As soon as I start it, pedal goes to the floor

I would bet on a bad master cylinder myself. The seals on the piston are allowing a certain amount of fluid to blow by them, causing the pedal to go to the floor. Since they do hold some pressure, the calipers do get engaged and squeeze the pads to the rotors. I would also change the brake fluid, as old brake fluid gets a little thinned out and allows the pedal to go closer to the floor (because it compresses a little when under pressure).

Pedal is stiff because there is no assistance coming from the brake booster. All that the dump valve does is redistribute pressure to the rear brakes. So a failure of that valve really would not affect pedal pressure. The only other thing that I can think of is your ABS module, if your truck has ABS to begin with. Most of those trucks in 1989 did not have it, but apparently it was an option, according to ProDemand. In this case, there are valves controlling pedal pressure and brake pressure. A failure there could cause loss of pedal pressure. Plus your ABS light would likely be coming on, as the module would detect an internal problem and set a fitting code.

I really think your problem is in your master cylinder though.

Try switching that ABS module. No pulse from the pedal when you locked it up is something of a give away. Especially if the light is burnt out. If the light is good, it will come on for a second or two when you turn the key on, and then go back off. Obviously, if the bulb is blown, it will not come on at all.

And you are welcome. I do what I can to help out because we all understand that sometimes you just cannot take it to a shop.

You and I are cut from the same cloth. I know what I know because I refuse to take my car to someone else and pay them to do something I can do. But yeah, I would suspect a problem either with that ABS module or the master cylinder. It pretty much has to be one of them.

Hi guys. Allow me to add a few observations that may or may not be relevant.

1. If you have rear-wheel ABS, which would have been typical for an '89 model, the dump valve doesn't always pulse like the valves do in a four-wheel ABS modulator assembly. If the system fails in any way, the block valve just won't apply, and the entire hydraulic system will be identical to that on a truck without the system. Bleeding won't be affected by the dump valve being there.

2. I noticed the comment about the brake light switch. If that is adjusted down too far, it can hold the brake pedal from returning fully. That will prevent the pistons in the master cylinder from coming back far enough to expose the replenishing ports that allow brake fluid to fill in, in front of the lip seals. Normally the symptom will be the warming and expanding brake fluid can't return into the reservoir, and since it's trapped, it causes the brakes to apply harder and harder as you drive. If there's still air in the system, that can prevent the brakes from self-applying as the brake fluid heats up, but it will cause the low brake pedal. When you try to bleed again, if you resort to pedal-bleeding with a helper, which I don't recommend, you'll never get a new bite of fluid when the brake pedal is returned, then pushed again. You'll just move the fluid back and forth as the pedal is moved. If you do gravity-bleeding, which is all I do, gravity won't be strong enough to pull the brake fluid past the lip seals in the master cylinder.

3. I didn't see any mention of checking the rear drum brakes for proper adjustment. If the shoes have to move real far to contact the drum, you'll have a low pedal, but that is misleading only on GM vehicles because without power assist, it can take three men and a boy to push the brake pedal. That falsely gives the impression the pedal feels normal with the engine off.

4. The biggest thing that is often overlooked is trucks and minivans have a wide range of loading variables from empty to fully-loaded. The stopping power of the rear brakes has to adjust for that. Too little rear braking force puts all the load on the front brakes with the rears doing nothing. Too much rear braking force causes easy rear-wheel lockup, which, by the way, was the reason for developing rear-wheel ABS systems which are only found on trucks. To address this wide range of loading, most trucks and minivans use a rear height-sensing proportioning valve. There's a real good chance you have the truck on a hoist or jacked up in such a way that the rear axle is hanging down. That makes it look to the height-sensing proportioning valve as though the truck is lightly loaded and very little braking power should go to the rear brakes. With little fluid flow during bleeding, air will get stalled at a high spot rather than get pushed out to the bleeder screws. You might get fresh, clean fluid coming out of the bleeder screws, but it's dribbling past the air that's stuck in the system. To address this, run the truck onto a drive-on hoist or put the jack stands under the axle housing, then let the truck down so the axle goes up.

Use these wondrous suggestions as some things to consider. If these don't apply, I'll go back and sit in my corner.

The angle of the frame isn't the issue. It's less-complicated than that. The concern is simply the rear suspension's ride height, but I'm pretty sure the height-sensing proportioning valve would have been addressed when ride height was modified. That simply means readjusting the link between the valve and the axle. If that hadn't been done, it would always look to that valve like there was so much weight in the box as to force the rear to go down 4". That would require a lot more braking power to go to the rear brakes, and since you don't actually have that weight in the back, that would cause easy rear-wheel lockup. The ABS dump valve would kick in at just about every stop. The typical symptom would be the brake pedal sinks slowly to the floor on all but the lightest pedal applications. Assuming the valve was readjusted a long time ago and has been working properly, my comment about it wasn't related to the modified frame angle. It applies to any truck or minivan having the brakes bled while the vehicle is on a hoist and the axle is hanging down.

The only thing the higher frame angle MIGHT get you is the engineers at GM, for some reason, mounted a lot of their truck master cylinders at a wild angle that allowed air to become trapped in the fronts of the bores. That could only be bled out at the wheels or by loosening the steel lines at the master cylinder, then bleeding there with a helper. Your higher frame angle could possibly allow that trapped air to wash back into the reservoir like it does on all other vehicles when the brake pedal is released.