Brake pedal goes to floor...

[golfcarr]

2004 1500 2wd 4.7 SLT

New to me truck (daughter has been driving) recently noticed the brake pedal goes to floor under light pressure while engine is running. With a normal press on the pedal (harder) the pedal is normal until the truck stops, then pedal sinks to floor while sitting still.
In other words, the brakes currently work as they should, problem is the pedal goes to floor and will not pass inspection. Only changes that have been made over the last 4 months are removal of an electric trailer brake controller. should not have any affect (or effect?)

Things I have done:
Manually bled the brakes with person pumping and holding while Im under the truck cracking the bleeders.
Visually inspected hoses and calipers for leaks (none)
Replaced master cylinder (twice... in case one was defective - auto parts store reluctant to swap)
Bled system manually, have run about 5 qts of brake fluid thru the system.
Purchased Mitivac pump and tried to bleed that way.... no difference.

Symptoms remain same, brakes work as they should, but pedal STILL goes to floor.

What am I missing other than a trip to a PROFESSIONAL brake technician ?

 

[Later2u]

The only thing left is the brake booster. Not sure what is needed to test it, but that would be the final thing to check before seeking help.

 

[SYKRAMMAN]

It definitely sounds like the booster is loosing vacuum and is the issue from how you explained it.


(Found this on the ol’ inter web cause I’m lazy lol)

For a simple check of brake booster operation, make the following tests.
Run the engine for one or two minutes, and then stop it. Depress the brake pedal several times at normal foot pressure. If the pedal goes down further at the first time, but gradually rises after the second or third time, the brake booster is functioning properly. Go to step 2.

With the engine stopped, depress the brake pedal several times. Depress the brake pedal and start the engine. If the pedal goes down slightly, the booster is in good condition. Go to step 3.

With the engine running, depress the brake pedal and then stop the engine. Hold the pedal depressed for 30 seconds. If the pedal height does not change, the booster is in good condition.

 

[golfcarr]

Thanks for the replies,
I will definitely perform those test on the booster (as they cost zero dollars....)
However, I have always been under the assumption that the booster assist with making the pedal "softer" or easier to press. So, when my pedal presses too easy or too far, I would tend to think the booster is working TOO good...

any other input is welcome ( I will keep this thread up to date with my findings and the ultimate solution for other who may have this problem down the road - because I have not found any answers on the entire world wide web)

 

[golfcarr]

Results...

According to the test- brake booster is good.

 

[golfcarr]

More info

After testing brake booster as described...
went for a test drive on the back roads, cruising at 40mph slammed on brakes:
Here is what happened (twice) the nose of truck dives down, feel pulsing in the pedal (maybe 1 second) then the back tires lock up and skid to a stop leaving some cool looking skid marks on the pavement.

Then I drive home and symptoms remain the same.

 

[Jason G Slabaugh]

If it has rear drum brakes, they are possibly out of adjustment. There could be a small leak from somewhere in the brake lines somewhere, along the frame. A bad brake booster usually gives you a rock hard pedal and hard to stop symptoms, not a dropping pedal. A bad master cylinder will cause a pedal drop. If you do have drum brakes on the rear could have a bad wheel cylinder. Start clamping off brake hoses one at a time and press the brake pedal each time. If it changes to rock hard and stays there thats where the problem area is.

 

[golfcarr]

It has disc brakes front and rear (drum emergency brake)

Advise on how to clamp lines without damaging and where to clamp it. Something like a small “C” clamp ? And clamp it closer to truck or closer to caliper or does it matter?

 

[golfcarr]

Was messing around with it out in the shop and pulled the top vacuum line off the brake booster vacuum check valve (?), the booster sucks hard - line small line i pulled dont have any suction, but makes the brakes WAAAAY better ! Not right, just way better. Cover the valve with my finger, brakes go to floor. take finger off, brakes get good again. There is another larger vacuum line that goes into that valve that I did not remove - I suspect it has all the suckage.

(yes, I think its like a 15 dollar part - I plan on replacing it just for kicks and grins)

Will let yall know

 

[golfcarr]

taking to a local mechanic... stay tuned

 

[golfcarr]

WELL . . .



Can anyone tell me symptoms of a faulty ABS module/HCU ?
Mine is making a funny sound (like a whistle) as the pedal gets about 3/4 down and make a different sound as the pedal comes back up at the same point...

 

[Simpsj2]

Having similar issue. changed calipers and then bleed lines but brake goes to floor and no engaging of calipers. Have air in lines but cant get out I presume

 

[golfcarr]

UPDATE... Saturday Nov 24th.

I am happy and frustrated at the same time.

I went to the u-pull junk yard and got a ABS module/HCU (29.99 and 50% off for black friday) Which i highly recommend if for not other reason - you learn how to remove stuff without F-ing up stuff on your own truck...

I unhooked and removed battery (required to get to bolt anyway)
Let the truck sit for 30 minutes without battery
removed inner fender well (required to get to other bolt)
removed electrical connector
removed lines to HCU

Installed new used part, put everything back together,
bled the brakes, yet again (im sick of bleeding brakes)

the pedal NO LONGER SINKS TO THE FLOOR ! ! ! (happy part)

the problem, now the ABS light in the dash blinks on and off... (frustrated part)

 

[Jason G Slabaugh]

Throwing an ABS code, get it scanned. Something must not be compatible, or missing with replaced module, or it had an issue to begin with

 

[ljonSFCA]

same issue here.. also started after using a cheap trailer brake controller. but I'm also getting strange hissing sound in the cab near the pedals.. maybe unrelated. did you ever solve the ABS light??