Ok, Code0, rid yourself of the idea that any kind of air would be strong enough to pressurize a hydraulic system. They generally run on 3000 psi, and bleed pressure in aircraft is supplied at around 50psi.
Ao pressurizing hydraulics is always done by pumps. Now where do those pumps get their power? Usually by the engines, the so called EDP or engine driven pump.
Now that engine fails: An electrically driven pump now is activated and pressurizes the hydraulic system. Easy.
Now both engines fail: Too bad, no elec power to run the elec pumps. Here the ADG or RAT comes into play - an air driven propeller or turbine is released into the slipstream of our now powerless aircraft. That thing usually supplies a generator that has enough power to energize some hydraulics and the most esential instruments.
An uncomfortable situation that any pilot would want to solve for the better. So descending through the appropriate altitude for APU start, you get it running, and voila: An even more powerful generator kicks in, you come back to full instruments and more of the hydraulics are now pressurized.
And you get bleedair - nice, it has gotten quite cold on our way down by now so the packs start running again, you all get to take off the O2 masks.
Even better: Now you can use that bleed air to start the engines back up. Or at least try.
You get it now? There is no link between bleed pressure and hydraulic pressure. It done by changing power into different forms and finally supplying elec pumps to build up pressure.
Nic