How much brake hissing is normal?

Topic: How much brake hissing is normal? (Read 1897 times) previous topic - next topic

How much brake hissing is normal?

December 01, 2010, 03:11:05 pm

I am familiar with the hiss that the brake system makes on air-brake equipped trucks. Yesterday, while driving, our new to us would discharge about every 10-15 seconds. Brake pressure was around 120 psi for both systems, and everything seemed fine to my newbie senses.

Is this normal? If not, how do I remedy it? Also, do I have tanks I must drain of condensate?

Matt B
1994 U-240

Re: How much brake hissing is normal?

Reply #1 – December 01, 2010, 03:22:24 pm

Matt, First maybe you can take a real good listen when the engine is off and nothing running, listen for an air leak, If your governor is spitting off every 10-15 seconds you have a MAJOR air leak and should be able to hear it with the engine running.
Maybe the issue is not air leaking but a terminology issue.
Heck it is hard to tell from your description, and with the air gauges not moving, Most amazing.
Maybe need another description of problem.

Re: How much brake hissing is normal?

Reply #2 – December 01, 2010, 03:37:39 pm

I agree with Dave.

First, your air gauges SHOULD move between CUT-IN PSI and CUT-OUT PSI-- generally a range of 12-15 PSI with 120 likely the cut-out PSI of the engine-driven compressor.

You either have a large air leak, or as likely, a problem with the air dryer check valve. Note: the air dryer needs servicing/replacing every couple of years. If an original Bendix AD-2 or even a replacement Bendix AD-9, consider upgrading to a more current Bendix dryer that is easier to service. A new-generation reman dryer from Bendix is a cost-effective upgrade.

Brett

Re: How much brake hissing is normal?

Reply #3 – December 01, 2010, 04:06:30 pm

Matt, my 93, U225 does the same thing. It seems to be related to humidity. I spent $400 for to have a new governor installed and it changed nothing. The pssssh you hear is the drying system purging itself. It does it frequently on some days and occasionally on others. Cummins checked mine out after they put the governor on and said everything is working properly. I think it is unique to the 225's and 240's without air bags.

Brett, doesn't your coach do the same thing?

Re: How much brake hissing is normal?

Reply #4 – December 01, 2010, 04:43:08 pm

Another description:

While driving at about 65 MPH, when we passed a high divider, I could hear what sounded like an air discharge, duration of a fraction of a second. The sound was like what a compressor makes when it reaches full pressure, though the sound was much shorter, maybe a fourth second or shorter. I have heard trucks make this sound. We could hear this sound about once every ten to fifteen seconds.

If I understand the systems correctly, it would not be an air leak - after six hours with the ignition off, playing with the air hose, and finding and venting the condensate valves, air pressure is 90 and 120. Does this make sense? When I shut the engine off, I hear a hissing for about 10 seconds at the engine front right side, but nothing else. The coach does not make the sound when it is idling - we noticed it only at cruising speed, when there was a sound reflective surface near the left side of the highway. Is the air pump engine driven?

Matt B

Re: How much brake hissing is normal?

Reply #5 – December 01, 2010, 04:47:21 pm

The air pump on a Cummins is engine gear driven. I presume the Cat is also. I think what you heard is perfectly normal.

Re: How much brake hissing is normal?

Reply #6 – December 01, 2010, 04:48:02 pm

Kent,

Seems to make sense. First hour we drove (to the Walmart to collapse from nervous exhaustion) it vented frequently. Maybe from high humidity or from sitting for a few weeks. Second day, we heard it once or twice in several hours, though we may have missed a few.

Matt B

Re: How much brake hissing is normal?

Reply #7 – December 01, 2010, 04:58:30 pm

For anyone with a new to them coach here is a good place to find knowledge: http://www.beamalarm.com/foretravel-links/foretravel-technical-help.htm#Air_Brakes Many of us including myself have started off driving an air brake equipped coach with practically no knowledge of air brakes or how to do an air brake test. Anyone that knows how to do the test will be a safer driver. Here is a website that includes the test. http://dmv.ca.gov/pubs/cdl_htm/sec5_a.htm By the way some states require a class B drivers license if you are driving a vehicle with air brakes or above a certain gross weight usually 26K lbs. I drove my coach 2 yrs. with a class C license before finding out that I should have had a class B.

