I would take the cylinders to a local hydraulic shop and see if they can determine the cause.  Then see if they can repair them and whether or not they think the repair will last.

If they don't think the repair will last I would buy the new cylinders from Bend Pak.  That way if they fail again at least they should be covered under some kind of warranty and/or parts to repair them will be available. 

Temperature shouldn't make any difference.  I've used heavy equipment many times in temperatures well below freezing and never had a seal failure in a cylinder.  In fact I can't remember EVER having a seal failure, including on the backhoe my neighbor has which is my age.  So if I had to pinpoint a reason I would probably think, based on the little we know, that the cylinders may have been failure prone (reason Bend Pak does not offer repair parts, and instead has new cylinders available), and this was probably due to sourcing cylinders from the lowest cost supplier (probably made overseas). 
 

Cylinders are not that bad to work on.  Just make sure you have a safe way of disassembling them.  Don’t use compressed air to make a parts cannon. You don’t want to launch the rod and piston into low earth orbit.   If they can’t be pulled apart by hand once the gland nut is removed rig both ends to something stationary and pull it apart with a come along or chain fall.

U-cup seals (poly-packs) are common.  McMaster Carr (think industrial Amazon but not evil) has them.
https://www.mcmaster.com/products/polypack-lip-seals/

Lots of things it could be, but the seals are most likely.    Could also be that the piston is loose on the rod.

What type of hydraulic oil are you running?

TKOPerformance wrote:

Temperature shouldn't make any difference.  
 

The instruction manual says the safe operating temperature range for this product is 41°F to 104°F, and it has always been used between that range.
A search on the internet now seems to indicate a rash of cylinder problems about the time I bought it in 2016. Dannmar who made the lift when I bought it said to use Teflon tape on the hydraulic fittings, but now BendPak wants you to use PTFE thread sealant. 

The ID of most cylinders is just honed steel tube, not polished & plated like the rod.  The surface that the piston seal rides on isn’t nearly as perfect as the rod seal.  Who knows what concoction of “rubber” was being used at the time to make the seals.  Take it apart and measure it up.  Worst case you put new u-cup seals in them and it still leaks down.  The seals are not costly.  I can’t think of another way it would have failed unless there is contamination / rust inside or if the surface finish of the honed tube is poor.

Maybe replace the oil when you are done just for good measure?

Mach1Driver wrote:

TKOPerformance wrote:

Temperature shouldn't make any difference.  
 

The instruction manual says the safe operating temperature range for this product is 41°F to 104°F, and it has always been used between that range.
A search on the internet now seems to indicate a rash of cylinder problems about the time I bought it in 2016. Dannmar who made the lift when I bought it said to use Teflon tape on the hydraulic fittings, but now BendPak wants you to use PTFE thread sealant. 

The knock on teflon tape is that the first thread on the fitting should not have the tape applied.  This is to preclude strips of teflon tape breaking off getting into the hydraulic system and causing problems.  As manufacturers can’t control the people applying the tape, they switched to recommending PTFE sealant.  (That's my understanding)

 

I personally would not use air filter foam…at least long term.

The equipment air compressors at work have the foam filters and they breakdown within a year or so.
I understand the heat it is exposed to at the head of the compressor inlet.
I believe there are better options available than foam.

I would use a crankcase vent filter.
This will allow free air flow as well as keep bugs out of system.

Yikes! Good move on scrapping it. Glad we still have you around Terry.