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Old 08-13-2022, 06:51 AM   #1
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Brake pedal goes to the floor

We have a 2015 E450 chassis with brake problems. Both the ABS and brake light are on. The pedal is solid until you start the engine and then it will progressively go to the floor. No visible leaks, no loss of fluid at the master cylinder, just flushed the system with new DOT 4, cleaned the ABS sensors on the front wheels. All of the brakes at the wheels look correct with no visible issues. The ABS sensor wiring looks good as well.

I am suspecting an ABS pump issue but that is just a guess at this point. Any advice would be appreciated. Thanks.
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Old 08-13-2022, 06:57 AM   #2
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Did you just flush the system then the problem started or was the problem there and that's why you flushed the system?
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Old 08-13-2022, 07:07 AM   #3
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It only takes the tiniest of leaks. Make sure all your bleed screws are gutentite. Sometimes you get a bad bleed screw that won't reseat well. Check it over and good luck!
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Old 08-13-2022, 07:08 AM   #4
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Did you just flush the system then the problem started or was the problem there and that's why you flushed the system?
The brake problem occurred and then I bleed and flushed the system to make sure it wasn't just an air problem.
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Old 08-13-2022, 07:19 AM   #5
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The brake problem occurred and then I bleed and flushed the system to make sure it wasn't just an air problem.
Suspect you have a small leak somewhere. If you live in a salty area begin by looking where lines are clamped into place. The salt hides in these clamps or looms. However, yours is a newer coach and this would be more common on older vehicles. Still.....
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Old 08-13-2022, 07:23 AM   #6
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Suspect you have a small leak somewhere. If you live in a salty area begin by looking where lines are clamped into place. The salt hides in these clamps or looms. However, yours is a newer coach and this would be more common on older vehicles. Still.....
Thanks but there are no leaks nor loss of any fluid
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Old 08-13-2022, 07:28 AM   #7
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Thanks but there are no leaks nor loss of any fluid
I am speaking from experience. I too thought the reservoir was staying full. As I said, it only takes a miniscule leak for the pedal to drop. Good luck!
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Old 08-13-2022, 08:15 AM   #8
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A defective master cylinder (MC) can and will leak internally. That means when you push the brake pedal down the seals inside the MC don't build pressure because the fluid is leaking past the seals.

In fact that was always a sign of a bad MC when pressing down on the brakes the pedal would be somewhat hard but then slowly fall to the floor. The sign of air in the system was a soft pedal. Fluid pressure would build up to a point, but the air would compress and the pedal would be soft.

Sometimes one could give the pedal several quick pumps and the pressure would build but again if constant pressure was applied it would again slowly drop to the floor. Over my 40 years working and teaching this stuff I've seen a dozen or more failed MC with the same symptoms. No external leaks but still no consistent pressure.

There may well be another reason for your symptoms but I'd find it strange based on no external leaks but still no buildup of brake pedal pressure. My diagnosis still says to follow the symptoms.

I've rebuilt many a MC with good results. A defective seal is fairly easy to replace. Todays rebuilding kits may provide a new internal cylinder with seals. That stuff changes like the wind. If the aftermarket folks can increase their sales by providing most of the parts in a kit they'll do it.

Years ago when we started seeing a lot of front CV joints failing we first took the axle out pulled it all apart, repacked and replaced the rubber CV boot around the CV joint. Within a few years it was easier and cheaper to buy a rebuilt front axle.
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Old 08-13-2022, 08:30 AM   #9
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A defective master cylinder (MC) can and will leak internally. That means when you push the brake pedal down the seals inside the MC don't build pressure because the fluid is leaking past the seals.

In fact that was always a sign of a bad MC when pressing down on the brakes the pedal would be somewhat hard but then slowly fall to the floor. The sign of air in the system was a soft pedal. Fluid pressure would build up to a point, but the air would compress and the pedal would be soft.

