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 Parking brake pops out at 58-60 PSI
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bus a groove
Senior Member

70 Posts

Posted - 05/06/2013 :  2:55:01 PM  Show Profile  Reply with Quote
we have a 1994 Thomas West-Coast-ER when you pump down the brakes the pp1 valve applies parking brakes around 58-60psi. We have checked the gauge and it is correct. We have changed the pp1 valve and the treadle foot valve along with a bunch of check valves and relays. We have checked lines and replace the air line from pp1 to spring brake control valve. We still have the same problem. Does anyone have any ideas?

Mechan1c
Top Member

USA
695 Posts

Posted - 05/06/2013 :  5:14:45 PM  Show Profile  Reply with Quote
Does this thing have an SR-1 spring brake valve?

Edited by - Mechan1c on 05/06/2013 5:15:08 PM
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57fan
Senior Member

United States
60 Posts

Posted - 05/07/2013 :  12:21:56 AM  Show Profile  Reply with Quote
being it was made after 1987 it should have a SR-1.

Lead, Follow or get out of the way!
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bus a groove
Senior Member

70 Posts

Posted - 05/07/2013 :  08:07:23 AM  Show Profile  Reply with Quote
yes it does have an SR-1 we swapped out a new one and had my mechanic double check lines were hooked up correctly
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slippert
Advanced Member

USA
383 Posts

Posted - 05/07/2013 :  10:04:41 AM  Show Profile  Reply with Quote
Depending on part # pp-1 valves come in 20,30,40,and 60 psi pop/release pressures, got to be sure you have the correct one.. I assume this problem started before you swapped the pp-1 valve the first time?
The dash valve uses system air pressure to overcome the spring inside the valve to stay in released position, if you don't have enough air pressure it pops out, at what ever pressure internal spring is rated for, i.e, 20 to 60 psi... if you have correct air pressure and flow coming to dash valve verified with a gauge in the supply line coming to pp-1 valve. then I would say you got wrong valve, and if old one was doing same, they do get used a lot on a bus(or should)and probably was worn out and started popping too soon.
Ck your part numbers and see what valve you have and what pop pressure is for that number...

I am talking about a pp-1 valve as they do come in more than one pop pressure,,

There are other dash valves that are only built to set at a certain pop pressure say 30psi

Edited by - slippert on 05/07/2013 10:05:39 AM
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bus a groove
Senior Member

70 Posts

Posted - 05/07/2013 :  10:51:04 AM  Show Profile  Reply with Quote
I really got my hopes on that hoping that would have been the case we have a bendix 284726N I just called them that PP-1 is set to pop off at 40PSI. I sent the bus to the dealership this morning but I may bench test a new PP-1 i have to make sure it is correct. Thank you slippert
quote:
Originally posted by slippert

Depending on part # pp-1 valves come in 20,30,40,and 60 psi pop/release pressures, got to be sure you have the correct one.. I assume this problem started before you swapped the pp-1 valve the first time?
The dash valve uses system air pressure to overcome the spring inside the valve to stay in released position, if you don't have enough air pressure it pops out, at what ever pressure internal spring is rated for, i.e, 20 to 60 psi... if you have correct air pressure and flow coming to dash valve verified with a gauge in the supply line coming to pp-1 valve. then I would say you got wrong valve, and if old one was doing same, they do get used a lot on a bus(or should)and probably was worn out and started popping too soon.
Ck your part numbers and see what valve you have and what pop pressure is for that number...

I am talking about a pp-1 valve as they do come in more than one pop pressure,,

There are other dash valves that are only built to set at a certain pop pressure say 30psi

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Mechan1c
Top Member

USA
695 Posts

Posted - 05/07/2013 :  12:51:28 PM  Show Profile  Reply with Quote
If you already did this, my apologies...

When you do a normal 121 air brake test using the air drains instead of the treadle, do you get the same early pop-off? (Test 1 drain wet, drain primary,apply brakes, drain secondary, record the psi for the PP1) and then reverse the test and do Test 2 (drain wet, secondary, apply brakes, drain primary, record the psi for the PP1...)

And do the air cans function like Bendix says they should with the tanks drained to simulate air loss/failure?
Test 1: A.front service brakes should apply, B.rear brakes should apply through modulation of the spring brakes-spring brakes will apply rear axle brakes, C. when treadle valve is released, the front brakes should release, and the spring brakes should repressurize and release the rear brakes.
Test 2: A.there should be NO activity on the front axle, B.regular service brakes should apply on the rear axle-Not Modulated Spring Brakes, C.when the treadle valve is released, the rear brakes should release.
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IBTMech
Top Member

USA
973 Posts

Posted - 05/07/2013 :  4:01:40 PM  Show Profile  Visit IBTMech's Homepage  Reply with Quote
My guess there is a restriction in an air line or fitting leading to the parking brake valve.

If it doesn't fit, FORCE it.
If it breaks, well, it needed replacing anyway.
Pullin' wrenches for 45 years.
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