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The new direct injection systems in IC engines work at pressures of close to 3000 PSI. Combustion pressures in iC engines peak at about 1500 PSI. Not sure about the hydraulic brakes. Engine lubrication systems, cooling systems, AC systems, power steering and the others previously mentioned demonstrate that modern vehicles have numerous "hydraulic systems" with numerous potential leak sources. Yet my 13 year old Maxima with 140 k miles leaves no trace of any failure in those systems on the floor of the garage.
You are absolutely correct about weight. A 4 ton portable hydraulic jack is a heavy machine, but it is very efficient at converting the pressure into useful work, and has survived the test of time for many decades. The drive I conceived will work with 1/8th inch wall thickness high tensile steel up to 3k PSI, more if you use an accumulator of higher pressure capacity. Modern racing sailboats work with pressures of 12 PSI, which would require double the wall thickness of the pistons and cylinders in a drive motor.
Containment of the high pressure portion of the circuit in the return low pressure vessel uses the same type system that an engine uses for lubrication, of course at much higher pressures.
Low pressure containment vessels also allow individual seals to be eliminated, with a designed in fluid loss of 1-2% to act as lubrication and cooling, with a filter in the low pressure return circuit and a radiator for heat dissipation. None of this is new technology except the drive itself. The rest is ancient technology, most of it dating back to the replacement of seals with rubber and synthetics going back to pre WW2.
While the battery point is certainly valid, the counterpoint is the first gen Honda Insight, which only had enough battery capacity to move the vehicle about a mile on battery alone. In the early Insight the battery was supplemental to the engine and acted as an energy damper in precisely the same way I see a hydraulic system operating, with the exception that the engine was directly connected to the motor, instead of being capable of independent operation. With what many would consider serious design flaws, the first gen Insight is still the mileage champ among production vehicles, with a 48 pound battery in a 2 k pound car. It would be interesting to convert one of those Insights into a hydraulic hybrid with a 100 pound accumulator system and drives in each front wheel, eliminating the transmission axles and brakes, as well as the engines induction control systems would be very close to weight neutral, while providing blistering acceleration and pulse a glide capability in the accumulator while maintaining a constant speed.
Anyway that is what I am trying to get built today. Maybe in a year it will be finished but it won't be an early Insight conversion in the first prototype, maybe the second if things sort out well.
Appreciate the interest.
regards
Mech
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