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 Post subject: 26L won't maintain
PostPosted: Fri Apr 12, 2019 6:01 pm 

Joined: Tue Jun 14, 2016 6:14 am
Posts: 33
Hello all,

I am having a issue with 26L not maintaining a brake reduction. Just replaced the automatic brake valve and the P-2-a. After making a reduction it will hold and then the automatic brake starts venting off slowly. The locomotive is the problem. It will maintain in passenger though. In freight no luck. I have crawled over looking for a air leak. Any Ideas?

Thanks in Advance


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 Post subject: Re: 26L won't maintain
PostPosted: Fri Apr 12, 2019 7:08 pm 
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Joined: Tue Feb 20, 2018 7:08 pm
Posts: 146
Location: Alberta, Canada
I was told by an old head engineer that a leaky gasket/o-ring on the line to the equalizing reservoir gauge is a likely cause of this problem.

Your problem is a uncommon but not rare occurrence on our (CN's) locomotives with 26 and 30 brake systems. Today we just flip it over to Passenger and keep going, I have never attempted a repair myself.

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 Post subject: Re: 26L won't maintain
PostPosted: Sat Apr 13, 2019 3:05 pm 

Joined: Sun Aug 22, 2004 11:06 am
Posts: 503
Location: NE PA
I will second the leak in the equalizing reservoir piping, O rings in Salem gauge test ports, Salem gauge mounting plate are not uncommon along with piping leaks. A loosely rolled newspaper rubbed against the piping with the engine shutdown may help amplify the leak.
Mike Tillger


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 Post subject: Re: 26L won't maintain
PostPosted: Sun Apr 14, 2019 3:47 pm 

Joined: Sun Aug 22, 2004 10:18 pm
Posts: 98
Location: Philadelphia, PA
Take a close look and make sure equalizing reservoir pressure is not actually rising. With the ABV set to freight position about a two pound rise in equalizing res pressure is enough to get the 26F to go to release. When the ABV is in passenger and the F control portion set up to graduated the issue will take more time to manifest itself.

I’m not close to my schematics but it’s also helpful to know how the unit is piped as related to J Relay type.

EBL


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 Post subject: Re: 26L won't maintain
PostPosted: Mon Apr 15, 2019 11:11 am 

Joined: Fri Nov 16, 2007 10:21 pm
Posts: 117
I'm assuming that what you are experiencing is that with the independent brake in release, you take a set with the automatic, the brake cylinder pressure comes up as normal, and then slowly bleeds off? It's also important to note whether the brake valve bleeds down with the brake stand cut out.

When we encounter problems of that nature, it almost always ends up being either a small plumbing leak, or a bad check valve. Air leaks can be hard to diagnose, because it doesn't take a very large leak to present a problem, and the leak may only be present for a few seconds. Lots of soap suds and eyes on the brake lines while someone else works the controls.

My vehicle mechanics have a smoke machine they use to find vacuum leaks on automobiles. I've contemplated using this for diagnosing air leaks on brake systems, but have not figured out how to go about that yet.

Eric


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 Post subject: Re: 26L won't maintain
PostPosted: Wed Apr 17, 2019 3:52 pm 

Joined: Tue Jun 14, 2016 6:14 am
Posts: 33
Thanks For the help. It was the gage orings. Works like a charm now.


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 Post subject: Re: 26L won't maintain
PostPosted: Sun May 17, 2020 8:09 pm 

Joined: Wed Dec 31, 2008 12:33 am
Posts: 32
Location: Southeastern PA
Resurrecting an old thread. I'm having the same exact issue with our GP38 26L system. All of the valves were recently replaced. When I make an application with the automatic brake valve in the FRT position, the BP reduces to the correct pressure called for by the ER. After they equalize, the ER continues to reduce at a slow rate with BP following. This only happens in the FRT position. When you do this in the PASS setting, it holds pressure. I've even tried making a reduction in FRT and then when the ER/BP pressure starts to fall after equalizing, I've put the valve in PASS. The reduction stops right away and the leak seems to stop.

Any places to start looking? I read above to check the gauge O rings. It sounds like the ER is a good place to look, but wondering why it only leaks in one position, but not the other. I would think an ER leak would leak in both settings. I had the system charged today with the engine shut off to try and hear leaks. Upon initial inspection, I could not hear any apparent air leaks. Is it possible this is internal leakage? This engine had this problem even before the valves were exchanged.

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 Post subject: Re: 26L won't maintain
PostPosted: Sun May 17, 2020 8:19 pm 

Joined: Sun Aug 22, 2004 11:06 am
Posts: 503
Location: NE PA
In addition to the mounting plate O rings for GWS gauges, you should also check the gauge test fittings O rings. Use dishwashing detergent mixed with water in a spray bottle, use enough detergent that when you spray it in your hand you get some suds/bubbles.
Mike Tillger


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 Post subject: Re: 26L won't maintain
PostPosted: Mon May 18, 2020 12:18 am 

Joined: Wed Oct 31, 2007 10:40 am
Posts: 91
Location: Chama, NM
RCman2626 wrote:
This only happens in the FRT position. When you do this in the PASS setting, it holds pressure. I've even tried making a reduction in FRT and then when the ER/BP pressure starts to fall after equalizing, I've put the valve in PASS. The reduction stops right away and the leak seems to stop.

I would think an ER leak would leak in both settings.


Due to the function of the brake valve in the PASS position, this is normal operation, even with an ER leak. Using the PASS position as a temporary work around to get a train in when an ER leak develops on the road is an option. However, you must be very careful not to move the brake valve handle even slightly toward the release position or you will initiate a release of the train without recharging the brake pipe fully.

It is possible you have a mounting gasket leak. More likely, it is in one of the fittings in the ER system plumbing. I would use the soap suds test on every connection in the ER system all the way up to the gauge connection. Due to the small volume of the reservoir and associated piping, it only takes a very small leak to cause this type of problem.


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 Post subject: Re: 26L won't maintain
PostPosted: Mon May 18, 2020 7:19 pm 

Joined: Sun Aug 22, 2004 10:18 pm
Posts: 98
Location: Philadelphia, PA
With the ABV pilot valve set in FRT here are the steps I would take:

1. Examine the equalizing reservoir gage, base and test O rings, and the associated piping.

2. Check the equalizing reservoir and pipes 5 & 15 from the ABV on down.

3. Check for external leakage at P2A. If no external leakage renew the portion.

4. Check for external leakage at the 26C ABV. If none change the portion.

EBL


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