Brake Pedal Travel issues. Need Help

[T-7ToNewOldB]

So Vacuum boosted brakes.

The pedal only moves a tiny amount before engaging the brakes. It takes very little foot pressure to stop quickly. Issue is the brake engages by maybe 1/2 inch of pedal movement and then grabs like crazy. Pedal will only move many another 3/4” before allowing no more movement.

Truck stops fine and can lock up all 4 wheels.

No spongy feeling at all.

Just that requires almost no movement, and then becomes like a rock.

Thoughts?

 

[DirtDonk]

Well, my first thought was "yay! What everybody else wants and does not get!" but even though you have not experienced it remaining on when you release the pedal, I would still check the piston-to-rod clearance between the master and booster.

Did you install it yourself, or was it bought assembled?
Either way, just to double check, unbolt the master from the booster (you can leave all the lines connected) and see if it springs out when you do. Or when you push it gently back to the booster, does it feel like it stops just before the mating surfaces are solid against each other?

I'm just wondering if the booster rod is just a bit too far out and touching down on the back of the piston before the mounting surfaces of the two meet up.
Otherwise, smooth, powerful brakes that you can lock up on demand are what everybody hopes for with Broncos.
Granted, too sensitive is too sensitive, especially on a Bronco, but at least you're starting from an advantage point, rather than the other end.

Paul

 

[T-7ToNewOldB]

OK, to be honest it’s my son’s 79 F-150! But I like the info on this forum so I left that detail out. Basically the same stuff as we all know. He bought it this way. It’s already got a bunch of Wild Horses stuff on it!

We haven’t taken it apart yet to see what might be wrong.

The odd part is that it just doesn’t move really at all between no brakes and locked up. Something not right, that always worries me with brakes.

 

[DirtDonk]

Yes, worrisome it sounds to me too.
For the year and model, it makes no difference. The concept of the rod adjustment is identical no matter the vehicle, as long as it has an adjustable rod and the proper parts.
But the too-quick lockup is strange. Maybe some contamination on the rear shoes? While I've personally never run across the condition, and still have a hard time grasping the whole concept of oil causing lockup, I've always read that gear oil or brake fluid on the shoes or pads can cause quick locking up. Especially in the rear, where any grabbyness is magnified.

It's also possible that the brake booster is failing. The rear poppet valve that opens up as you push the pedal may be tired and opening fully too soon.
Or maybe the PO put in an F350 booster instead, trying to get that extra bit of power.

Sounds like you'll have to dig into it a bit to see what's going on. It's even possible that a replacement master cylinder had residual pressure check valves installed.
I think when a proportioning valve fails it usually causes the opposite problem, but I don't know all that much about them. Mine did cause the rears to lock up, but that's only because it cut off all flow to the fronts and so the rears were doing all the work.

Got a pic of this thing? Most of us love seventies Ford trucks, and especially the '78/79 Broncos. Be great to see it.
And post up a pic of the brake area too, just in case we can see something different about the booster and/or master.

Paul

 

[T-7ToNewOldB]

Loves his truck but is selling to buy a different year that he likes better. In a nod to how much EBs have appreciated, he hopes to get $10k for this. Everything works, floor pans rusted out and replaced with plastic one. Rust pretty minor otherwise.

Has full blown programmable air horn!

 

[T-7ToNewOldB]

Figure there has to be something wrong with the booster or interface with the master cylinder. The lockup by itself might have other causes but the brake pedal itself stops right away like it hits a brick wall. Think we have to bite the bullet and take it apart to see what is going on.

 

[Apogee]

I've had a booster fail in such a way that it lost all modulation, meaning it went from zero boost to full boost with nothing in between. On the booster I was working with, it ended up being a crack in the air valve housing on the input rod side of the booster...the fix was to replace the booster.

 

[T-7ToNewOldB]

I've had a booster fail in such a way that it lost all modulation, meaning it went from zero boost to full boost with nothing in between. On the booster I was working with, it ended up being a crack in the air valve housing on the input rod side of the booster...the fix was to replace the booster.

Makes sense that this is either a mechanical misalignment between the pedal/booster/MC or a booster issue.

I’d put money on the booster at this point. I’ll post once we resolve this. Will be a couple weeks before we can get to it.

 

[DirtDonk]

The boosters on these trucks worked well for years, but are now becoming a common failure point. The F350 booster was a common upgrade, but became too common apparently and none of the manufacturers are making them right now. When you go into a parts store to get one, they tell you to bring your old one in and then they will have it rebuilt. And it'll "only" take a few weeks!
Lots of full-size owners are swapping out to modern boosters from 1990's and 2000's trucks instead. And also going hydro-boost.

Both are custom installations for now, but it's at that point that the old stuff is just not readily available.
I'm guessing, but I'd have to say that the reason the aftermarket suppliers gave up on them instead of reproducing the old stuff is that Ford had something like 19 different combinations (no, not kidding) of pedal/bracket/booster setups between '73 and '79 on the full size trucks. Nobody can believe it, but that's exactly what they did. I think there were at least four or five different boosters, and nobody ever seemed to get the right one the first time.

More and more discussions on the full-size forums then, about later model swaps.

If you're lucky, yours is still available, or you have a local rebuilder that can do it for you. I still have one locally and am going to take advantage because my F350 booster is toast. I'll have it perhaps to sell, because I'm supposed to be installing my MOAB kit soon. With my newly revised schedule though, we'll see how long that takes...%)

Paul

 

[T-7ToNewOldB]

Paul,

Thanks. He is going to sell the truck but wants the brakes to work properly before he does. Has his eye on a different year, not sure which one.

Hugh