I have to apologise yet again for being late to this "party" but I've been very busy with "stuff" resulting from my brother's passing.
This jack's main hydraulic element is one you find in many brands of these small "hobby" jacks. I suspect there's a factory somewhere in China, Taiwan, or wherever knocking these out by the million which the jack manufacturers then buy for their production. Here's a couple I own at present:
The yellow one was bought just a few years ago from Halfords but the blue one must be 40 years old if it's a day and they are, to all intents and purposes, identical. The blue one has a red pumping element because it's original one failed and I fitted the element from a similar Sealey to keep it going - the cylindrical bit that does all the work, contains the main ram, oil reservoir, smaller pumping piston, over pressure relief valve and a screw type valve which allows you to either pump or relieve the pressure to the main ram. I've attempted to"mended" a number over the years with varying success. The biggest problem for anyone trying to mend them is that there's no spares backup - I've not found any suppliers, but if anyone knows of one please do let me know with a PM.
If you remove the hydraulic element it looks like this:
The screw type valve can be clearly seen on the left (the older ones, like this one, have a slot in the end so you can turn it with the jack handle but most now seem to have a flattened end with a suitable end on the jack handle to engage with it) Regardless of what type of end they have by far the biggest problem I've found is that people screw them up too tight before pumping. This, typically, results in damage to the thread, but you can helicoil them if the valve body hasn't been damaged. Also a machine shop can easily make a new one up if needs be - local college did one for me as an exercise for a trainee - cost me a bottle of Famous Grouse. In the side of the barrel is a plain slotted screw (still with all the paint on it) this is the pressure relief valve which limits the max weight the jack can lift. Try to lift something too heavy and this valve lifts internally against a spring and allows fluid to return to the reservoir instead of extending the main ram. I suppose there might be a wee bit of dirt under it's seating which could give you the problem you're experiencing. However, if you remove it very important count exactly how many turns it takes to come out as it's spring loaded and that controls the blow off pressure. Screw it in too tight and you're putting yourself in danger of being able to overload the jack! At the top is the pumping piston - you can just see the rubber O ring on this one, which you shouldn't be able to see but this is a very worn one. In fact it's out of my old friend which I swapped the whole pumping element on as it was too worn to rebuild - I've done a wee feature on this somewhere on the forum. On the other end is the main ram:
Which is nearly always slightly damp with oil but shouldn't be really "wet" or dripping. This one was fine. This is something worth looking at if you're buying one as I've found the quality of machining of the ram itself varies quite a bit. You want one where the finish is as near mirror like as possible. I've seen them so rough that you can feel the ridges left by machining if you run a finger nail down the extended ram which must significantly reduce the life of the seal? For this reason I'd be buying from somewhere like Machine Mart so I could check the actual jack I was buying before handing over my hard earned.
So, what about repairs? Well, surprisingly perhaps, I've not yet come across one with a significant leak on the main ram seal. I did dismantle the one featured here in the pictures and it was a right old struggle to get it undone! Like wise the over pressure relief valve. Haven't had a problem with one of these either. Easy to check. You can do a rough check by sticking the jack under something really heavy or an immovable object and pumping it up until it stops. Pump it some more, which will take more force, and you'll hear the relief valve make a noise as it lifts and the fluid bypasses it (you can feel it through the handle too) It's difficult to check if it's blowing off at the right pressure unless you have some calibrated weights to load it with. The big thing is simply whether it blows off at all because if it doesn't and just allows you to attempt to lift something too heavy then the main seal could blow which would result in the jack collapsing instantaneously - nasty!
Apart from problems with the screw type operating valve (more on that in a minute) The little pumping plunger on top does wear. I've replaced just the sealing O rings before but what really scuppers them is when. like my old blue one, the actual metal of the plunger and little screw in cylinder it runs in wear to excess. That's what condemned my old blue one, the cylinder was so worn the piston slopped sideways in it so even with new O rings it still leaked. The piston itself isn't too bad, just scored a wee bit, it's the cylinder which has the real problem and I did play with the idea of getting the college to machine a new one for me - I think the piston is made from something pretty hard but the cylinder is just mild steel? Trouble was, I couldn't get the wee cylinder to unscrew - you can see the marks left on it by my Stilsons! Anyway, looking on my local Gumtree I found the Sealey 1153CX jack advertised and he lived not even a mile away from me. The jack looked almost new and he took £30 for it. Got it home and realized the pumping element was the same as used in the old blue jack so I just swopped it over - which explains why the blue jack has a red pumping element - just in case that was "worrying" anyone?
So, what do I conclude from the one's I've worked on? Well, by far the most usual problem is the wee screw in operating valve. If you unscrew it from the casting, the "business" end looks like this:
The rubber sleeve is there to stop oil leaking out and direct it back to the reservoir when it's unscrewed to allow the main ram to retract so may make the jack "messy" and loose you a little oil but it doesn't serve any other functional purpose. Take a careful look in the hole it came out of:
and you may be able to see that there's a plunger at the bottom of the hole. It's a sod to try to "pick"it out but comes out relatively easily if you just pump the handle with the screw removed and the main ram blocked so it can't move. the inner end of this plunger seats against the casting when the screw is tightened down and this blocks off the return to the reservoir for the oil allowing the oil to drive the main ram. Take a very careful look at the end of this wee plunger and it's seat inside the hole. If damaged at all then you're on a looser. Over tightening can damage the seating and I think is one of the reasons for failure. However it may just need cleaning and a new rubber to stop it dribbling. My guess would be that it's this which is most likely to be causing your problem. Might just need a thorough clean if you're lucky. I've seen these jacks with damaged threads and even one where the side of the casting had cracked due to over tightening. People need to be aware that these valves should really just be "nipped up" but I think they believe they need to be "really really tight" because of the weight they are lifting. You have been warned folks!
Of course, as Green Viper says above, the O rings and any seals need to be oil resistant. I have a wee industrial electric motor rebuilder near me who, I suppose because they are often electrically driven, has diversified into overhauling pumps. He has boxes full of ordinary and oil tolerant seals which he kindly lets me rake through when he's not too busy. It can be difficult to find oil resistant O rings but a local engineering works or factor can be a good place to look too.
Hope that was helpful. By the way, I've noticed the Pumping elements seem to come in about 3 different capacities/sizes depending on size of jack, All originating somewhere in the far east would be my guess?
