No leaks in breaks still no pressure - 1988 Jeep Comanche

An Oil Pressure Sensor/Switch is just that, a 'pressure sensor', and yes they do have a leak history. It may sometimes leak from the threaded area that it is installed by, and in most cases can easily be fixed by tightening it a little (use caution to not strip the threads in the block or breaking it off, if that happens then you have even more problems). Otherwise a leak may be from the switch mechanism itself, if so then it is best change it right away and it is actually very easy to do so. Remember it is a a 'pressure sensor', so if it is leaking that means it can easily become a stronger leak, to include a total leak, meaning pumping oil out of the engine at the range of 40-60 PSI.

fuel filter, computer

slave cylinder.... clutch fluid doesnt leak from a gear box

check the brake pads. When they start to wear down, they can have extra play in the pedal causing you to have to press further or pump them. If they are still loosing pressure, check the fluid levels and check again for leakage all through the lines, both front and back.

Sounds like a brake line is cracked and leaking. Stop driving that car until you fix this. Break fluid leaks can cause sudden loss of hydraulic pressure and a complete break failure.

does the truck have rear drum brakes on the rear? if so then the fluid is leaking out of the rear wheel cylinders. or the master cylinder is leaking fluid into the brake booster.

You probably have an internal leak in the master cylinder. Brakes are no joke have this corrected asap. Topping it off isn't the cure. Becareful!

your problem could be the Master Cylinder,first check the wheel cylinders for leaks if there are no leaks,then bleed the brakes.if that dosen't work replace the Master Cylinder there are steps to bleeding the Master Cylinder instructions come with it.

U are going to need a new brake master cylinder, the internal seals are leaking and allowing the fluid pressure to leak down, this why the sinking pedal

The engine is low on oil. but assuming it is not.
The engine is either idling to low .
The engine oil pressure itself is low. The engine oil pressure switch is leaking or not reading the oil pressure in the car.
I assume there is not knocking noise from the engine.
I would check for any oil leaks on the engine .
IF there are no leaks , I would change the oil pressure sesnor or switch first .
This could lead into major engine work.

Good luck