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Old 10-04-2017, 08:28 AM   #1
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Brakes overheating

Have a '97 Winnebago Itasca with the GM P37 chassis, 7.4L turbo diesel.

Severe brake overheating on both front wheels. Back duals OK.
Can drive it only a few miles on open road and temp. on the front 2 brakes
will reach 300 degrees.

A transmission/brake shop had it for 2 weeks and claimed they did all they
knew to do, finally had me come get it and said "first time we ever failed to
find the problem".
Any ideas?
Thanks for your help,
Phil
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Old 10-04-2017, 08:44 AM   #2
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Phil;

Are you brakes air or hydraulic. If they are hydraulic I have an idea I will share. I will wait for your response until I know what type of brakes you have.

Don
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Old 10-04-2017, 08:58 AM   #3
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Correct me if i'm wrong -- this is what I recall on the P series.

Your MH has hydraulic brakes. Disc in front and drum in rear. The power assist for braking is done by "Hydro Boost" (similar to the vacuum power booster on your car, but using hydraulic pressure from power steering pump).

If the above is incorrect - then ignore everything below

See this link for diagnosing the Hydro Boost. Especially the apart about "slow release". If the brakes are releasing slowly or dragging, they will get hot.
Hydro-Boost Power Assist Systems: Operation Diagnosis and Repair

Another thing that can happen (although usually only one brake gets hot - but you could have two bad lines) is that the rubber brake lines from chassis to brake caliper tear internally. They look good on the outside but are damaged internally. Then when you step on the brakes this internal tear acts like a flap and prevents all the pressure from relieving when you take your foot off the brake pedal (the pressure eventually goes to zero but more slowly, so the brakes heat up). You can check this by jacking up the wheel and spinning it before and after stepping on brake to see that is free.
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Old 10-04-2017, 10:53 AM   #4
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Is the pedal hard and right at the top? I have had this problem when replacing the master cylinder. It seems the new master cylinders have the port spacing a few .001 off and the brakes hold some pressure when the pedal is released, to fix it I placed one flat washer on each of the 2 bolts holding the master cylinder on to increase the pedal free travel. The washer goes between the frame mounting the master cylinder and the cylinder, placing the cylinder forward about .060. This was on a p-30
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Old 10-04-2017, 02:10 PM   #5
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Brakes overheating

The brakes are hydraulic; disc in front, drum rear.

Brakes work fine stopping vehicle; no hard nor soft spots in pedal travel.

We will review procedure for checking proportional valve., also check
Hydro-Boost power Assist System.

Also, based on another recommendation, will bleed all brakes and use
synthetic fluid as replacement.

Thanks for your ideas, and will advise you what problem finally turns out to be.
Phil
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Old 10-04-2017, 02:51 PM   #6
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Quote:
Originally Posted by arktvlr View Post

Also, based on another recommendation, will bleed all brakes and use
synthetic fluid as replacement.

Thanks for your ideas, and will advise you what problem finally turns out to be.
Phil
I would advise against going to the synthetic fluid. They should, and have worked just fine with DOT 3 fluid.
Checking the lines for restriction is a good tip. Also, have you done any work on the system lately?
Happy Glamping.
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Old 10-04-2017, 03:37 PM   #7
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Quote:
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I would advise against going to the synthetic fluid. They should, and have worked just fine with DOT 3 fluid.
Checking the lines for restriction is a good tip. Also, have you done any work on the system lately?
Happy Glamping.
I agree - do not switch to synthetic fluid (I assume you mean DOT5).

It is not worth the trouble. DOT 5 can not be mixed with DOT 3 or 4, so you would have to completely flush the system, and DOT 5 is hydrophobic which means water/moisture doesn't disperse in the fluid and will form pockets. Not a good thing in a brake system. DOT3, DOT4 and DOT5 - DOT3, DOT4 and DOT5 | HowStuffWorks
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Old 10-04-2017, 03:49 PM   #8
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Brakes overheating

Have bought Valvoline synthetic DOT3 & 4 (on the label).
Any technical reasons why we should not use it since we have it?
Thanks,
Phil
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Old 10-04-2017, 06:35 PM   #9
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The Valvoline brake fluid will be fine, the prior post are thinking of silicone dot 5 fluid, which is a no-no.
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Old 10-04-2017, 06:50 PM   #10
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I had a similar problem, I had worked on the Motorhome, my front brake was dragging and overheating, I would bleed pressure off the wheel cylinder and it would work until I used they would start dragging again. It turned out that I had kinked the line, fluid would pass through the kink but would not release.
I would look for a damaged line near where repairs were made.
I have heard that flexible lines will deteriorate and plug the lines.
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Old 10-04-2017, 07:14 PM   #11
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For sure jack up front wheels, even if you can only jack one side up at a Time, the wheel should spin fairly easy, have a helper mash brake and release, wheel should spin freely again.

Air in brake system will not cause this issue, so bleeding wouldn't help unless you have a soft pedal.

That age of chassis, if there orginal, I'd be suspect of rubber flexible brake lines to calipers, as posted, may look good on outside, but not allow fuild to return..... could also be sticking calipers to slide, hopefully the brake shop you used check that,

Do not mix DOT 5 with anything, you can use the DOT 3/4 but 5 is not compatible.

If all this checks good, you'll need to look backwards towards the master cylinder, follow the brake lines to frame till you find pressure differential valve and/or porportioning valve.

Since you don't mention any power steering issues and unit stops ok, I'd not focus on the hydro-boost component.

You may want to take it to a med/heavy truck repair shop that can hook up pressure gauge at valves, and brake bleeder screw to be sure .....but usually the caliper hose and slide are the 1st place to look.

Good luck!
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Old 10-04-2017, 09:42 PM   #12
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Quote:
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,,,I have heard that flexible lines will deteriorate and plug the lines.,,,
My first thought considering the age of the chassis. The flex lines can collapse internally. During braking they work fine but will limit the return flow of fluid and release of the caliber. Externally the lines will look fine but internally not so good.

And stick to the original brake fluid type.
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Old 10-05-2017, 07:51 AM   #13
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Use a caliper spreader to push back the piston. They should move pretty easy. If you have to use a lot of force the rubber brake hoses are most likely collapsed .
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Old 10-05-2017, 11:32 AM   #14
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cheap fix is to replace the calipers and lines. i would also look at the brake proportioning valve., or some call it the equalizer, or splitter. this valve equalizes the front to rear brake pressures. these sometimes plug up from old fluid coagulation.
they are cheap and easy to replace. just do a good job bleeding. this may sound like the shotgun approach, but under the circumstances and the age of the coach, it wouldnt hurt to do it anyway.
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