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Brake seizing problem

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MWSMITH2:
     I have a 32 ft 1975 Landau class A motor home, built on an extended Dodge M500 chassis.  I am the 2nd owner and have been driving this vehicle fo 30 years.  Intermittently, while driving down the road, and with no brake pedal input, my brakes will start to slowly seize and slowly bring me to a full braked stop.  When I put it in park and step on the brake pedal, it is unmovable and rock solid.  The master cylinder cannot be compressed one bit. This seems to happen after some recent required brake action...like for road construction, but not always. 
    I have been fighting this problem for several years now.  I have changed master cylinders twice, both hydrovac drums with the “slave” cylinders...the front one twice, the metering/warning light Valve once,  and all wheel cylinders within the last two years. My dash vacuum gage shows good idle vacuum h18-23 inches) and the vacuum check valve seems to hold good.  None of this has remedied the problem.  It affects both the front disk and rear drum systems.
    The only way I found to get moving again (without manually getting out and bleeding the wheel cylinders) is to shut down the engine, wait a few seconds, then press hard on the locked pedal and restart the engine.  This causes the pedal to respond normally by moving down an inch or so and the backs will release so I can roll again. 
     The local “pros” don’t know what is the cause or they don’t want to work on something this old.  I did all my own maintenance until I turned 80 this year and got lazy.
     I would appreciate any suggestion as to what might be the culprit.

mytoolman:
Mr Smith I have had a similar situation once upon a time ago. Although in my case it was isolated to just the front brakes holding pressure when the pedal was released. How did you determine that BOTH the front and the rear are engaged when this this engaged brake situation you described happens? In my case it was a failed rubber brake hose that had its inner liner failing. A piece of the inside wall of the hose was separating and blocking off the flow of brake fluid keeping the brakes on.

Oz:
Have you checked your emergency brake adjustment?  If the brake work was done but the emergency brake wasn't adjusted, that can definitely cause your rear to lock up. 

Eyez Open:
I experienced the very same issue, actually prior to the very first drive in yrs i had gone thru most of the systems..meaning rebuilt. Part of that was grabbing the rubber brake line and checking for cracks/plyabilty...there was none. But the inner walls had decomposed.

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