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Old 05-25-2017, 08:27 PM   #1
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Brake fluid leak @ master cylinder

While descending a pass from 7000 feet to 4000 feet the ABS light on the silverleaf came on. While making the final approach into town the brake pedal became quite soft and almost went to the floor. After getting settled in, I extended the generator compartment and saw brake fluid on top of the generator and on the inside of the front cap. I also saw brake fluid on the fittings and also on the opaque reservoir up higher. Both caps were in place and there was fluid in both reservoirs but down maybe an inch from the top. Has anyone else experienced a master cylinder leak like this? Is it time for a new master cyl to cure the problem? Thanks. We're currently in Worland, WY heading to Yellowstone.
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Old 05-25-2017, 08:35 PM   #2
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When was the last time you changed brake fluid/flushed system? Moisture build-up in system can cause all kinds of issues--especially if you "stress" the system, eg, stop and go traffic on a hot day or descending a mountain grade with not enough Jake brake and a bit too much service brakes.....
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Old 05-26-2017, 06:32 AM   #3
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It's been 3.5 years since the brakes were flushed. They'll need to be flushed again now and I might as well go ahead and get the 1.75 master at the same time, wouldn't you agree? Hard to examine the master without pulling it since it's mounted up so high.
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Old 05-26-2017, 06:35 AM   #4
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First, IMMEDIATELY clean the brake fluid off all painted surfaces.

As Old Scout said, if brake fluid is old/hydrated, you could easily exceed its boiling point under protracted brake application.

When the liquid turns to a gas, it can PUSH BRAKE FLUID OUT THE RESERVOIR BREATHER.

Next is to change the brake fluid. New fluid boils about 500 degrees F. Hydrated brake fluid boils at 286 degrees F.
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Old 05-26-2017, 06:38 AM   #5
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Quote:
Originally Posted by Janet and Todd View Post
It's been 3.5 years since the brakes were flushed. They'll need to be flushed again now and I might as well go ahead and get the 1.75 master at the same time, wouldn't you agree? Hard to examine the master without pulling it since it's mounted up so high.
Just because fluid came out the reservoir does not mean that the master cylinder is bad.

But, changing to the 1.75 master cylinder is still something you may want to do.
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Old 05-26-2017, 06:45 AM   #6
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So how do I clear the ABS light from the Silverleaf? Will it clear automagically when the fluid is flushed? And should I ask the techs to be careful about the sensors when removing the inside duals? And for next time... what is the never exceed RPM for the ISL? I used the high jake and was in forth. I was concerned that if I put it in 3rd that it would over rev the engine. Thank you one and all for your help with this.
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Old 05-26-2017, 08:07 AM   #7
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Cummins ISL (this for the 2003, but suspect the RPM's are the same):

Idle speed: 600-800.
Peak Torque 1200 lb-ft @1300RPM.
Peak HP 2100.
Governed RPM 2330.
Overspeed 2600.

So, to answer your question, 2600 RPM.

Suggestion for next time you find yourself in that situation:

Down arrow to the next lower gear. The Allison will NOT shift down until it will not overspeed the engine. Firmly apply the brakes (service brakes/brake pedal) until you are slowed enough to allow a shift to the next lower gear.

The correct gear for descending a grade is the gear (and use of engine brake) that holds your speed in EQUILIBRIUM. That is, you are not slowing down nor speeding up. That gear may be 2nd or 5th, depending on the grade.

You can safely use transmission up and down arrows as much as you want. Same with engine brake-- Off, low, high.
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Old 05-26-2017, 11:26 AM   #8
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Good info Wolfe10. Thanks for sharing. I apparently fried the ABS sensor on the drivers side rear. Hopefully I won't need new pads and or rotors.
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Old 05-26-2017, 02:01 PM   #9
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The manual for the 2005 recommends inspecting the brake fluid every 10K miles and replacing as needed. The "as needed" part is subject to the opinion of the person doing the inspection. Two coaches with the same mileage and the same age fluid could have fluid with very different characteristics depending on driving habits and other conditions.

My local shop for hydraulic work charges about $100 to flush out the old and replace with new. Doing this once a year may be overkill but I like to have my rig stop when I need it to. Going two years should be the max. It will be one of the least expensive things you will ever have performed, but may be the most valuable. At the same time, the other hydraulic components can be inspected.

Those who are mechanically inclined may wish to do the work themselves. While that sounds tempting, I usually have more important things to waste my time on.
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Old 05-26-2017, 06:06 PM   #10
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If Brake Fluid is escaping out the top of the reservoir it could be caused by your brake fluid being Hot.

Remember that Fluids Expand when they get hot. If the brake fluid reservoir was full when cold and the fluid gets hot, it will expand and overfill the reservoir and escape out the top through the vented cap.
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Old 05-26-2017, 06:23 PM   #11
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Yup--I am sure brake fluid can expand when heated but when water turns to steam--wow!
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Old 05-26-2017, 06:47 PM   #12
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Besides having the water in the fluid turn to steam and pushing the fluid out the master cylinder, you now have a compressable gas in your brake lines. That's what causes the low pedal you felt.

Once cooled down, things seem to go back to normal again, because the steam turned back to water.
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Old 05-26-2017, 06:50 PM   #13
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Great summary TB--explains exactly why the brake pedal goes to mush in this situation....
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Old 05-26-2017, 06:50 PM   #14
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Okay, they didn't have the correct cap replacement on their flusher so we had to the flush the old fashioned way. Got a lot of air out the rears and he was able to flush both ends of the rear calipers. There might be more air in there because the pedal does not feel as stiff as before. I'll repeat the flush asap at a for reals truck place and be more aggressive on the downshifting and taking it slower downhill. Thank you all for your generous time and advice.
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