This pattern is very specific and useful:
Firm / pumps up with engine off → soft / spongy with engine running
That usually means the hydraulic side is basically OK, and the problem shows up when power assist is working (or something is expanding / leaking under assist pressure).
You already replaced a lot of the front hydraulic parts. Next steps are more targeted.
───
What this symptom usually means
┌────────────┬───────────────────────┬──────────────────────┐
│ Engine │ What you’re testing │ Typical result if │
│ state │ │ good │
├────────────┼───────────────────────┼──────────────────────┤
│ Engine off │ Pure hydraulic │ Firm pedal after a │
│ │ system, no assist │ couple pumps │
├────────────┼───────────────────────┼──────────────────────┤
│ Engine │ Assist + higher │ Still firm, just │
│ running │ system pressure │ easier to push │
└────────────┴───────────────────────┴──────────────────────┘
If it only goes spongy with the engine running, suspect:
1. Booster / assist problem (or bad install)
2. Flexible rubber hoses ballooning under assist pressure
3. Air still trapped (especially rear / load-sensing / RABS plumbing)
4. Master/booster pushrod adjustment after the new parts
5. On many diesels / HD F-350s: Hydroboost, not vacuum
───
1. Confirm what assist system you have
1992 F-350:
• Gas: usually vacuum booster
• Diesel (7.3, etc.): usually Hydroboost (power-steering hydraulic assist), not vacuum
If this is a diesel and a “vacuum drum/booster” was fitted or mixed up, that can create weird pedal feel.
If it is Hydroboost, also check:
• Power steering fluid level/condition
• Belt/pump noise when braking
• Steering effort changes when braking hard
• Leaks at Hydroboost, cooler, or steering box
───
2. Classic booster check (do this first)
A. Engine off pump-down test
1. Engine off.
2. Pump brake pedal 5–6 times until hard.
3. Hold firm pressure on the pedal.
4. Start the engine while holding the pedal.
Good booster: pedal drops slightly (about ¼–½"), then holds firm.
Bad booster / internal problem: pedal sinks, goes mushy, or won’t hold.
B. Look for fluid in the booster
With a vacuum booster:
• Pull the vacuum hose at the booster
• Check for brake fluid in the hose or booster port
• Fluid there = master cylinder rear seal leaking into the booster (even a “new” master can be bad or mis-installed)
Also check if the booster is wet with fluid at the master mounting face.
───
3. Very common after new master + booster: pushrod length
If the booster pushrod is too long:
• Master cups are preloaded
• Compensating ports don’t open correctly
• Pedal can feel weird, spongy, or drag once vacuum/Hydroboost is applied
If too short:
• Excess free play
• Soft/long pedal
After replacing both master and booster, this is one of the top remaining causes.
Quick check:
• With engine running, light pedal pressure should not immediately drag the brakes hard.
• Wheels should spin freely after release (safely jacked up).
• There should be a small amount of free play at the top of pedal travel before resistance.
───
4. Ballooning flex hoses (often missed)
You said new lines. That often means hard lines only.
Old rubber flex hoses (front and rear) can:
• Look fine externally
• Expand under power-assist pressure
• Feel like air in the system only when the engine is running
Check:
• Have someone press the pedal hard with engine running
• Watch each flex hose
• If a hose swells like a balloon → replace that hose
Replace all aged flex hoses as a set if any are original.
───
5. Bleeding may still be incomplete
“Back to front” is good practice, but on these trucks also mind:
• Rear drums out of adjustment (big soft-pedal feel)
• RABS rear anti-lock (if equipped) can trap air
• Bench-bleed the master on the truck again if needed
• Pressure or vacuum bleed is better than pedal bleed alone
Rear drum adjustment (very common soft pedal)
If rears are drums:
1. Jack rear, support safely
2. Adjust star wheels until slight drag, then back off a little
3. Make sure parking brake isn’t holding shoes partially applied or misadjusted
Out-of-adjustment rears make the pedal travel long and feel spongy even with good fluid.
───
6. Rule out master cylinder internal bypass under assist
A new master can still be defective.
Test:
1. Engine running.
2. Firm steady pedal pressure for 30–60 seconds.
3. Pedal should not slowly sink to the floor.
• Sinks = internal master bypass / seal issue (or external leak)
• Holds but still spongy = air, hose expansion, or rear adjustment more likely
───
Most likely remaining causes (in order)
Given what you’ve already replaced:
1. Flex hoses ballooning (if not replaced)
2. Booster/Hydroboost issue or bad new unit / bad check valve
3. Pushrod adjustment between booster and master
4. Rear drums out of adjustment / wheel cylinders seeping
5. Air still in rear circuit or RABS valve area
6. Defective new master (less common, but real)
───
Practical next diagnostic sequence
1. Identify gas vs diesel (vacuum vs Hydroboost).
2. Do the hold pedal + start engine test.
3. Inspect for brake fluid in vacuum hose/booster.
4. With engine running and helper on pedal, watch every rubber flex hose for swelling.
5. Re-adjust rear drums properly.
6. Recheck pushrod free play after the new master/booster.
7. Pressure-bleed again, rear first, then fronts; crack bleeder while holding firm pressure and watch for air.
───
Quick questions that narrow it fast
Reply with these and the next step gets much more specific:
1. Gas or diesel?
2. 2WD or 4WD?
