If you have manual brakes (no vacuum booster on the firewall), the engine running or not running should have no bearing on the operation of the brakes, since if it's a manual (non-vacuum boosted) brake system, there would be no vacuum from the engine associated to the operation of the brakes.

If you put the old MC back on without bench bleeding it, it will have taken on air inside the bore after it was unplumbed and removed from the truck.

I'm not clear whether the fluid level in the MC is dropping or not. You had mentioned that air was getting into the system. If air is getting in, it would have to displace the brake fluid to do so --IF additional air is getting in, fluid is leaking out. IF so, this means there's a loss of brake fluid somewhere in order for the air to occupy the space that was being taken up by the fluid.

IF there are no fluid leaks in the brake system, the only way the pedal could sink like it is would be that all the air hasn't been evacuated from the brake system or, the MC is bypassing internally (the compensating port in the bottom of the MC isn't sealing off and fluid is being forced from the MC bore and returning right back into the reservoir).

A bypassing MC will build pressure (for a little bit) if you pump the pedal. Assuming there are no fluid leaks and assuming all the air is bled from the system, a simple check to test for a bypassing MC is to push down on the brake pedal with normal force and just hold steady pressure on the brake pedal for a good two minutes. If the pedal stays steady and at the same height (relative to the distance off the floor) the MC is good. IF the pedal begins to sink towards the floor, the MC is bypassing internally, which would indicate a defective MC.

IF it's an issue of air in the system the pedal will also drop. IF it's an issue of fluid loss (leak) the pedal will sink to the floor.

 

That's was a lack of my explanation, I meant when the truck is in motion, the pressure disappears. But I am having the issue of being able to build up pressure and after a few minutes it's gone. I'll test that out

 

Again, if the brakes aren't vacuum boosted, the engine running, not running or the vehicle in motion should have no bearing on the ability to be able to actuate the brakes.

I forgot to ask earlier but, is this a new MC you bought and installed or is it a rebuilt MC?

 

Who's helping you bleed the brake system ? As it's sounds like you've still have air in it.
You can't pump up much pedal while air is still in it. And once you remove your foot you'll loose all the pressure you had pumped up on the pedal. ]
As you said it did the same with old M/C No hard pedal.


Orich

 

One major obstacle that needs to be corrected is to get the pressure differential valve back to center, to turn the brake warning light off and to restore full fluid flow to the front and rear brake circuits through the brake pressure differential valve.

Ford procedure for doing this:



When it comes to bleeding out the brake system, I never have help doing this. It's just me. However, Speed Bleeders, not dogs, are a man's best friend. I don't know the diameter and thread pitch of what an F250/F350 brake bleeder is but, if they are 3/8"-24 like an F100 (or F150 Dentside front brake calipers/rear wheel cylinders), the Dorman part number for them is 12701.

Once the pressure differential valve spool is centered again, having a valve lock tool to keep the spool centered, while bleeding the brakes, will be a MAJOR asset.



https://musclecarresearch.com/brake-valve-tool

 

We have went through 3 quarts of fluid and every line we bleed out still spits out bubbles no matter how much bleeding is done, that's why I was thinking theres an air leak. Even after we managed to get lines to not spit out bubbles after a few pumps, we test the pedal and it still slams to the floor unless we build up pressure. This is becoming a HUGE annoyance. The front brakes are brand new and rear are decent but not leaking or bad. But there's a never ending supply of air bubbles. Mostly seems that the fronts are what isn't working as I feel a slight braking but it's just a small dragging and the rears never spit out bubbles.

 

I don't know what method you're using to bleed the brakes --open the bleeder, push down on the pedal, someone closes the bleeder, lift the brake pedal repeat or, if you're using a vacuum bleeder like a Mighty Vac (?).

In any case (particularly with a vacuum method of bleeding the brakes), air can be pulled in from around the threads of the bleeder screw, giving the false impression the air bubbles are coming from the brake system when in fact the air is being pulled from around the bleeder screw threads.

