my truck failed safety inspection last weekend, lack of brakes, I noticed driving the 4 miles to the station it didn't seem great but I'm used to adjusting my driving style,

when I pulled into the station to back in a spot I noticed the pedal went to the floor before you feel any resistance to stop,

history,
94 f350 dually,
my brakes where also "not right", usually I would have to give a quick, small pump and then I would have a firm pedal, my rear brakes would lock up occasionally ( didn't thin it was supposed to with RABS )

18 months and about 2k miles ago while having the front suspension rebuilt I had them address the brakes, new master cylinder and everything new at the wheels, the brakes seemed much better, no more quick pump, but still not "new car" good.

the old mechanic at the inspection station took a quick look after I said everybody with this generation has "soft brakes" and he quickly said "you've got no brakes" and deduced it was probably something with the vacuum booster,

At home I pulled the vacuum line off the booster while running and it had a very strong vacuum and my vent controls worked so I really didn't think I had a vacuum pump issue.

everything is clean and no leaks at the wheels or master cylinder,

is there a way to check the vacuum booster itself ?
after that I assume it the master cylinder but being so new and it doesn't even have a coating of dust on it yet I hate to think that's the problem. Is there a way to check master cylinder is working correctly ?

I also tried pumping the pedal at home and first push goes to the floor with a "swoosh" sound and then after a few pumps the pedal does get slightly firmer.

appreciate any advice, I just don't want to throw parts at it,

Scott

 

If the vacuum booster wasn't working, or vacuum pump not working, you'd have rock hard brakes. The booster part makes the pedal 'softer', think of stepping on the pedal when the engine is off, there is no 'boost' so the pedal is hard. I'm not saying something couldn't be wrong with the vacuum side of things, but that tends to lead to a harder pedal than normal, or possibly sinks while holding the pedal.

You have an issue on the hydraulic side of things. Have you checked the level of brake fluid? Are there any leaks on the lines? It might just need a good bleeding if there is some air in there. Thats a good place to start. The ABS valve in these is below the cab on the driver side frame, it is common to have issues with the valve failing and 'leaking' internally. Many people bypass the line around the ABS valve because they barely did anything when new, and cause soft pedal issues when old.

 

After they changed the master cylinder, did they bleed the rabs module under the cab? It also has a bleeder screw iirc. Double check for leaks too.

Quite awhile ago I had a super soft pedal and hiss/whoosh noise when stepping on it, and the pedal would go just about to the floor before any braking would happen. I changed my master and booster as a complete unit, and it solved that. Which one was guilty I don’t know, but it was like stepping on a plum before lol

 

I bet it is your Booster.
I was chasing the same symtoms you are describing.
I see the light turn red down a ways, put a slight drag
on the brakes, down shift and apply the Brakes to stop,
and rock had like nothing there.

Checked the vacuum with a gauge at different places,
and seemed to be the Vac Pump. Changed it and seemed
good, the it goes back to the same problem.

Had a shop go through the brakes to see what they could find,
said it all looks good. leave shop, get down to the HWY,
and no Brakes again.

I did have the Whoosh from under the Dash when pushing on
the peddle. older guy at NAPA said sound normal.

I did run a vac gauge inside the cab to watch the vacuum.
Apply brakes, and it would drop quite a bit, and was slow to
build back up.

I did swap in another good valve into the Booster where the Vac
line connects to see if it was leaking.

The Booster was replaced several years before so really didn`t
think it was the bad guy. I finally picked up a new (Rebuilt) one
from O`Realy`s. Kid looks up year etc.....gives me the box and
off I go. Double check the part# and what is in the box before
you leave. get home and was wrong one. Studs didn`t line up with
holes in FW etc... Finally get the right one.
6.5 miles back and fourth (home to store) adds up.

Take her for a spin and damn near hit the Windshield it worked so good.
Finally I get one that works, and no more Whoosh under the dash.

I did install a Stewart Warner vacuum gauge in the spot where the
Tach lives as I don`t have a Tach. Still need to get it plumbed up.
That is the best add on gauge to have.


Goat

 

Charlie,
I'm confused by this statement, "you apply the brakes and they are rock hard had like nothing there,"

do you mean you push the pedal and it goes to the floor or that you push the pedal and have no vacuum assist ??

my pedal is never rock hard, as in, I can't push it with my foot,
my pedal is always super soft and first push goes to the floor

 

I reread what I stated, guess that could be confusing.
First application when I said I usually just put a light drag on the peddle,
ease off, truck is slowing down because I down shifted into 3rd.
2nd application if I have to stop, is like no Vacuum assist. Vac is more or less gone.
I never had the peddle go to the floor.
Saying it was rock hard, pushing the peddle was like pushing on a brick.

Hope I explained my self well.

Bottom line I was loosing vacuum when pushing down on the peddle
probably at a faster rate than it should. I hooked up my HF Vac gauge, ran into the cab,
it was slow to recover, before the 2nd app. This was even after changing the pump to
a new one.

Edit: What was strange is the vacuum would come up to 22 on the gauge, and then
after applying the brakes, it would be something like 17 or so. Wouldn`t built up to 22.
Next time the vac would drop quicker using the brakes, but would build back to 22.
Something strange going on inside the Booster.

When I installed the "New/Rebuilt" Booster, that is when I had vac assist and the truck
sat down on it`s nose. It did not take much peddle to stop.
it was the brakes have been in the 8 years I have owned it, and think this is the 3rd? Booster
that has been on it....and 2nd Vac Pump.

I kept the old pump need to check to see if it is producing Vacuum, and how much. It may
not have one of the problems, and the Booster all along.

This was frustrating, I would have vac, then would not, and then kind of sort of have it.

Works great, when it seems like it takes longer to stop, I know the rear shoes need adjustment.

Edit: I was going through my pile of recipts. Back in 1/20/20 I replaced the Booster at 293875 miles.
I changed it this past spring maybe in June, my mileage now is 332K, so maybe take 6K off that, that
isn`t very good long givity on these rebuilt boosters.

Charlie

 

i heard of a super soft pedal/pedal going to the floor no one could diagnose that turned out to be a bad booster.
they replaced every part in the brake system EXCEPT the booster, because "a bad booster gives hard pedal".
the guy said screw it that is the only part left. put on a new booster and poof! had perfect brakes again.

 

thanks for the ideas, I took a look last night under the dash and the booster nuts are much easier to access than I remember ( or at least compared to my daughter's 89 Grand Wagoneer!)

once I get the mental strength up to tackle this I'll probably do both the booster and master cylinder together, I'll post once I can't get it back together

 

remember to bench bleed the master before installing in the truck

 

well the truck passed it's safety inspection today,

unfortunately ( or fortunately ) I didn't fix it, with other projects going on and not wanting to chase a problem I took it to the mechanic,

he just bled the brakes and the pedal came back ( at least enough to pass inspection ! ) they still don't feel GREAT but definitely get me stopped,

I'm just confused by how they could get air in the system and go from good to terrible so quickly,

.

 

when you use your brakes, they get hot
if any moisture is in there, it boils making it into a gas
the gas will condense to liquid but the o2 in air wont go back to liquid always

This is why we flush brake fluid

Then add in a nasty bump when stopping and your MC sucks in a lil air bubble

Think of a check valve, old rusty fittings can allow vacuum in ( air intrusion) but not allow 3000 PSI out