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Sounds like the booster is on its way out. or the master cylinder. As far as brake hard parts your best bet is to go to an auto parts store and tell them the year. If they have more than one option take it apart and meassure or bring old stuff to them for copare.
First thing I would do is disconnect the battery cables. A truck that wont run wont kill you !A truck that wont stop WILL kill you ! Then start repairs to the brake system. Most auto parts stores sell a master cly booster as a unit. Check them for prices. Also remember your truck is 20 years old (young) and may need some other items replaced like the calipers, rotors and wheel cly's. How long have you had this truck and what type of repairs have been made to the brake system in the last 5 years. We all like to see photos, Show us what you got.
I would expect to see a leak at the back of the master cylinder if it's bad, but that's not always the case. Can you crawl under the front to look at the pads on the disc brakes and see if they're thin? Even if you take it to the repair shop, ya still want to do a bit of troubleshooting 1st to make sure you don't pay for repairs not needed.
I guess question number one is, is this a new problem... or has it been this way a while? Did you just buy this truck, and it doesn't have good brakes? Are you losing fluid from the master cylinder ( is the level ok? does it stay there?)If you are not losing fluid, you are not drawing in air. The vacuum caused when you release the brake is nowhere near the pressure exerted by the fluid when you apply the brake.I agree that if you are not losing fluid, you have a problem with either the booster, master cylinder, or rear adjustment.The exception to this is if you purchased the vehicle with the brakes this way. If this is the case, you may have air in the brakes due to improper bleeding by the last owner.Good luck!
If that is the case, it may just need to be bled. Otherwise, the self adjusters may not be working, causing the back brakes to become out of adjustment.Hope he has good news for you!