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Topic: Parking Brake Effectiveness (Read 1206 times) previous topic - next topic

Parking Brake Effectiveness

New topic, old subject.  Just did a search for "parking brake" and found a lot of discussion as far back as 2009, some of which I had written, but not the answer I was looking for.  Last time I moved the coach, applied the parking brake at about 5 mph and it came to a stop, but not like a hard brake application.  Then from a stop on level ground with the parking brake on and transmission in first gear, applied power.  At 1000 RPM the coach began to move.  Would you consider this normal, or abnormal?  Since there are 2 spring brakes (one for each drive set of wheels), its possible that one could be broken or weak.  How could this be determined?  My service brakes are normal, and pads and rotors are in good condition.  I don't go under the vehicle without chocks and blocks. 
Jerry Whiteaker former owner 96 U270  36' #4831 Austin,TX-Owner Mods LCD TV w/front cabinet rebuild - LCD TV bedroom - Dual Central AC, either can cool coach w 30 amp - Skylights at roof AC openings - Drop ceiling for ducting of AC - Shower skylight white gelcoat/wood/epoxy frame - Air Springs/Shocks replaced - 2014 CRV - 8K Home Solar - Chevy Volt

Re: Parking Brake Effectiveness

Reply #1 –
Yes, old but important subject. Rather than even trying to go into detail, please go to the PDF here at: http://www.truckstopper.com/Automatic%20Slack%20Adjuster.pdf Good diagrams for operation, wear checks, lubrication, etc.

You can print this info out as it is a long article. Wear checks and maintenance on the air brake system should be on the "to do" checklist especially on a vehicle that sees seasonal use. This is especially true of an RV that has been driven in a harsh winter climate with salty roads.

The big no-no here is trying to take the brake canister apart to check on the condition of the spring. Without caging the spring, this can result in a fatal injury. Anyone trying to take the canister apart should really know what they are doing. Not difficult but like petting the lion in the zoo, better be acquainted pretty well.

Pierce & Gaylie Stewart
'93 U300/36
Pierce and Gaylie Stewart
'93 U300/36 WTBI
Detroit 6V-92TA Jake
1140 watts on the roof
SBFD (ret)

Re: Parking Brake Effectiveness

Reply #2 –
Do most shops have the equipment and expertise to work on our brakes?
Dick, '03 U320 40' Tag, 2 slides, Coach #6075
Full Timers
2009 Honda CR-V

Re: Parking Brake Effectiveness

Reply #3 –
Quote from: Dick S –
Do most shops have the equipment and expertise to work on our brakes?
Best answered with a maybe.  Most trucks have drum brakes.  I bought pads  from Austin Brake and Clutch (sells parts for large trucks) and they  recommended someone that did work for the fire department.  Most fire trucks have air disk brakes like we do, so ask the fire department who they use for brake work.

By the way, I park on some 2" thick planks to get the tires off the dirt at the storage yard.  The planks are tapered on the ends and it takes as much power, maybe a little more than it does to move with the parking brake on to get on the planks.  I will go to Austin Brake and Clutch and ask about the situation.  Several months ago I cut a 3/4" x (forgot the length) square piece of wood to wedge between the dash and pedal to hold the service brakes on in case I ever get stuck on a slope where the parking brake is not enough to keep it from rolling.  I keep it on the floor next to the drivers seat, and maybe it would give me enough time to chock the wheels, although I hope someone else will be with me to do the chocks.
Jerry Whiteaker former owner 96 U270  36' #4831 Austin,TX-Owner Mods LCD TV w/front cabinet rebuild - LCD TV bedroom - Dual Central AC, either can cool coach w 30 amp - Skylights at roof AC openings - Drop ceiling for ducting of AC - Shower skylight white gelcoat/wood/epoxy frame - Air Springs/Shocks replaced - 2014 CRV - 8K Home Solar - Chevy Volt

Re: Parking Brake Effectiveness

Reply #4 –
Quote
Do most shops have the equipment and expertise to work on our brakes?
ABSOLUTLY NO........ I had a shop screw up my rear brakes and they had a reputation as being competent on buses and trucks.  What they did was FAIL to properly lube and adjust the brakes after they replaced them.  I later confronted them and they just shrugged it off.  BIG MO In Austell, GA knows nothing about disc brakes. Bill W in Tampa was able to adjust them properly.

