in this case, a high-quality (preferably permanently malleable) sealant in a reasonable quantity should be used.
The only correct way to repair an oil leak caused by a worn or deteriorated elastomeric seal is to replace it with a new one!
Fixing damaged O-rings with silicone sealant is like mending broken windows with transparent scotch tape - it does hold it in one piece and keeps the rain out, but it's something you wouldn't like to see in your house!