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 Wabco HPB
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earl1412
Active Member

11 Posts

Posted - 02/10/2014 :  6:28:21 PM  Show Profile  Visit earl1412's Homepage  Reply with Quote
Do I understand correctly that the "master cylinder" on a Wabco HPB system is strictly a pressure sending device to be interpreted by the HCU and is not able to apply hydraulic pressure directly to a disabled brake system? (step through braking).
Is this even legal in the US?
Please correct me if I'm in error here.
Thank you,
earl1412

bwest
Administrator

United States
2184 Posts

Posted - 02/11/2014 :  2:13:48 PM  Show Profile  Reply with Quote
I will put a link to the mechanic's forum, that will be the best area for this question.

Bryan
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mastertech
Advanced Member

211 Posts

Posted - 02/13/2014 :  1:00:46 PM  Show Profile  Visit mastertech's Homepage  Reply with Quote
That is correct. The system has 2 independant circuits and will apply the parking brake just like an air system if a major failure occurs. It also will not let you release the parking brake until both sytems are pressurized.
I dont know about legal.
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earl1412
Active Member

11 Posts

Posted - 02/15/2014 :  07:04:05 AM  Show Profile  Visit earl1412's Homepage  Reply with Quote
quote:
Originally posted by mastertech

That is correct. The system has 2 independant circuits and will apply the parking brake just like an air system if a major failure occurs. It also will not let you release the parking brake until both sytems are pressurized.
I dont know about legal.


Thank you for the response.
I have up front face time with the SAHR parking brake system, and am not impressed that when you need to tow a vehicle with a completely dead battery system, you can't move the bus until you cut the short cable to allow release of the brake shoes. At least with air brakes you have the option of using the spring release tool to take pressure off parking brakes to allow towing.
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mastertech
Advanced Member

211 Posts

Posted - 02/18/2014 :  6:48:22 PM  Show Profile  Visit mastertech's Homepage  Reply with Quote
I generally cut the connector between the cables and keep extras on hand . Most of the time just charge up the battery and release with the switch or if it has a failure in the system override it with the wabco software to release the brake.
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bwest
Administrator

United States
2184 Posts

Posted - 02/19/2014 :  05:39:21 AM  Show Profile  Reply with Quote
Isn't there a nut on the cable that you can run up to back off the brake?

Bryan
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earl1412
Active Member

11 Posts

Posted - 02/20/2014 :  7:40:13 PM  Show Profile  Visit earl1412's Homepage  Reply with Quote
quote:
Originally posted by bwest

Isn't there a nut on the cable that you can run up to back off the brake?


Yes there is,but it is generally corroded and too much pressure will shear the hollowed out threaded piece that goes into the SAHR unit.
Just had one come back from IC telling me they couldn't get the piece unstuck, so they put a new cable and SAHR unit on to the tune of over $740.00
Bus went in for a cracked EGR cooler, and was hydro locked. (warranty)
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