Sometimes one could give the pedal several quick pumps and the pressure would build but again if constant pressure was applied it would again slowly drop to the floor. Over my 40 years working and teaching this stuff I've seen a dozen or more failed MC with the same symptoms. No external leaks but still no consistent pressure.
That makes sense. I know that could put the brake light on but could it trip the ABS warning light as well? We have 2 code readers but neither read ABS faults. The readers just say no stored codes and no pending codes. Thank you for your suggestion. I thought about getting 2 side caps and see if the pedal stills goes down with the master cylinder lines blocked. That would pretty much prove it is the master cylinder. I am going to try to bleed the ABS unit as that would seem next easiest. I just got a flare wrench on it and I can get the outgoing lines loose. Again, thank you very much.
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Old 08-13-2022, 09:57 AM   #10
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We have 2 code readers but neither read ABS faults. The readers just say no stored codes and no pending codes.
Get Forscan. It can read all of the codes that are stored in all of the modules. It is available for download at www.forscan.org. Their page also shows how to select an OBDII adapter to plug into the vehicle. I bought mine from Amazon for less than $30.
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Old 08-13-2022, 10:02 AM   #11
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I've been out of teaching for 17 years. The ABS systems were around before I retired but I'm sure they are a lot more sophisticated these days and only a current TECHNICIAN would know how to answer the total involvement to the brake system.

The ABS system basically uses a sensor at each wheel which monitors the wheels speed. The ABS controller keeps each wheel close to but not locked up by modulating the fluid pressure. When your wheel locks up the only friction being created is the rubber against the road. The skidding wheels are basically uncontrollable.

I checked my current automotive book and little is mentioned of much connection too the ABS and the regular brake system. However, if there's an issue with the MC (low fluid, low pressures, no pressures) it will be reflected by the ABS system and it may set a code. There may also be some other connections in the newest systems.
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Old 08-13-2022, 10:27 AM   #12
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So we get heck from time to time for relating our related but not specific to problems in helping one another. But here goes.
Most of my vehicles are of northern culture and are/were driven through the salty winter slush. So I've had three of them fail with rusted brake lines. The most recent was the '05 Escalade. Pedal almost to the floor. Service brake message and traction control disabled warning. Ancel reader was used to "AUTOBLEED" the brakes after finding no stored codes. Pedal stayed the same and messages remained. Did a 30 minute battery disconnect. No joy. All the while the brake fluid reservoir appeared to stay topped off. In desperation, I took it to the mechanic who found and replaced two rusted lines much as how I described in a previous post. So, maybe someone lurking around the brake posts can learn something. I am done. Bye now!
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Old 08-13-2022, 10:46 AM   #13
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So we get heck from time to time for relating our related but not specific to problems in helping one another. But here goes.
Most of my vehicles are of northern culture and are/were driven through the salty winter slush. So I've had three of them fail with rusted brake lines. The most recent was the '05 Escalade. Pedal almost to the floor. Service brake message and traction control disabled warning. Ancel reader was used to "AUTOBLEED" the brakes after finding no stored codes. Pedal stayed the same and messages remained. Did a 30 minute battery disconnect. No joy. All the while the brake fluid reservoir appeared to stay topped off. In desperation, I took it to the mechanic who found and replaced two rusted lines much as how I described in a previous post. So, maybe someone lurking around the brake posts can learn something. I am done. Bye now!
I get it. We live in Florida now and this is a Florida rig but prior to 2019, we were full time in western Pennsylvania on 67 acres out in the boonies with lots of snow, ice, and salt. Added to that I was a tractor-trailer mechanic in the late 1970s and early 80s and seen some diverse issues over those years. The condition under the RV is very clean and I would be surprised with any rusted lines and no indications of leaking fluid but anything is possible. It just seems the more likely now is an issue with the master cylinder and/or the ABS system. We do appreciate your suggestions and thank you for taking the time to do so.
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Old 08-13-2022, 11:16 AM   #14
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I would double triple check that its not the ABS pump, because they are expensive. It sounds like a MC problem. Your idea to block the lines will verify if it is. Rock Auto probably sells a MC for $200 I bet.
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