Re: How much brake hissing is normal?

Reply #8 – December 01, 2010, 05:46:30 pm

NO, it is not normal for the dryer to purge every 10-15 seconds. On the U240 there is very little air use (air brakes only, no air suspension), so the cut-out PSI is not reached very frequently-- certainly not every 10-15 seconds!

Brett

Re: How much brake hissing is normal?

Reply #9 – December 01, 2010, 05:59:53 pm

I just remembered something! For that first drive, a floor mat was stuck underneath the edge of the brake pedal. It didn't seem enough to cause any braking action, but it did keep it from being fully up. Could this be the cause of the discharge?

Matt B

Re: How much brake hissing is normal?

Reply #10 – December 01, 2010, 06:56:59 pm

Quote from: oldmattb – December 01, 2010, 05:59:53 pm

I just remembered something! For that first drive, a floor mat was stuck underneath the edge of the brake pedal. It didn't seem enough to cause any braking action, but it did keep it from being fully up. Could this be the cause of the discharge?

Matt B

The only time brakes hiss is when they are released, if a floor mat is holding the pedal down you could have dragging brakes. The other hiss comes from the air dryer when the governor cut out pressure is reached. If you get water out of the tank drain after a days drive consistently the air dryer probably needs service. A little bit of water is normal and how humid the air is will have some effect too. Stand outside the vehicle with the engine running, if you get a hiss from the air dryer frequently like every few minutes something is wrong.

Re: How much brake hissing is normal?

Reply #11 – December 03, 2010, 06:04:03 pm

I got a chance to go out and tinker some. At idle, the dryer vents when pressure is reached and is then generally quiet. At 15 RPM, it vents about every fifteen seconds. I adjusted the governor slightly (peak pressure per the dash gauge was about 130) down to about 120 pounds. I did this mainly because I was so pleased to have found the governor on my own. The dryer is still venting like before.

I examined the dryer for model number and could find none, though it is likely hidden by stuff. It was every bit as dirty as the oil tank and the other stuff, so it has likely been there a while. The service records I got with the coach do not mention it being replaced, so who knows!

In any case, the plan is to replace the dryer (old) and the governor (cheap). Which should cover the problem I am having, if it really is a problem.

With a driveway full of Toyotas, I have not done much wrenching in a while. It was fun.

Matt B
1994 U240

Re: How much brake hissing is normal?

Reply #12 – December 03, 2010, 07:00:57 pm

The air suspension also makes hissing sounds, easily thought of as the air brake system. I hear mine every time I use the retarder full on to slow around town.

Re: How much brake hissing is normal?

Reply #13 – December 03, 2010, 07:07:24 pm

Last summer we took a brief tour through Yellowstone National Park in our coach. Weather was beautiful and we were traveling relatively slowly, so we had the windows open. We could hear the air bags venting as the suspension made adjustments on the curving and hilly roads.

Re: How much brake hissing is normal?

Reply #14 – December 03, 2010, 07:07:58 pm

Quote from: Tom Lang – December 03, 2010, 07:00:57 pm

The air suspension also makes hissing sounds, easily thought of as the air brake system. I hear mine every time I use the retarder full on to slow around town.

This may not apply to a 1994 U240. I don't think these had air suspensions/leveling.

Re: How much brake hissing is normal?

Reply #15 – December 03, 2010, 07:51:26 pm

Quote from: Peter & Beth – December 03, 2010, 07:07:58 pm

Quote from: Tom Lang – December 03, 2010, 07:00:57 pm
The air suspension also makes hissing sounds, easily thought of as the air brake system. I hear mine every time I use the retarder full on to slow around town.
This may not apply to a 1994 U240. I don't think these had air suspensions/leveling.

Very true, but everyone reads these posts for guidance.

Re: How much brake hissing is normal?

Reply #16 – December 03, 2010, 08:13:56 pm

Matt,

Have you opened the tank drains? If you find ANY water, you must service the air dryer immediately. I service mine every eighteen months, as FT recommends.

Re: How much brake hissing is normal?

