I strongly agree with looking into repairing what you have instead of assuming they need to be replaced.
I don't have such levelers so my thoughts are fleeting ones. But I have read of other people stating the levelers need regular cleaning and lubrication. If you could extend them fully down somehow (with encouragement), clean the exposed tubes with mineral spirits or kerosene. If they don't feel smooth as silk, then follow up with 0000 steel wool to make them smooth. Then clean again to remove the impurities you introduced with the steel wool, followed with a thin lubricant. WD40 comes to mind but I am sure there are better products.
I imagine a working but leaking unit is repairable.