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Old 10-18-2004, 02:24 PM   #1
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Exclamation Help - MH Brakes Locked Up!

Dennis (87MH) is having some trouble on the road. Just got off the phone with him and he asked me to post this message seeking answers to some issues he had with his MH brakes locking up on the trip from Texas to Georgia (he did make it to the campground finally).

Here is the background:
  • 1987 345 MH
  • Recently replaced the rear brake discs, calipers, shoes along with new flexible lines and a new master cylinder (read about it here)
Here is the issue:
  • Rear brakes gradually locked up to the point where the MH could not longer move. He "unlocked" them by releasing line pressure from the rear caliper bleed valves.
  • Next time the problem occured it was with the fronts - fixed it the same way
  • It continued to happen on the fronts over and over again.
  • He avoided hard braking and supplemented braking with the Toad brakes and the MH parking brake but he mentioned that the issue did not seem to coincide with actually using the service brakes on the MH. In other words - if he never touched the brakes at all they would eventually lock up
His questions are:
  • What could be causing this?
  • Is there a simple mechanical solution?
  • Is this related to the master cylinder?
  • Should he just replace the master cylinder with a a GM Factory cylinder?
I'll be calling him back this evening so any and all ideas and suggestions would be appreciated. I'll also (as many of us) be seeing him this weekend so we can bring parts and tools if necessary - but knowing Dennis he won't want to wait until then
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Old 10-18-2004, 03:07 PM   #2
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I'm sure if it had a 'Brake Lock' he would have mentioned it. But on the off-chance that it does have a brake lock, that would be the first thing to check.

Otherwise, it's got to be the new MC.

Can't be dirt or a pinched brake hose since he did a complete brake overhaul.

Could the booster be busted?
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Old 10-18-2004, 04:07 PM   #3
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My first guess would have been the flex lines, but since they've been replaced I agree with the master cylinder diagnosis.

The MC pistons have to retract to a certain position to open two very small ports to the reservoir. In this idle position, fluid can pass either way - from reservoir to cylinder or from cylinder to reservoir. Rest pressure in disc brake circuits is zero. If the pistons don't come back far enough the ports remain blocked, the pressure can't bleed off, and as the fluid expands the brakes lock up.

The new MC may be slightly off in some dimension, or it may have been assembled incorrectly at the factory. Also, if the pushrod is adjustable, it may be misadjusted. If I recall correctly, I read on a GMC MH website that pushrod length differences as small as .010" (!) were critical on those vehicles.

Bob McKeown
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Old 10-18-2004, 05:07 PM   #4
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Disclaimer - I know 3/4 ton trucks, not motor homes!

MC to me simply because it happened on front & back. Unless there is a proportioning valve between the MC and the brakes, how could the same problem affect both the front & rear brakes?

Since 1968, Federal law says front & rear brakes are supposed to have independent systems. The MC is the only common link.

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Old 10-18-2004, 06:40 PM   #5
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I am not a certified brakeman, but my suspicion was drawn to question of what would cause the pressure to build when not applying the brakes...My first suspicion was the Master Cylinder. I like Bob McKeowan's description (above) of the MC function.

Another possibility could be the wrong sized drums and pads were installed. Check brake drums for proper thickness and roundness, and lining thickness. Also, if the drum is out of round, it will catch on the linings and cause what you describe.

Regarding the rear wheels, another thing that may help or be of cause is the emergency brake tightness of the cable, frays inside the cable hanging up, rust, and weak springs within the rear brakes themselves. That might be worth checking out as well.

On the "maiden voyage of the Oddysey", Josephhh and I experienced drag and brake lock on a single wheel on his 1979 MH. We did a complete overhaul of one of the calipers, but as it turns out the brake lines needed replaced due to the natural deterioratin of the interior lining of the brakelines--the juice could not flow back and forth properly. Unlike this situation our problem was isolated to a single brake caliper, so it doesn't seem to be a likely cause of the problem.

Best of Luck and BE SAFE!!!
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Old 10-18-2004, 06:41 PM   #6
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Only few possiblilites that could effect both systems at once.

