Air over hydraulic brakes not locking up in reverse

[Carl Boger]

I just got back from filling my MH and running it around about 20 miles to heat up the engine and trans.  Everything was fine until I got back home.  I turned the coach around and started to back up some ramps to help level the coach.  As I was backing up, probably riding the brakes the coach kept rolling!  I had to put the transmission back in drive and the brakes stopped it again.  Back in reverse, still rolling.  Put it in drive stopped it set the parking brake and got out since I didn't want to take a backwards ride down the hill behind my house.

Brake fluid is full, but I will verify again.  I had good air pressure built up.  No problems on the 20 mile trip or the 22 point road turn to get the coach turned around on the side of my house.  Any ideas what this might be?  Did I simply need to pump the brakes to get more pressure on them,(last thing I wanted to do at that moment)?

Thanks for any help, planning a weekend trip on Sat so not much time. 

[Carl Boger]

Ok, see if this makes sense to anyone.  Both sides of the reservoir had DOT 3 in them, down a little but not empty. 

I topped off one side and ran out of DOT 3.  Will get more tomorrow.  I fired up the MH and ran the air pressure up to full pressure.  I then started applying the brakes over and over.  With full pressure the brakes feel firm as I get down toward the minimum pressure before the alarm sounds the pedal feels spongy.  Alarm sounds repeat with same results.

I feel that the 22 point road turn at idle got the air pressure low enough that the brakes would not grab on the down hill slope in reverse, but when I put it in drive going uphill there was enough friction to stop the MH.  I if I would have let the air build back up by waiting or increasing the rpm's I would likely have had brakes.

That and going to the last thing I "fixed"  I just added the Super Steer Trim kit and Safe-T-Plus.  The trim kit taps into the air system and that may also be taking away from the Air over Hydraulic braking margin. 

Really just one more new learning experience if this is indeed fixed!   Thanks Everyone,   Carl

[Gerald Farris]

Carl,
There is at least a 95% chance that your braking problem was caused by low air pressure. The continually braking at idle speeds depleted your air pressure and gravity with the transmission in reverse was more force than you could stop with the depleted air supply.

Gerald

[Carl Boger]

Thanks Gerald!

Glad to have some support to my theory of why it happened.  I won't lie, it scared me pretty bad.  Once I got it parked I considered it quite appropriate to come inside and have a cold beer!

[Eric Maclean]

Carl
That is why your not supposed to pump the pedal on an air brake system because the is no mechanical connection between the pedal and the master cylinders on your air over hydraulic system when the air pressure is depleted there is no braking.
However when you reach the point that the low air warning buzzer you should still have brake action it would be interesting to see at what air pressure you lose effective braking.
On a conventional air brake system you should have enough air to make two brake applications obviously this isn't the case on these air over hydraulic systems.
This would lead me to believe that the low air warning should be set at a higher pressure on these coaches for safety sake.

At the point were the service brakes become inaffective the parking brake valve should dump it's air and apply the parking (spring brake)
I would do a complete fan down test and record your air pressure readings .

Hope some of this helps
Eric

[Carl Boger]

Thanks Eric,

I am not sure what a fan down test is?  I believe that the air alarm goes on and off at about 90 psi, not positive though.  I am pretty sure with all the back and forth jockeying I ran the air pressure down.  Now that I know its a problem I will tr to avoid that situation. 

Add another thing to the list to look over BRAKES! 

[Steve Huber Co-Admin]

All,
Apparently I've got this backward. I was under the impression that loss of air pressure would cause your brakes to lock up.
Steve

[Carl Boger]

Steve,

A major loss of air pressure will cause your parking brake to lock up, I think I have read that at 40 psi that will happen but don't hold me to that number. 

In my case I still had about 90 psi or so.  I was still applying some breaking pressure, just not enough to be effective.  Just under that 90 psi my alarm comes on.  I am not sure if there is a way to raise the alarm psi to 95+
 but I do plan to be more aware of this when I have to do multiple stops at idle speeds.  If I would have simply stopped and revved up the engine for 20 or so seconds this probably would not have happened.

[Fred Brooks]

   It is a straight forward test to prove the park brake activation and prove the "pressure protection valve" is operational.
On level ground chock the drive wheels. Start the engine and build air pressure until you hear the receiver/dryer cycle. (120 pounds). Release the park brake. Start to pump the brakes and document the pressure at which warning lights and buzzers occur. At 40 to 35 psi the park brake should "pop up" and engage. The pressure protection valve should be supplying air from the accessory tank (green needle) over to the dedicated rear brakes tank (red needle) as you apply the brake pedal.
   Hope this helps, Fred

[Mike Shumack]

I'm following along on this thread, but I want to be sure to understand what chassis you have Carl.
Do you have the "Magnum Air" chassis (the B-Series chassis with the 3126 engine), or the Magnum "Big Cat" (C-Series chassis used with the patriot Thunder with the CAT C12 engine)?

