Hydromax brakes - rear dragging/don't release - bad master cylinder?

[Neal Strickberger]

1996 Beaver Monterey with Bendix (now Bosch) Hydromax (hydraulic boost over hydraulic), no ABS, about 60kMi, Magnum chassis with air ride, Cat 3126-300.
Great running, no issues, just put about 2000 miles on it on 3 short trips.

Short freeway run, driving fine, braking fine, smelled burning brakes.
Both rear brakes overheated, enough to melt the piston boots. Fronts seemed fine. IR temp gauge pointed at the disks (after about 45 minutes) fronts 100°F and 90°F, rears both about 400°F.

The upside: this short drive was the best possible scenario to have a brake failure, not on a road trip, no loss of control, no fire, now in a space with easy access to parts and tools.

Lifted the RV, with pedal released and engine off both rear brakes were applied enough to hold the wheels to prevent rotation by hand. Loosened a bleeder on one rear caliper and some fluid came out under pressure and rear the brakes released. Applied and released brake pedal, rear brakes stay applied again and didn't release until opening the bleeder. Fluid that came out looked clear, not cloudy, rear pads look about 1/8" wear from stock (replaced by previous owner 2-3 years ago 5kMi ).

Suspicion is the master cylinder, not the booster as pressure is held even with engine off (and backup pump) off and the fronts are not dragging. Maybe a clog in the lines, though the fluid looked good.

Does anyone have experience with dragging brakes on one circuit only?

Thanks!

[Marty and Suzie Schenck]

Neal, the slides that those calipers ride on (and the locking key) need to be lubricated with a HIGH TEMP grease regularly. If not, the parts rust and the calipers stick and will not return to a not pressure position. I was able to find a high temp spray and extended the nozzle tube so I could grease the slides through the holes in the wheels and never had to disassemble them again to grease. That was on my 1998 Monterey. Marty

[Gerald Farris]

Neal,
The hydraulic hose that leads from the frame to the rear axle is the first place that I would look, especially if if there is only one hose like most non-ABS automotive applications. The lining in the hose can fail and act like a check valve to fluid flow. Since you can duplicate the condition, it will be easy to loosen the line above the hose to be certain that pressure is being retained above the hose before you replace the master cylinder.

Gerald

[Neal Strickberger]

Problem was the frame to differential hose exactly as you proposed, when the upper fitting was loosened (not easy - required a bit of heat) the calipers stayed locked, loosening the lower fitting and they retracted.

Now to find replacement hoses - I'm sure these are standard - but can't find part numbers. Need the 3 rear hoses and will replace the fronts as well. Ideas?
If I can't cross reference will have new ones made up.

Big lesson here - old brake hoses can fail internally with no exterior indication - and lead to a seriously dangerous situation. Flush brake fluid every few years...

Thanks Gerald!

Neal,
The hydraulic hose that leads from the frame to the rear axle is the first place that I would look, especially if if there is only one hose like most non-ABS automotive applications. The lining in the hose can fail and act like a check valve to fluid flow. Since you can duplicate the condition, it will be easy to loosen the line above the hose to be certain that pressure is being retained above the hose before you replace the master cylinder.

Gerald

[Gerald Farris]

Neal,
There is almost no chance that you will be able to find a prefabricated hose to fit your application. The hoses were custom made at the chassis facility when your chassis was built and any part #s on the old hoses will be meaningless in locating a new hose. But, at least you know what the problem is with your brakes now.

Gerald

[Neal Strickberger]

Gerald,
I expect you're right, though there are other RV and bus chassis that use disk brakes on an Eaton differential, searched and called but none seem to be the right combination of fittings. So I got replacement hoses made (at Pirtek SFO - never use them! Took 3 tries, sloppy work with kinks, expensive, don't care - next time will find a truck shop with old guys).

Reinstalled new hoses, same issue. Crack the connector at the master, brakes release. Very strange, it seemed repeatable that the center hose was holding pressure before, now at the master? Checked the brake switch, backed it out to make sure it wasn't preventing the pedal from releasing. No issue on the fronts.

Bought a HydroMax master (2" bore, remote reservoir) for $299 from Industrial Brake and Clutch (best price I could find), will see if this solves it.

Neal,
There is almost no chance that you will be able to find a prefabricated hose to fit your application. The hoses were custom made at the chassis facility when your chassis was built and any part #s on the old hoses will be meaningless in locating a new hose. But, at least you know what the problem is with your brakes now.

Gerald

[john brunson]

Please share contact data with industrial brake & clutch

[David T. Richelderfer]

I found this on their website...  San Antonio, TX.

http://www.industrialbrakeandclutch.com/contact-us.html

[Joel Weiss]

Over the past two years we've replaced all six of the brake lines on our MH.  As I had posted last year our rear lines were causing the drive axle brakes to lock in a manner very similar  to what has been posted in this thread.  This year we found a rusted through spot in one of the front lines and had those replaced.  Those front lines are not easy to get to so be prepared for a difficult job.  At least I feel good that the brakes should, hopefully, work, as designed, for the next couple of years.

[john brunson]

Joel,

where did you obtain the hoses?

/john

[Joel Weiss]

The hoses were fabricated by shops in Elkhart and San Antonio where we had the work done.  They can be made in a couple of hours and cost ~$50-100 each.