Spongy brakes that react slow, then are good

Here it is, a year later, and I'm still fighting these brakes.  They were pretty good since my last FTE Bullnose forum post, but lately the LR has been locking up easily in the morning on our dirt road. To fix the overly-eager LR brake and poor parking brake action. we just did new shoes and a hardware/spring kit. We also lubed the parking brake cable to that wheel. Adjusted and bled, that wheel no longer tends to lock up. I suspect old return springs were the culprit.

Everything is new except the calipers and metal lines.
New drums, shoes/springs/hardware 
New MC, vacuum pump, booster (rebuilt)
New rubber lines and pads in front
New pressure differential valve (1 wire model)

But we're getting intermittent brake warning light, and poor pedal response. The brakes suffer a delay. You push, the pedal is mushy as if the system were not bled, then about 2 seconds later the brakes start working, front and rear. They feel fine, in fact. But those first two seconds are disconcerting. This caused one of our sons to damage the original XLT Lariat bright trim grill last night (tips welcome on where to find one).

The fact that the brake warning light comes on indicates that the pressure differential valve has detected what it thinks is a leak. That's my understanding of (one of) its function(s). Its little piston moves fore/aft to the end of the truck that has the imagined or real leak and blocks it off. But if that were the case, why do the brakes feel good after a couple of seconds?

Way back in 2018, I mentioned an old-timer who was a Ford service manager when Bullnoses came out. He said to stand on the brake pedal when stopped, to reset the pressure differential valve. I tried, no luck. Then there is a bleeding procedure he recommended, again back in the OP.  He said that bleeding one end of the truck can trip that pressure differential valve. We did, in fact, take that truck to @calvinhg 's shop so he could vacuum bleed the rear. Perhaps we need to bleed the front now, and follow the former service manager's suggestion?

From my 2018 FTE post: https://tinyurl.com/y42bvg4h
"We talked to an older gentleman who worked at Ford for 30 years. He says there is a bleeding procedure that can usually restore normal operation, if the brake warning light is working. The pressure differential valve has a secondary duty of shutting off pressure to a circuit that has a leak. So if it's only braking on the rear, the valve has decided that the front has a leak. This could caused by a genuine leak, or by air in the system. Once the pressure differential valve has tripped to shut off one end of the truck, it needs to be re-centered. To re-center it, he said to use the brake warning light. Step lightly on the pedal with the brake warning light on. Crack a bleeder on the end of the truck that IS braking. When the warning light goes out, STOP bleeding. He says that will re-center the proportioning valve to normal op. "

That is the procedure for recentering the shuttle.

Some people don't know that they should press or pull the button/knob on the differential switch while bleeding.

There was some contention over this a short while ago, because 250/350 brakes differ from 150 brakes in that regard. (push v pull)
Reference was made to Chris' (Ctubitis) post back on FTE.
He lost his Ford documentation when Photobucket went to a pay for play model.

I know little about the vacuum pump system on diesels.
How are the lines, reservoir before the booster?

Bill (85LebaronT2) is probably your best source here on this forum.

The vacuum line from the pump to the booster does, indeed, receive the steely Germanic baleful glare of suspicion from my baby blues.  It's 33 years old, and may be failing internally. But its failure doesn't fit the symptoms I have.  Vacuum is there in the booster. Were that vacuum line to fail, there would be a shortage of vacuum in the booster and/or poor recovery time.

What we have here instead is poor attack time when asking for a little brakin' please.

There's a reason people do hydro boost conversions.
Just ask Gary or Jonathan

Have you used the shop manual section here: Documentation/Driveline/Brakes and then the tabs on Master Cyl's & Boosters, Factory Shop Manual Sections, and then the Brake Booster - Vacuum Pump?

There's a chart in there that shows how much vacuum you should have.  Have you checked that?  A low level of vacuum is the problem I would expect to give the symptoms you have.

We did a good bleed today. I think. The old fashioned way. We got a little air out of both ends.  We used this tool from Carolina Classics (see pic) to lock the fershlugginer brake differentiating valve in the center position while bleeding. I think. At least the pin went in easily. Then we bled using the Michael Jackson (man/boy) method us old fossils learned in HS Auto Shop class. The MC is nearly new, so I'm not worried about destroying its piston seals by going past its normal stroke.

But the brakes are essentially the same still.

I plan next to replace the vacuum supply line. Napa sells it by the foot. It's 36 years old, after all, and if it's sucked shut inside somewhere, it might be a factor. But strictly speaking, even if it's restricted, enough vacuum should accumulate in the booster after a minute or so to make that first brake attack good. It's not good, it's delayed. Recall than I have a new vacuum pump. I should probably test it too.

Still, that old rubber hose bugs me, sitting there like a boomer, born back when Bon Jovial was on the AM radio.

That's an interesting tool!  Hadn't even heard of that.  Cool!

If the new hose doesn't help, then it is time to check the vacuum on that new pump as suggested by the TSB.

I cut a piece off that hose to take it to NAPA. It's not old!  Cut real easy, flexible. Guess I was wrong. But I took it to NAPA anyway to make sure they agreed that it's vacuum hose. It is.

I verified the pump and check valve while I was at it. The pump seems to suck great, and the check valve is solid. I heard no leak in the booster, it's new anyway.

So it's all pointing to a hydraulic problem now. Maybe that fershlugginer valve is all the way to one end. The brake warning light has never come on and stayed on throughout this affair - always intermittent. So the tactic of "center the valve by bleeding one end until the light goes out" has not been possible for us.

You have that jazzy little tool to hold the shuttle centered.
Why would the valve cause this?