Re: Parking Brake Effectiveness

Reply #5 –
Quote from: Jerry Whiteaker –
Last time I moved the coach, applied the parking brake at about 5 mph and it came to a stop, but not like a hard brake application.  Then from a stop on level ground with the parking brake on and transmission in first gear, applied power.  At 1000 RPM the coach began to move.  Would you consider this normal, or abnormal?  Since there are 2 spring brakes (one for each drive set of wheels), its possible that one could be broken or weak.  How could this be determined?  My service brakes are normal, and pads and rotors are in good condition.  I don't go under the vehicle without chocks and blocks. 
I finally got around to working on this problem. 

I was almost at the point of PMing James T when I decided I should first check the free stroke of the automatic slack adjusters on the rear wheels.  I first went under the coach with the parking brakes set just to make sure that both were actually doing what they are supposed to do.  Happened to notice that a little bit of red under all the dirt on the air chamber push rod was showing.  Hard to notice, really, due to dirt.  H'm mm isn't red usually a warning! 

Released the brakes (blocked and chocked of course), marked the push rod with a piece of masking tape flush with air chamber, reset the parking brake and measured again.  A stroke of 1 7/8 inches.  The left wheel was almost the same at 1 3/4 inches.  I then measured the free stroke where you insert a pry bar in the clevis and move it by hand until the pads contact the rotor.  That turned out to be 1 1/4 and 1 1/8 which is out of limits for brakes that have been in service and the pads broken in.  According to the Meritor brake maintenance manual the free stroke should be between 3/4 and 7/8". 

By now I am really puzzled, don't I have automatic slack adjusters, why aren't they working correctly?  How come both wheels are like this?  I checked the free stroke on the front wheels and they were okay.  I put a wrench on the adjusting nut and turned it counterclockwise to shorten the stroke.  The nut turned very easily and the stroke was shortened.  I adjusted it to 7/8" on both sides. 

The pull pawl was working correctly,  I could not turn the adjusting nut in the other direction with out pulling the pull pawl.  That is what it does when the automatic feature is reducing the slack from brake wear, it can only turn in the direction to reduce the slack.  At this point I went home and did some internet research.  There is a red mark on the air chamber push rod, if it is showing when the brakes are applied firmly, the stroke is out of adjustment and needs to be corrected before driving.

I found in this article that it takes 6 full hard as possible at 100 psi brake applications to cause the automatic slack adjusters to do what they are supposed to do.  Light brake applications do not get the job done.  www.mpi.mb.ca/PDFs/AirBrakeManual/ABM_Section8.pdf 

Today I returned to the coach and did that.  Then I tested the parking brake by putting the coach in gear and trying to move forward.  This time it took 1300 rpm instead of 1000 before it would move.  I also confirmed that the slack was further reduced from 7/8 to 3/4".  I feel a lot better now that the parking brake will hold when it is supposed to.  I can only surmise that I was not doing my part with hard brake applications to get the automatic slack adjusters to function properly.

 I took the CDL B class driving test and while air brakes were discussed for 15 minutes, that didn't come up.  Here is another article about air brakes.  www.mpi.mb.ca/PDFs/AirBrakeManual/ABM_Section8.pdf  It is mainly for truckers of course, but much of it applies to us.  Every thing works about the same up through the slack adjuster.  Most trucks have the "S" cam drum brakes and we have disk brakes.  We don't have to worry about the expanding drum, but excess heat will still ruin the rotor.  Automatic slack adjusters did not come into use until about 1991.  I now understand why the older truckers do not trust automatic slack adjusters. 
Jerry Whiteaker former owner 96 U270  36' #4831 Austin,TX-Owner Mods LCD TV w/front cabinet rebuild - LCD TV bedroom - Dual Central AC, either can cool coach w 30 amp - Skylights at roof AC openings - Drop ceiling for ducting of AC - Shower skylight white gelcoat/wood/epoxy frame - Air Springs/Shocks replaced - 2014 CRV - 8K Home Solar - Chevy Volt

Re: Parking Brake Effectiveness

Reply #6 –
The simple answer is:
IF you have good pads, good dry (Non oiled) rotor, proper lubed and free acting caliper and proper adjusted slack adjuster, you have good brakes.
No need to make it complicated.

