So I’ve access to an old, not used for years, trolly jack. Obviously I would not get under a car supported just by any trolly jack but if it appears you work fine, is there any reason not to use it?
It is not good practice to get under anything that is supported by just a trolly jack no matter how old or young. You need some axle stands under there as well, just in case. I usually put the wheel under the sill as well, better a crushed sill than a chest.
Trolly jacks are for ‘lifting’ only.
Stay safe.
Indeed 
The jack should only be used for jacking up so provided you can jack it up to the correct height before supporting properly and the jack doesn’t start to come back down by itself or leak there shouldnt be a problem. You should never get under a car that is only supported by a jack, new or old in case the hydraulics fail - if the jack is suspect at all then why not invest in a new one? I highly recommend sgs engineering, have had mine for many years. Once jacked up a fixed solid support that you can rely on is what you need. I have multiple sets of axle stands but rarely use them, I always have a selection of concrete building blocks and a box of cut timber kicking around as I trust their stability more but this is a personal preference and not a recommendation.
I can’t see any problem with using an old jack, of course check it doesn’t leak and it holds the car up without slowly coming back down, some do.
I regret giving my old one away, much better made than the new stuff (reasonably priced models) you get now. I still ended buying another, 2 jacks I find are better than one.
I won’t lecture you on the safety aspects, you are aware of those.
Another vote for SGS, if you end up in the market for a new one 
I’ve cheapo from Halfords 2 ton. If buying again I’d go cheap S/h. As long as it works there is not much to go wrong and you will likely get a quicker action and greater height. One proviso, you may need small wooden raisers just to get the jack under in the first place.
I’ve two sets of axle stands. They have their place but, like Deano, I often prefer blocks and various bits of wood.
Especially as my drive falls away in 3Directions…
I was given an old 1.5 ton jack and I really like it. It has less mechanical advantage than my newer jack, but takes less pumps to raise.
It’s not low profile enough to get under my mx5, but I use it on the other half’s hatchback.
Many many moons ago, I got an old 4 ton scissor jack off the market, figuring it would be enough for my 850cc Mini. It collapsed.
The Mini or the jack? !!!
I had a jack suddenly collapse when raising a car. The oil seal in the small “raising” piston failed, rather than the anything in the main cylinder. A jet of hydraulic oil shot out and hit me straight in the face, which was bad enough, but what shocked me more was how quickly the jack dropped. It was instant. Secondary support definitely a must.
The trolley jack should only be used to lift the vehicle onto and later off, a solid support item, such as axle stands.
As long as it does this, no problem. 
I’ve a SGS 3T and recently bought a low level 2T from Halfords.(Needed after fitment of coilovers)
Both great at their jobs , £90 and £55 respectively.
I have two low profile trolly jacks but they been shelf’d for some 4 years now as I getting a little older I have gone for the easy option I use 2 X 3 ton 12volt battery jacks once up I replace them with axle stands …
cost me under £50.00 for the two well worth it as my old bones get a little rest… 
Interesting. Got a link to where they came from?
wow thats clean…
I have to vote AGAINST SGS…I have a 1.5t Aluminium jack. After only a few months one of the caster bearings disintegrated. I asked for a new one…I was even happy to pay…but the cost was more than a new jack and they were not in stock.
I’d buy a decent brand (Sealey, Snap-on, etc) if you a) want it last and b) be able to buy spare parts.
Sorry to hear about your issue. Can’t speak for their aluminium jacks but my steel low entry is a tank and I’ve had no problems with it with loads of use.
Yep, I’ve had no issues and also have another Clarke jack that’s given solid service over a number of years.
I also can’t fathom if there was an issue a few months in, why that wasn’t sorted/resolved under warranty? Sounds more likely to be a faulty manufacturing batch than anything. Problems or issues do occur with any manufacturer from time to time of course.
