Two weeks ago I found the brake pedal suddenly did not keep the normal height and went down to the bottom. It gets back to normal height right now but if I press down the pedal softly it still travels down to the bottom.

If a certified mechanic has looked at the system and spent time checking the brake pads and fluid condition and lines etc. and found no other problems, then it could be a problem or maybe the master cylinder fluid is beginning to by pass allowing the pedal to drop.

What is happening - the proportion valve / combination valve is stopping you from bleeding the rear brakes.
check out this video to point you in the right direction.
https://www.youtube.com/watch?v=eF3KSkPCTBM

You may need a new brake boost or you have air in the lines

If the pads are being applied and you are not pushing on the pedal, it could be something going on with the ABS.

Check that the pedal height isn't sagging due to low brake fluid level. Also check for adjustment of the brake light switch or the BPP (Brake Pedal Position) switch.

Brake Pedal Position (BPP) Switch Removal & Installation To Remove:

1. Disconnect battery negative cable.
2. Disconnect brake pedal position (BPP) switch electrical connector.
3. Remove self locking pin.
4. Remove switch from brake master cylinder push rod.
5. Remove push rod spacer.
6. Remove push rod bushing.

To Install:

1. Install push rod bushing.
2. Install push rod spacer.
3. Install switch to brake master cylinder push rod.
4. Install self locking pin.
5. Connect electrical connector.
6. Connect battery negative cable.

brake pedal height is determined almost exclusively by the rear brakes adjustment. turn the starwheel adjuster and spin the tire by hand until the tire begins to drag. more for firmer higer brake pedal. it is only too much if you smell the breaks while driving. an initial smell is normal as the shoes burn into position. sometimes you can drive in reverse and apply the brakes sharply for an "automatic" adjustment. but this could take many attempts and will only work if everything is clean and well lubricated.

in no air or leaks in the system
replace the clutch

1988 chevy r 3500 1 ton changed out driver front brake caliper
brakes are soft peddle goes to the floor tried to bleed but no brake fluid will come please help

sense its a deisel i would go with the vacume pump, diesels don't put out any or very little vacume, if you don't have vacume than booster won't work hense hard peddel. when you say you let up on peddel and hit it again you got better break tells me it is not getting enought vacume to booster. i would go for the vacume pump.

My 03 trailblazer does the same thing and the service dept. told me that it was normal for this model to do that, it has never failed to stop but if your sitting at a red light you can feel the peddle go down to almost the floor but it stops fine and it has done this since day 1 and they say thats normal on the trailblazer.