1991 4.0 Jeep Cherokee brake booster, can an o-ring be replaced if the booster leaks, and how?

You have a blown power booster. It is leaking vacuum and you will have a hard brake pedal and find it tough to stop your vehicle. Replace the power booster and you will have brakes and no more leaking air sound.

Sounds like air in the brake system. Will need to bleed the brakes to remove the air. May possibly have to bleed the master cylinder as well.

I don't see why not.
Technically, the hydraulic brake system works independently from the ABS. The ABS system uses the regular hydraulic system to function.
So your regular hydraulic system should work even with the ABS system in place.

Sounds like your brake booster is leaking vaccum and will probably need to be replaced.

replace it , may as well do it before it quits altogether

it sounds like the diaphram in your brake booster is leaking.booster assembly needs replacement.

if you cant find any leaks in lines or cylinders or calipers u probaly have a bad booster
see lots come threw my shop like that but also their is a vacum line that runs to the booster and it hooks to a plastic valve that goes in to the booster those get gloged and you loose pedal as well try replacing that before you bye a new boooster

then you will need a master cylinder or a brake booster

check under dash for a leaky vaccum switch. if booster was faulty brake pedal would be very hard to depress.

check under dash for leaking vaccum switch. usually manufacturers use vac switches to release the e-brake automatically when applying the brake.