J71 Parking Brake Pump Cycles On/Off Very Rapidly

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1994-2003 P32 Chassis J71 Parking Brake Pump Cycles On and Off Very Rapidly.

When the pump cycles rapidly, the pressure is building then dropping rapidly. This means the pump is able to build enough pressure to open the contacts on the pump pressure switch shutting off the pump motor. However, it also means the system is not able to hold the initial pressure it built. Fluid loss is equal to pressure loss. In other words, the escaping fluid is dropping the pressure enough to close the pressure switch, reactivating the pump motor to regain pressure. The rising pressure opens the switch again, and repeats this process. This happens very quickly over and over, turning the pump on and off rapidly.

Possible causes of fluid loss are:

  1. External leak (Fluid on the ground) Problem is obvious...find what is leaking, then replace that part.
  2. Internal leak (Fluid leaking back into the reservoir) Possible causes are:
    1. The valve, or valve seat damaged.
    2. The spring was not installed properly into the recess of the bushing.
    3. A weak spring not pushing on the poppet valve hard enough to keep fluid from leaking past.

Both the Poppet valve, and spring are inexpensive. Having this in mind, replacing these items are the first step we recommend after verifying they are installed properly. Labor is expensive, these are wear items, and are easily replaced. It makes sense to start with these, especially if these are able to be installed by the coach owner.

Replacing your poppet valve and spring.

  1. Place your coach into park and turn off your ignition switch.
  2. Remove the pressure switch in the side of you pump assembly.
  3. Remove the bushing that the switch was installed in.
  4. When removing the bushing you will find a small spring behind it.
  5. Remove the spring.
  6. Use a pencil shaped magnet and insert it into the bushing cavity in the pump. Pull back on the magnet and it will bring the poppet valve out with it.
  7. Inspect the spring and valve for damage, as well as the seat for the poppet valve in the pump housing.
  8. Install the new valve into the pump housing using just your finger.
  9. Install the spring into recess in the back of bushing using some wheel bearing grease to hold the spring in place as you install it into the pump housing.
  10. Reinstall the pressure switch and test pump operation.
  11. If the pump is still cycling, then plug (#4 JIC Plug) the metal line going into the pump on the side of the pump closest to the yellow reservoir cap (94-03) if you have a 2004 with a black cap, you will have to replace your pump...Period.
    1. If the pump stops cycling after installing the plug, then replace the solenoid valve.
    2. If the pump continues to cycle after this test then you will have to replace the pump.

If you need to find a wiring schematic for your coach, you can find them listed here https://www.ultrarvproducts.com/Workhorse-Manuals

You can find the poppet valve here: 773776

You can find the spring here: 410462



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