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Old 08-02-2011, 02:46 PM   #1
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brakes go to floor

I dont understand.. On my 72 m300 chassis, I did the new brake, bled them with no problem, seemed like the pedal was getting hard. so i started it and the pedal went to the floor,shut it off and still goes to the floor... figured it was the powerbrake. seems not to be the problem. theres no leaks, master is full, brakes are adjusted... im lost..
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Old 08-02-2011, 03:00 PM   #2
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My guess would be the master cylinder has failed.
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Old 08-02-2011, 03:02 PM   #3
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Quote:
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My guess would be the master cylinder has failed.
I will replace it tonight, hopefully that is problem
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Old 08-02-2011, 05:20 PM   #4
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Just wondering if you should bleed your master brake cylinder and brake lines one more time to make sure there is no air in the lines before buying a new master brake cylinder
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Old 08-02-2011, 05:28 PM   #5
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As Aguyfromcalg suggesed if it were me I'd bleed the system again - starting with the rear wheels. If that doesn't do it I'd suggest you disconnect the out pressure line from the master cylinder to see if fluid comes out when the pedal is pushed. They used to sell master cylinder rebuild kits but I don't know if they are still available.
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Old 08-03-2011, 06:21 AM   #6
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My guess would be the master cylinder has failed.

That would be my guess too. Could be the seals are allowing the brake fluid to leak past instead of going through the brake lines it just cycles back into the master cylinder, that's why you don't see a it leaking.
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Old 08-03-2011, 10:13 AM   #7
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On a motor home the brake system is a tad more complex (generally) than on a car.

That said: I know how to bleed the brakes on a car.

The first time they need it on my motor home I'm going to have to watch a professional do it.

What am I saying? You missed something... (And I know not what it was)
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Old 08-03-2011, 10:26 AM   #8
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On a motor home the brake system is a tad more complex (generally) than on a car.

That said: I know how to bleed the brakes on a car.

The first time they need it on my motor home I'm going to have to watch a professional do it.

What am I saying? You missed something... (And I know not what it was)
Well the system is relatively close to a car, its only an m300 chassis, so essentially a large dually truck, Im going to install the new master thursday (as I have current obligations tonight) There is a power booster located directly on the master with the master facing reverse direction (as you would think of it). the booster (according to my knowledge) does not contain any fluid and is basically a glorified push rod for the master cylinder.
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Old 08-03-2011, 10:36 AM   #9
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screen shot from manual, sorry about size

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Old 08-03-2011, 06:59 PM   #10
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Your single booster Master-Vac system (not the later dual cylinder Hydrovac system) is very simular to a car system. While your problem could be a sticking master cylinder piston it sounds more like the control valve in the booster is not releasing [Test 2, sub-paragraph (c)] on the page you posted. Here is an old Popular Mechanics page that describes the operation of a Master Vac unit like yours.
Popular Mechanics - Google Books

One reason I think it is the booster is that you were able to pump the breaks up initially and your problem occured once you applied vacuum to the booster. While the extra booster force could have caused the master cylinder to bind up, the typically failure would be the booster.

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Old 08-04-2011, 09:13 PM   #11
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Dave,
You were right it was the booster. I'm going to call alretta truck tomm to see if they stock them. I would like to rebuild but this one looks pretty chewed up. So I'm going to do new booster and master cylinder. If alretta doesn't carry them, anyone know where I can find one?
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Old 08-05-2011, 11:38 AM   #12
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NAPA
With Master Cylinder: NBB 50314 ($182)
https://www.napaonline.com/Catalog/C...+50022+2022002





W/O Master Cylinder: NBB 5473143 ($146):
https://www.napaonline.com/Catalog/C...+50022+2022002



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Old 08-05-2011, 12:06 PM   #13
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NAPA
With Master Cylinder: NBB 50314 ($182)
https://www.napaonline.com/Catalog/C...+50022+2022002





W/O Master Cylinder: NBB 5473143 ($146):
https://www.napaonline.com/Catalog/C...+50022+2022002



Dave
Youre always on top Dave. I actually just ordered one from autozone
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