Power drawbars on commercial machines often use an "air over hydraulic" system similar to what you describe. You can generate as much force as you want, with proper design. The idea is to use a small air cylinder to move a small hydraulic piston which pressurizes the hydraulic fluid, to move a larger hydraulic cylinder. The small cylinders, of course, must have much longer travels than the large cylinders, as both must displace the same volume of fluid.
A brake caliper can easily tolerate in excess of 1000 PSI.
Regards,
Ray L.