Reply #17 – December 03, 2010, 08:50:37 pm

My dryer periodically does the same thing Matt has described and it has never had any water in the tanks. I'm not sure he really has a problem.

Re: How much brake hissing is normal?

Reply #18 – December 03, 2010, 10:17:58 pm

Based on my understanding of how the air dryer purges itself, it would seem to be excessive cycling. When the governor reaches its set air pressure it will unload the compressor and that triggers the air dryer purge valve. Once the governor unloads it keeps the compressor from adding more air until it drops to the low pressure setting when the cycle starts over. Mine would not fully close after purging the first time and the pressure would take for ever to build up. It was really bad at 8000 feet. After I replaced the purge valve then everything worked properly. I could always tell what was happening by the air gauge which would stay about 120 until I did something like using the brakes. When the air pressure dropped to about 90 then it would rebuild and I would hear the air dryer purge at 120.

It is too bad that it only occurs at speed so that you cannot tell what is causing the issue.

Re: How much brake hissing is normal?

Reply #19 – December 08, 2010, 08:38:33 pm

A final word for those who were following the thread:

It was the dryer that was venting frequently, causing the noise. I set the RPMs at about 1200, and walked around and listened. I looked the dryer over for a model number while it was still in the coach, but could find none.

The old dryer was a Bendix, but I could find no model number on it. I had ordered a Bendix AD-9, it arrived today and I replaced it this evening. No big complications. It took a while to get the brackets loosened enough to get the dryer out. I had to pull it down out of the coach to get off one of the lines, which was against the rear of the coach. One fitting was slightly relocated, so I had to think-out which fitting should be tightened first.

Cranked up the FT, the dryer vented when the pump reached 115 psi, and no more vents. Shut off the engine and no hissing from a new leak. Looks like the job is done.

Matt B
1994 U240

Re: How much brake hissing is normal?

Reply #20 – December 09, 2010, 09:39:59 am

Matt, if it was the dryer working improperly, couldn't you have simply cleaned or replaced the purge valve rather than the entire dryer? Also, I thought you had not found there was a problem with holding air pressure. Is that correct? What made you purchase a new dryer?

Re: How much brake hissing is normal?

Reply #21 – December 09, 2010, 10:09:42 am

Kent,

While he could have replaced the check valve, dryers DO require servicing.

And the cost of a reman or even new dryer is not that much more than the cost of an overhaul kit if you count the labor involved. Yes, some have spin on filters that don't take much time to service, but it sounds like he may have had the original Bendix AD-2 dryer. Not worth fooling with.

I do the same-- every 3-4 years I just buy a Bendix factory reman dryer.

Brett

Re: How much brake hissing is normal?

Reply #22 – December 09, 2010, 10:13:39 am

Interesting, what kind of cost are we looking at?

Re: How much brake hissing is normal?

Reply #23 – December 09, 2010, 10:26:16 am

Under $200 for complete replacement.

But there are several different brands on the market with some variation in how they are serviced.

Start by seeing what brand and model you have. Only thing they all have in common is the need to be serviced/replaced every couple of years. More frequently on newer coaches (use a lot more air) and those operated in humid conditions.

Brett

Re: How much brake hissing is normal?

Reply #24 – December 09, 2010, 10:50:45 am

What he said!

I have pretty good service records, but no indication that the dryer had ever been serviced. Even after removal, I can find no identification other than a tiny serial number, and a big "Bendix Air Dryer" sticker on the side. I took great trust in Brett's age advice and just updated the whole unit.

Newbie question:
It sounds like we are talking about two things at one time. I am guessing the check valve is the little screw-in valve that protects from over-pressure, like one a small shop compressor. The purge valve is built into the dryer, and vents liquid, vapor, and impurities at the end of each compression cycle. Do I have that right?

Newbie question two:
Can the AD-9 be serviced without removing it from the coach? It looks really tough.

@Kent There were no significant air leaks, but the purge valve was activating ever ten seconds or so. Best I could tell, I had to remove the dryer and disassemble to access the purge valve. I could not ID the old dryer, so I followed Brett's advice. Upside - same day R&R, pro rebuild, updated and improved equipment. Downside - just some dollars.

Matt B

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