3. Rear drums or rear discs?
4. When engine is running, does the pedal sink slowly under steady pressure, or stay up but feel mushy?
5. Were the rubber flex hoses replaced, or only hard lines?
6. Any brake fluid smell/wetness at the booster, or power-steering issues when braking?
───
Bottom line:
New master, booster, calipers, hard lines, and fluid with no change strongly points away from “just needs more bleeding of the fronts.” Focus on assist system behavior, flex hose expansion, rear adjustment, and pushrod setup. Those are the usual leftover causes on a ’92 F-350 with this exact symptom.
Good — gas 460 means this is a vacuum booster truck (not Hydroboost). That keeps the diagnosis simpler.
What to focus on now
With engine-off firm and engine-running spongy, on a vacuum-assisted 460 F-350, the usual remaining causes are:
1. Vacuum booster problem (even if “new”)
New units can be bad, or the check valve / vacuum hose can be wrong.
Do this test:
1. Engine off, pump pedal 5–6 times until hard.
2. Hold firm pressure.
3. Start engine while holding the pedal.
• Normal: pedal drops a little, then stays firm
• Bad: pedal gets soft, sinks, or feels mushy after start
Also:
• Inspect the one-way check valve in the vacuum hose at the booster
• Confirm strong vacuum at the hose with engine idling (should hold vacuum when hose is pinched/tested)
• Check for brake fluid in the vacuum hose or booster — that means master rear seal is leaking into the booster
2. Rubber flex hoses ballooning (very common miss)
If only hard lines were replaced, old flex hoses can swell under vacuum-assist pressure and feel exactly like air.
With engine running and a helper on the pedal, watch:
• Both front flex hoses
• Rear flex hose(s)
Any hose that expands/swells needs replacement.
3. Booster-to-master pushrod length
After replacing both master and booster, wrong pushrod length is common.
• Too long → ports don’t uncover properly, weird/spongy/dragging feel once vacuum is applied
• Too short → long/soft pedal
There should be a small amount of free play at the top of pedal travel.
4. Rear drums out of adjustment
Most 1992 F-350 gas trucks are front disc / rear drum.
Loose rear shoes = long travel and spongy feel even with good fluid and new fronts.
Adjust rears until light drag, then back off slightly. Also confirm parking brake isn’t misadjusted.
5. Residual air / incomplete bleed
Bleed again, but prioritize:
1. Master cylinder (bench/on-truck)
2. Rear right
3. Rear left
4. Front right
5. Front left
Pressure bleeding is better than pedal-only. If equipped with RABS (rear ABS), air can hide near that valve.
───
Best order to attack it
1. Hold-pedal / start-engine booster test
2. Check vacuum hose + check valve + fluid in booster
3. Watch flex hoses for ballooning under hard pedal (engine running)
4. Adjust rear drums
5. Verify pushrod free play
6. Pressure-bleed again rear-to-front
───
Most likely on your truck
Given new master, new vacuum booster, new front calipers, new lines, new DOT3, and still spongy only when running:
┌────────────┬──────────────────────────────────────────────┐
│ Likelihood │ Cause │
├────────────┼──────────────────────────────────────────────┤
│ High │ Old flex hoses swelling │
├────────────┼──────────────────────────────────────────────┤
│ High │ Rear drums out of adjustment │
├────────────┼──────────────────────────────────────────────┤
│ Medium │ Bad check valve, weak vacuum, or bad new │
│ │ booster │
├────────────┼──────────────────────────────────────────────┤
│ Medium │ Pushrod adjustment after master/booster swap │
├────────────┼──────────────────────────────────────────────┤
│ Lower │ Defective new master still bypassing under │
│ │ assist │
└────────────┴──────────────────────────────────────────────┘
───
One more detail that helps a lot
When the engine is running and you hold steady firm pressure on the pedal for ~30 seconds:
• Pedal slowly sinks → master internal bypass / seal issue (or external leak)
• Pedal stays up but feels mushy/springy → air, ballooning hose, or rear adjustment much more likely
If you can answer that (sinks vs stays up but spongy), and whether the rubber flex hoses were replaced, the next step is basically pinned down.