1. Master cylinder or booster. Since it had a recent brake job there is often the possiblility that the master has traveld farther then normal and run over some crud in the end of the bore resulting in damaged seal or other problems. Whay may have happend is the crud is now restricting free movement of the pistons in the master and they are not fully returning to the point where they can bleed off the fluid like mcneon pointed out.

A second possible issue is there is a pin/rod in the booster that locates into the center of the master. If by some chance the booster retracted faster then master (goes back to curd in bore) that pin may not have centered and is actually applying the brakes a little. I actually have had this happen after a master cylinder was replaced on a company vehicle (I didn't do the work). That pin didnt center and it locked up the rear brakes.

The only other thing I can come up with is a reach. In a proper brake job they would bleed the system to the point of having new fluid in the whole system. Maybe a reaction of different fluids and it caused the rubber brake lines to swell internaly and pinching off the lines enough not alowing hte calipers to retract. Its a reach but it has happend.
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Old 10-18-2004, 06:46 PM   #7
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Quote:
Originally Posted by 59toaster
The only other thing I can come up with is a reach. In a proper brake job they would bleed the system to the point of having new fluid in the whole system. Maybe a reaction of different fluids and it caused the rubber brake lines to swell internaly and pinching off the lines enough not alowing hte calipers to retract. Its a reach but it has happend.
He replaced the brake hoses with teflon lined aftermarket parts.
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Old 10-18-2004, 06:51 PM   #8
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depending on whose MC was put in, a replacement should be able to be gotten at a dealer in GA. There is a Motohrome rv dealer just befor Chattanooga on I 75, and many Chevy dealers not too far from our camp spot. If it makes sense to go a bit out of the way...the Three Way Camper dealer in Marietta, was an Airstream dealer, and has a reputable service dept if needed. Just some tid bits incase they could be of some help. Plus with all of us up there this weekend, amybe we can assist in the diagnosis.
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Old 10-18-2004, 06:57 PM   #9
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All things seem to point to the MC. I agree in that it has affected both circuits and this is the only "new" part shared between the systems. Certainly something easy to fix.

Also, a few points of clarification in the interest that it jogs some more ideas. The 87 345 has front and rear disc brakes. Only the tags are drum brakes (Dexter electrics). Also the parking brake is very different in that it's a drum brake mounted on the output shaft of the transmission. It's a drum brake (from a chevy pickup) and is cable controlled.

I also think Dennis completely replaced the brake fluid after the rebuild.

I relay all of this information to Dennis tonight. Thanks to all those who have posted. We've all either been in his shoes or will someday so keep those ideas coming!
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Old 10-18-2004, 07:04 PM   #10
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Alan,
Agreed! If you are going to have a problem it certainly helps to do it when you'll be close to parts sources and surrounded by Airstreamers for the weekend!

BTW - if anyone has a brake bleeder perhaps you could bring it with you to CC this weekend. Dennis mentioned that he left his brand new super duper one in his garage...back in Texas.
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Old 10-18-2004, 07:31 PM   #11
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Quote:
Originally Posted by ALANSD
depending on whose MC was put in, a replacement should be able to be gotten at a dealer in GA. There is a Motohrome rv dealer just befor Chattanooga on I 75, and many Chevy dealers not too far from our camp spot.
There's a new Camping World Franchise in Chattanooga too.

Camping World Chattanooga, TN @ 6734 Ringgold Road
Chattanooga, TN 37412 1-866-999-1286 1-423-892-0144
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Old 10-18-2004, 09:01 PM   #12
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mcneon mentioned the push rod, reminded me I had the problem of brakes going to the floor after total rebuild took 3 to 5 brake applications for the failure to show up would pump up and then fail again. I finally found a spacer/washer missing between the mc & firewall. In effect the push rod distance was changed by a few thousands of an inch and created a big problem. The pushrod distance is very critical. Having said that I would suspect an improper MC rebuild first.

Garry
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Old 10-19-2004, 09:31 PM   #13
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Guys,
I passed on everyone's information and ideas to Dennis today. Many thanks from Dennis to everyone for their input. I think he plans to try adjusting the pushrod on the booster first and then the backup plan is to replace the MC.
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Old 10-20-2004, 09:05 AM   #14
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Thanks for the update on Dennis. I was impressed with the outpouring of knowledge on this subject. This is a great website with great people!
Best of Luck to Dennis too!
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