How many PSI do you loose on brake application?

[zubzub]

As I continue to tighten up the brake system and run checks seems to me I loose about 10 psi in one brake aplication .  Does this sound about right?  Keep in mind I am talking static test with the engine off.  Thanks, Patrick.

[Len Silva]

From my recollection, that seems OK.  Assuming no leaks, the amount lost during a static application with the engine off depends entirely on the capacity of the tank(s).

More importantly, how much does the pressure drop past the initial drop with a sustained application?  If you start with full pressure (engine off) and it drops a certain amount, you should be able to hold the brakes on for a long time without further drop.

[Dreamscape]

I thought I read somewhere it should be no less than 5 psi in 60 sec. but then again I forgot what I had for breakfast too!

You're close I think.

Paul

[akroyaleagle]

Try this. It is the DOT test!

AIR BRAKE TEST SINGLE VEHICLE

With ENGINE OFF, AIR PSI at MAXIMUM, wheels chocked, spring brake released,
and key on.

1)  Air Leakage Rate

   a. Watch the air supply gauge for 60 seconds, the air loss should be no
       more than 2 psi
   b. Apply the foot valve fully, Watch the air supply gauge for 60 seconds,
       the air loss should be no more than 3 psi.

2) Air Warning Light
 
  *Apply and release the foot valve until the air warning light comes on, this
    should happen before 60 psi.

3) Spring Brake

   *Apply and release the foot valve until the spring brake pops out, this should
     happen between 20 and 45 psi.

4) Air Build Up Rate

   * Start the engine to build air pressure, when air pressure reaches 85 psi, time
      build up rate to 100 psi at idle. It should take no more than 45 seconds.

5) Governor Cut Off

    * The Governor should cut off at about 110 to 125 psi.
       You will hear air release from the system.

6) Spring Brake

    a. Remove the chock blocks.
    b. Place the vehicle in drive.
    c. Attempt to pull the vehicle forward, the vehicle should not move.
                     d. Place the vehicle back in neutral.

7) Service Brake

    a. Release the spring brake.
    b. Place your foot on the foot valve.
    c. Place the vehicle into drive.
    d. Put the vehicle in motion, when the vehicle reaches the speed of 5 mph
       apply the service brake to stop the vehicle. The vehicle should stop!

[zubzub]

thank you  akroyaleagle that about covers it.  One little note
" b. Apply the foot valve fully, Watch the air supply gauge for 60 seconds,
       the air loss should be no more than 3 psi"
I presume this means after the initial drop?

[bevans6]

yes, apply the foot valve fully, observe the pressure at that point, and look for any drop.  I personally accept a  zero, or unreadable drop of pressure, since that is the norm on my bus and if I started getting a drop I would want to find out why.  I figure a small drop could change  to a catastrophic drop at any time if I don't know what is causing the drop.

For me the two keys, beyond the "do the brakes work" test, are building air and losing air.  I track the compressor recovery test (85 psi to 100 psi, done after fanning down below 85 PSI from a full 120 psi pressure) every time I do the daily inspection in a log book, so that I can notice any change or degradation in compressor operation.  I also log the fact that the pressure loss test is zero readable.  It can be weeks or months between drives, so I like to keep the diary.  It reminds me to check tire pressures, and I keep fuel fills and mileage info in there too, as well as brake adjustments and any repairs.

Off topic, sorry...

Brian

[JackConrad]

" b. Apply the foot valve fully, Watch the air supply gauge for 60 seconds,
       the air loss should be no more than 3 psi"
I presume this means after the initial drop?

Yes.  Jack