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Old 10-16-2021, 02:30 PM   #1
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Hyd System Pressure?

I own a 1998 Safari Sahara, with a 3621b CAT, and hydraulic over hydraulic disk brakes.

The Parking Brake warning light switch is leaking only when he parking brake is released.

I pulled it off to get a part number, but no luck. The only text is "Hobbs Corp" "Springfield IL" on the back of the switch there are 2 terminals, one marked "NC" and the other marked "O".

A google search quickly reveals that it is a Hobbs (duh) pressure stitch, similar to the pressure switches on small aircraft with a Hobbs meter for tracking engine operating hours. I also determined that it is likely a Normally Closed switch.

There are also Honeywell and other options on the market.

I need help determining the proper operating pressure for the switch.

Does anyone know the appropriate switch operating pressure to select?

Has anyone actually measured the hydraulic pressure at the port when the parking brake is released?

I am concerned that it I purchase a switch that can't handle the operating pressure that it will fail soon. I am also concerned that if I purchase a switch with an operating pressure too high that the light will never shut off.

Alex
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Old 10-16-2021, 03:30 PM   #2
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Your parking brake is hydraulic ?

Most driveshaft parking brakes are air release.
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Old 10-16-2021, 10:15 PM   #3
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Buy a Hyd gauge and screw it into the port temporarily. Use a 5000 psi gauge to be safe.
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Old 10-17-2021, 03:55 PM   #4
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Quote:
Originally Posted by twinboat View Post
Your parking brake is hydraulic ?

Most driveshaft parking brakes are air release.
My parking brake is manual, aka a cable. The cable attaches to a hydraulic valve then goes down to the parking brake drum on the output of the tranny.

I don't know the function of the hydraulic valve that the parking brake attaches to. I can see that the hydraulic valve is actuated when I set or release the parking brake. The hyd valve has a pressure switch that illuminates the parking brake warning light on the dash when the parking brake is set.

As far as I can currently tell, when I set the parking brake the hydraulic pressure is released which allows the Normally Closed valve to close and turn on the parking brake warning light.

Alex
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Old 10-17-2021, 03:57 PM   #5
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Buy a Hyd gauge and screw it into the port temporarily. Use a 5000 psi gauge to be safe.
I will if I have to, but I was hoping someone on the forum has checked the pressure before.

I am trying to avoid buying a hydraulic pressure gauge, and then store it away likely to never be used again.

Alex
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Old 10-17-2021, 10:06 PM   #6
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My park brake is the same as yours. The cable goes to a hydraulic valve. Pressure disengages the brake, springs engage it. Was common in olden times.

FWIW the Hydromax specs min 1000psi and up to 2200psi working, so that may give you a range to check for since it's all the same hydro circuit.
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