I get a rumble feeling when i hit the brakes

Try to change a new brake booster may help. http://www.fiverhope.com/

it is 20 years old, the brakes should have been changed at least 4 times by now. if you hear the metal wear tang hitting the rotor, or a dragging or scraping sound when you hit the brakes, it is time again.if you can feel the brakes through the brake pedal, it is time again.

not sure if you mean while driving or when parked with the ignition off but here are a couple reasons on why the brake pedal may be high and low at different times. If the vehicle is running the brake pedal will feel the same just about every time. If the car is off and you step on the brakes it will get harder and harder to push down since there is no vacuum assist. Another reason would be the brake pad thickness. New brakes the pedal will feel higher where as worn brake pads the pedal may feel lower. Another reason would be if you have air in the brake lines. this will make the pedal feel mushy and soft at first and firm up as you pump the brakes. Hope this helps.

Check if there's any Leakage underneath the car and all around the brake hose or galiper/drum, maybe you broke a line or busted a seal in a galiper or a brake cylinder if you have rear drum brake, when you did hit really the brake pedal ! Or check if there's any leak near the Brake Master Cylinder, it could be a bad Brake Master Cylinder ! Check if you didn't Loose a Pad too.. !

Bleeding isnt going to help a hard pedal. If the pedal is mushy, and there is no air, the Master cylinder could be bad. If the pedal is HARD, there is no air. Press on the brake pedal several times with the engine off (5-6 times) now hold your foot on the pedal and start the engine, you should feel the pedal slightly give way under your foot. If you dont, and have a HARD pedal, the brake booster could be bad, but have professionals check it out before spending any money.

When you say the pads and rotors look good, did you actually remove the wheels and check them? A lot of times the outside pad will look ok, but the inside pad can be right down to the metal backing plate. If you can get under the truck, run your finger across the rotors on the inside of the rotor. It should feel smooth. If it feels rough, the inside pad is gone. There really isnt any other reason you would both hear and feel grinding in the brakes.

Try the following to confirm if is the servo unit .( engine off,pump few times the brake pedal to become stiff.Keeping pressed the pedal start the engine and if servo is working you will feel the pedal slightly goes down.If not will not move at all-remain stiff.) Also if you do have the check valve close to servo not working you can realize it by the engine idle ( erratic ) and the servo not working.The other possibility if the vehicle is fitted,the ABS unit itself.

sounds like the vaccum booster is bad. check vaccum line going to booster for cracks. vaccum booster is located behind master cylinder.

have you check? the master cylinder. sometine when you get the rotors cut they dont come out right because they may be warped or to thin. if you did the job yourself you might have put the calipers in on the wrong side. this happens alot . if your rotors front and rear are fine then switch the calipers to the other side.

Can you hear or feel a "click" in your gear selector when you hit the brakes? If you don't, the solenoid that disengages the lock could be sticking.