Brake pedal sinks all the way to the floor

[jja]

Just noticed that today during the yearly technical test. I was doing the brake tester and it was all nice and fine with the front wheels. Then we went to the rears, the rollers started spinning the tires and I started slowly depressing the brake pedal while looking at the tester dials. At some point the readings stopped going up and remained level while the brake pedal kept going down until it literally reached the floor. The braking force was just enough to pass the test but it really didn't feel right. No leaks are visible on the brake system and fluid level is normal.

I never had a brake pedal do that on any vehicle so far. I cannot remember whether I ever tried it on the KJ before.In normal driving that never happens. The truck comes to a halt and stays that way at stoplights (it's an auto) with the pedal feeling nice and firm against my leg. So I never tried to push it further down. I even did a real emergency braking the other day and I am positive the pedal never reached the floor.

So, I am kinda dumbfounded now. Brakes seem to work just fine but the pedal behavior does not feel normal. Is it something all KJs do? It's a '07 3.7 with ABS/ESP.

 

[daves06lrenegade]

My 06 was a "salvage" vehicle and had damage to the left rear brake line... After replacing the line and manually bleeding the brakes I had a soft pedal that would ALSO stop the Jeep but would drop to the floor if pressed harder... Turned out that the ABS pump had air in it that normal bleeding would not remove...I didn't want to take to a dealer ($$$) and found that the ANCEL AD-610 scanner/reader could exercise the ABS module I bought it for $110 on Ebay... Ran the brake system through the bleeding process and I then had firm brakes ... DId you have brakes worked on recently? You might not have noticed the soft pedal...
Dave

 

[Billwill]

Weird that this only happens only when the rear end was on the rollers.

Could be an ABS problem....the rear axle turning while the front wheels are stationary could confuse the ABS system!

Maybe the brake booster has a problem.
Check the vacumn pipe going from the intake manifold is firmly on at the Brake Booster and there are no holes or tears in the pipe.

Normally you test the Brake Booster by depressing the brake pedal firmly while engine is OFF, then while keeping your foot firmly down on the pedal start the engine....brake pedal should drop by about a half inch at most under your foot!

If you go for this test again...subtly pull up the handbrake while testing the rear brakes

 

[turblediesel]

I've seen pre-ABS systems act like that due to worn out master cylinders. The rubber parts wear, harden, or disintegrate and the fluid just moves around without exerting any force. Called an internal leak.

 

[jja]

So, has anybody tested that actually? Would the pedal reach the floor when applying constant pressure on your trucks? It does not drop down immediately, it takes a sensible amount of time.

No work has been done on my brakes for more than an year now.

Weird that this only happens only when the rear end was on the rollers.

The front end exerts enough force to bring the rollers to a halt long before the pedal reaches the floor.

I've seen pre-ABS systems act like that due to worn out master cylinders. The rubber parts wear, harden, or disintegrate and the fluid just moves around without exerting any force. Called an internal leak.

I am kinda thinking about that yeah. Either that or the ABS pump doing the same.

 

[JeepJeepster]

I assume with the rear wheels spinning, the ABS should have been going crazy since it thinks the rear tires are spinning while the front is not spinning. Im rather confused by the whole 'Technical Test.' Is the Jeep running during this?

The only thing that can make the pedal go to the floor is a bad master cylinder. What tires are spinning n such does nothing. This is all assuming that the ABS is not activated.

 

[jja]

I assume with the rear wheels spinning, the ABS should have been going crazy since it thinks the rear tires are spinning while the front is not spinning. Im rather confused by the whole 'Technical Test.' Is the Jeep running during this?

The only thing that can make the pedal go to the floor is a bad master cylinder. What tires are spinning n such does nothing. This is all assuming that the ABS is not activated.

I think they are spinning the wheels in different directions. That's why the ABS does not engage. The Jeep is running during the test and I am sitting in the driver's seat. No ABS lights or anything.

I really wonder how it worked with the front brakes though. It is one and the same MC, right?

 

[BazW]

Turblediesel has the answer I would say. Fluid bypassing the rubber seal in the master cylinder

 

[Billwill]

I think they are testing the braking performance.

In the old days in SA the Tester would drive the vehicle at a certain speed then hit the brakes. He would then measure how far the vehicle went before stopping to decide if brakes are within specs.

 

[jja]

The more I play with it the more it looks like something that happens by design.

When the engine is off I cannot get the pedal to the floor for sure. An internal leak in master cylinder would have allowed me to do that.

I can only do it with the engine on and only if I press it with constant significant effort for like 7-8 seconds. That's what happened during the test too. Under normal usage that, i.e. while driving or sitting at a traffic light that never happens. I.e. I press the pedal strong enough for the truck to come to a stop and it stays firm against my foot while the truck stays in place. It is only if I keep on pressing with much larger force than needed that I can bring it to the floor.

So, can some of you, guys, make this test on your trucks and let me know how it works out? Preferably it should be a gaser since diesels are different in that department.

 

[daves06lrenegade]

After bleeding my 06 the brake pedal does not go to the floor when I'm holding it with the engine running... Sounds like the vacuum power brake module may be failing... and I don't think the master cylinder is at fault as you can't get the pedal to the floor with the engine off (no vacuum) and defaulted manual brakes...

 

[jja]

After bleeding my 06 the brake pedal does not go to the floor when I'm holding it with the engine running... Sounds like the vacuum power brake module may be failing... and I don't think the master cylinder is at fault as you can't get the pedal to the floor with the engine off (no vacuum) and defaulted manual brakes...

OK so there really is a problem then and yeah it looks like the vacuum booster might be the problem.

So, I tested the vacuum booster as suggested above, i.e. holding the brake pedal while starting the engine. If the engine has been running in the last couple of hours the pedal would sink a bit when the engine starts. If it has last been started a day ago that does not happen. Is it the same for you guys?

 

[DadOSix]

I’m not sure myself. Used to be the only thing a bad booster would do is make it seem like you have manual brakes. The ‘boost’ is supplied by vacuum applied to a spring loaded diaphragm. All the boosters that I replaced on the heavier trucks were failed diaphragms. You could hear the booster hiss like a big vacuum leak when the brake pedal was depressed. Not sure if this is helpful in your case.