What does your Parking Brake setup look like? Is your parking brake the (disc brake) "Caliper" Style or "Drum" Style (both would be mounted to driveshaft).

[Eric Maclean]

Carl
Sorry I took so long to get back to you
Fred is correct on the fan down procedure
The important numbers are
1) warning devices should activate at or about 60 psi and no lower than 55 psi ( below 55 is a bad warning switch)
2) the park brake pop off should occur in or about 40 psi and no less than 30 psi ( below 30 is defective parking brake valve)

The next part of the fan down is pressure build up
With engine running at idle let the gauges come up to 80 psi and watch to see how long it takes to go from 80 psi to 100 psi
This should take approximately  two minutes if the compressor is sized correctly and working properly.
The compressor should end cycle (stop pumping ) at or about 120 psi as Fred stayed 100 psi is considered a fail on the govenor if the pressure goes above 145 psi the air governor should be replaced but some are set higher 125 or 135 on some newer highway coaches. If your not reaching 120 psi the air governor on the compressor can be adjusted to increase the pressure or they are fairly cheap to replace personally I have my governor adjusted to 135 to allow me to have enough pressure to top up tires at 120 psi.
After the pressure has build again the next part of the test is the apply drop test
Shut the vehicles engine off
With the wheels chalked release the parking brake while watching the air pressure the gauge should go down slightly and stop if the gauge continues to drop after 10 seconds or so you have a leak in the park brake air lines or chamber.
Next do a hard brake application and hold the gauge pressure should drop approx 3 psi per axle and no more than 6 psi on a  rig with full air brakes this is an indication of brake adjustment (more brake chamber travel equals more air used)
Hold the pedal down for two minutes and watch the pressure, the pressure should not drop any further than the initial drop if it does there is a leak in the air lines or one of the chambers ( or in your case one of the pressure converter chambers)

Now with that all done start the vehicles engine again and set the parking brake
Fan down the air pressure and record at what pressure the air governor kicks the compressor back in should be approximately 20 psi below kick out IE if you cut out pressure is 120 psi then the cut in should be 80 psi or say cut out at 130 psi cut in at 90 psi
This spread is built into the governor itself and can not be adjusted if the spread is too wide the governor is defective and should be replaced.
Sorry to go on so long hope this helps explain the fan down and it's function
Eric

[Joel Ashley]

Thanks to Fred for reminding us of that test;  I read it somewhere a few years back, printed it, and stuck it in the document sleeve in the driver’s left side panel... and promptly forgot it.  And also thanks to Eric who, as another experienced mechanic, has stepped up to add much to the technical pages here. 

We are fortunate to have quite a few knowledgeable contributors now, and I hope new members realize the value of signing and staying on, even in light of recent membership fee increases.  Although we must concurrently be FMCA members also, I have hardly ever found it necessary to use that Forum;  this one usually supplies all the answers I need.

Joel

[Carl Boger]

Thanks Fred, Mike, and Eric

 Fred and Eric,I can certainly get the fan test done, will try to get it done tomorrow if nothing comes up.

Mike I do have the Magnum Air chassis B-Line and I have a drum parking brake. 

I do appreciate everyone's help here.  For me personally I want to make this coach as like new as reasonably possible without just throwing money at it.  Making it dependable and reliable are the top priorities.  Thanks again everyone.

[Carl Boger]

Just to update this post.  This is just from memory as I did not write the numbers down so may not be 100 percent accurate, but close.   

On my Coach the air alarm goes off some where near 70-75 psi, it will then air up until 115 or 120 psi when the air dryer releases.  Pumping the brakes the air alarm comes back on at around 60 psi after that I believe the parking brake sets at 40-45 psi. 

My initial problem was letting the air pressure get down to about 60 psi and not building it back up.  When running down the road my psi runs between 95 and 120 or at least in that range. 

Thanks For everyone's help!

[Eric Maclean]

Carl
Those numbers all look good to me .
If I were you I'd still like to know at what pressure you lose effective braking  just for reference.
It shouldn't happen before the low air warning but it may.
Any way it appears your pressures are good and you warning buzzer is working as intended.
Good news
Eric