Spongy brakes to no end

[SasquatchOnDeck]

a couple weeks ago, i backed up a hill a steep hill, parked (with parking brake), dropped my tailgate, and rolled a considerable log into the back of my 89 K3500. when i started it up an released the Peebrake, the brake light on the dash didnt go out... i shrugged, went to put the truck in gear, and the brake pedal went to the floor. It went almost all the way before there was any resistance. If i let it idle in drive, it would take 10 feet or so to stop from less than 10mph.
I've since replaced the vacuum booster and master cylinder, which was bench bled. I've bled the brakes at each wheel. so. many. times. the pedal is still spongy, and goes to the floor to stop the truck. i know OBS brakes suck, but neither of mine ever sucked this bad. the brake light is still on the dash, usually only when i press the pedal. there are no leaks, and the rubber flex lines are questionably old (ive had the truck a year) but don't visibly swell when brakes are pressed.
Any ideas here? Based on my reading, tomorrow im gonna get it up around 35 on gravel and lock them up a few times (it will still lock the brakes on pavement if that helps diagnose an ABS problem... lol), from what i've heard that's the poor man's ABS bleed, if you can even do it from OBD1.
Also, i have an ALDL cable and Tunerpro RT, but don't really know what im doing with it, if there's anything brake related in the data stream, please tell me where to look and i'll throw that in here ASAP.

 

[SasquatchOnDeck]

Tore up some gravel and re-bled brakes, and tightened a fitting on the side of the master cylinder that looked like it wasn't in quite all the way, but gave me no real reason to beleive it was leaking. A couple times it stuttered and the brake light went out, brakes were strong but touchy and jittery... then back to the same old problem. Ordered the s10 proportioning valve, gonna keep throwing hard earned money at it until it decides to act right.

 

[Dravec]

Hoses. Replace all three of those and wheel cylinders, and possibly calipers, first. Also check all along the frame for any pinhole leaks in the hard lines.

Edit: I suck at spelling

 

[Schurkey]

1. An '89 should have a cast-iron-body RWAL ABS unit. It'll have a bleeder screw right on it. There's no need to be dicking-around locking the brakes on gravel to bleed that ABS unit.

2. If the brake light on the dash only lights-up when you push the pedal, you've almost certainly got air in one side of the dual hydraulic system.

3. It takes more work than many folks recognize to get a master cylinder completely bench-bled. Good chance there's air trapped in the master that can't get out until you jack-up the back of the truck so as to tip the master cylinder "down" in front--then tickle the brake pedal to get the air bubbles to move from the cylinder to the reservoir via the fluid transfer ports.

4. HOW are you bleeding the brakes? Pressure bleeder? Vacuum? Reverse-injection? Gravity? Two-man pump?

5. Have you had the drums off to verify no leakage at the wheel cylinder(s)?

6. A stuck-open accumulator solenoid valve in the ABS can mimic "air in the system". The pedal goes way down, the braking power sucks 'cause the rears never apply. The dash light may come on when braking due to the lack of pressure in the rear hydraulic circuit.

 

[SasquatchOnDeck]

1. (pic attached) The bleeder on the left? i've turned that fella for miles over the last two weeks. no air, just solid fluid flowing. i've also played with the little nub on the right, and got it to release fluid.
2. Sometimes it will stay on after for a while. so do you mean side as in driver/passenger or side as in the front or rear chamber of the master cylinder?
3. I bench bled it with the provided kit, then went and bought one with barb fittings and hoses, and bled it IAW autozone's youtube tutorial, i definetely gave it 50+ full length pumps. also, having the rear up like that is what started the problem, may revisit that hill tomorrow and pump the brakes a few times for science.
4. two man pump
5. trying to avoid tearing into rear drums at all costs, but it looks like i'm going to have to
6. i just got the 90s S10 prop valve in, going to put it on and sweep the legs out from under the ABS system entirely. That sounds about right , i dont think i was really getting much from the back, and i could still lock the front up.

 

[1998_K1500_Sub]

Did you get this sorted out?

 

[SasquatchOnDeck]

Did you get this sorted out?

I wish. Theyre pretty spongy about halfway down, then all the sudden, real strong. like throw your body forward strong. I'm dreading that it's likely a wheel cylinder issue as that's the last things i haven't touched other than front calipers, and it's not my daily, so im choosing to procrastinate... so kinda, lol

 

[Supercharged111]

I wish. Theyre pretty spongy about halfway down, then all the sudden, real strong. like throw your body forward strong. I'm dreading that it's likely a wheel cylinder issue as that's the last things i haven't touched other than front calipers, and it's not my daily, so im choosing to procrastinate... so kinda, lol

I was gonna say, it really sounds like wheel cylinder. It would also explain why the issue came on so suddenly. But you're not seeing fluid leaking?

 

[SasquatchOnDeck]

I was gonna say, it really sounds like wheel cylinder. It would also explain why the issue came on so suddenly. But you're not seeing fluid leaking?

Not a drop. part of me wants to just shell out and convert it to disc. I always have hated drums

 

[Supercharged111]

Not a drop. part of me wants to just shell out and convert it to disc. I always have hated drums

Meh. Good working drums are just fine. If you have an 8.5" axle though, you are unlikely to have good working drums.