Here again, whatever size your bleed screws are, replace them with Speed Bleeders (they have a built-in check valve). They will save you a LOT of aggravation and the time it actually takes to get the air out of your brake system --and you won't need the assistance of someone else to bleed the system with the Speed Bleeders installed.

 

I did both. We tried the normal bleeding attempt for about 2 hours yesterday to no avail. Did the same for an hour today and then we used our vacuum bleeder which still pulled air bubbles. Yes we are bleeding correctly

 

I've delt with a lot of Ford brake systems. This doesn't mean I know everything and in your case, it's difficult to diagnose what's wrong when the vehicle isn't actually in front of me to put my eyes and hands on it to see what is or isn't happening. I can only try to help with a diagnosis based on the accuracy of the symptoms given.

If you're dealing with a rebuilt MC, get rid of it and get a brand new casting (not a rebuilt MC). If you have the regular wheel cylinder bleeders, get rid of them and install Speed Bleeders. If your pressure differential valve spool is shifted off-center, recenter it. Once it's recentered and the brake warning light goes off, put a valve lock tool in the valve body to keep the pressure differential valve spool centered while you continue to bleed the brakes out.

I can only offer advice based off the same experiences I've had with these brake systems and what the measures were to correct them. Doing the same things as you're doing are going to keep coming back to the same (non-productive) results and the aggravation that ensues.

Of all species, humans have the greatest ability to learn from the mistakes of others but the least willingness to do so.

 

I've heard of guys apply some grease to the threads on bleeder valve itself TO aids in not sucking air pass them all while your thinking it coming out of your Brake lines/cyls.
Using a hand vac pump setup.
Me, I've done the gravity bleeding for like 40 yrs and never had any problems.
Other then getting it to flow at first. But I'd just open the right rear bleeder 1/4 turn then stomp the brake pedal a little & let off slowly then check the slow bubbles flowing.

Yes it take may be 30-45 minutes old school way. But a few times, I'd forget to bring my hand vac. pump and where, I live you can't walk away with any tools laying around on the ground or they'll be gone when you return. So let the little bubbles slowly flow with a pump on the pedal ever so often when no more bubbles with bleeder closed then go onto the next wheels until all 4 are bled and a high pedal.

Sometimes if your shoes aren't adjusted up good you'll be fighting the low pedal.
So may sure you've got them up with on heavy dragging.

Some times a person try's to hard can't see the problem their having..
Which is a good time to walk away and thing about what's happing that may be doing wrong.


Orich

 

Ordered a new MC from oreillys yesterday and it's gonna take 5-7 days from Wednesday to come in. We also tried gravity bleeding and the left rear and front right never had more than a few drops come out over the span of an hour

 

Well update, the OLD MC brakes the same it had before, but I didn't realize that my foot was so close to the floor. So it's a good thing I decided to replace MCs before the MC ultimately failed, and probably at a bad time

 

Common thing with old M/C failing when pushed to the floor.

Why cause they got a lot of crap inside that settled in them deeper in the cylinder bore.
So when the cups into get pushed into the settlement it will cause it to,
1, Stick in the bore or leak like hell.

In the brake shop we never pushed the pedal to the floor for that reason.

We did 3/4 pumps on the pedal at max.
Or we'd have to rebuilt it for free.
Orich

 

The constant bubbling says there is an inlet for air. If there's no obvious leakage I would say the MC casting is cracked near the rest position in the bore. It was traded in for a reason. The odd ones make it through to to the next poor guy. Once you start pressing, you are pumping the leaked air out and the piston seals the crack off as it is past it, but you will not really see a brake fluid leak for a long time if ever. The new MC should solve this.
Never seen that one. Seen cracked castings around the bypass valve which makes them 'give up' too soon in the throw, but not all the way at the beginning of the bore. Could be the machinist broke a tool on that one and passed it anyway.

 

I bought my "Quick Bleeder"(speed bleeders) set from O'Reilleys. Made by Dorman, and the part number is 12701. (This is for a 69 F250)