Re: Parking Brake Effectiveness

Reply #7 –
Quote
From Jerry:

"I found in this article that it takes 6 full hard as possible at 100 psi brake applications to cause the automatic slack adjusters to do what they are supposed to do.  Light brake applications do not get the job done.  www.mpi.mb.ca/PDFs/AirBrakeManual/ABM_Section8.pdf"

From the article: "Disc Brake Adjustment
There are a number of makes and models of air disc
brakes, each with a different adjustment procedure.
It is therefore recommended that you consult the
manufacturer's manual for adjustment or service."

Meritor's manual recommends against manual adjustment of automatic slack adjusters.

Re: Parking Brake Effectiveness

Reply #8 –
Question from a real dummy here.  If I do six very hard applications of the break pedal I assume this is while stopped.  And since my leg cannot measure 100 PSI would you say that a really hard effort  -- like trying to mash the brake pedal through the floor -- would be ok?
George Stoltz.  Retired from full-time living in a great Foretravel and now are back to living in a traditional sticks and bricks in Florida.

Re: Parking Brake Effectiveness

Reply #9 –
George,
I think we're talking about 100 psi tank air pressure...but I can't be 100% certain.
Peter & Beth Martin
No Forrest? What have you done?
MC# 15890 until Dec 2016; FMCA #F329677
Cincinnati, OH

Re: Parking Brake Effectiveness

Reply #10 –
Quote from: Duane Budd –
From Jerry:
"Meritor's manual recommends against manual adjustment of automatic slack adjusters.
I looked in Meritor's Maintenance Manual; I didn't see that.  Can you give a page # or where you found that?  Page 46 & 47 is about measuring and adjusting the stroke.  I was quite surprised to find the stroke out of adjustment on both rear brakes.

 
Quote from: George Stoltz –
Question from a real dummy here.  If I do six very hard applications of the break pedal I assume this is while stopped.  And since my leg cannot measure 100 PSI would you say that a really hard effort  -- like trying to mash the brake pedal through the floor -- would be ok?
That's what I did while stopped.  I made sure the air pressure gage was at 100 psi or above and pushed the brake pedal down as far as it would go with the parking brake off.  I also read someplace else that 4 applications was enough, which is correct?  If I could do it over, I would have done the brake applications first to see if the stroke was shortened before manually adjusting the stroke.  I do intend to check this more frequently in the future and if it happens again (stroke gets out of adjustment too long) will replace the slack adjusters. 
Jerry Whiteaker former owner 96 U270  36' #4831 Austin,TX-Owner Mods LCD TV w/front cabinet rebuild - LCD TV bedroom - Dual Central AC, either can cool coach w 30 amp - Skylights at roof AC openings - Drop ceiling for ducting of AC - Shower skylight white gelcoat/wood/epoxy frame - Air Springs/Shocks replaced - 2014 CRV - 8K Home Solar - Chevy Volt

Re: Parking Brake Effectiveness

Reply #11 –
100 psi brake application, means stand on brake pedal to apply the 100 psi to the brake air chamber .

If they recommend not checking nor adjusting the slack adjuster, I have lost all respect for who ever wrote that manual  :o :o :o

Re: Parking Brake Effectiveness

Reply #12 –
Thank you, Dave.
George Stoltz.  Retired from full-time living in a great Foretravel and now are back to living in a traditional sticks and bricks in Florida.
 

Re: Parking Brake Effectiveness

Reply #13 –
Peter,
That makes sense. thank you.
George Stoltz.  Retired from full-time living in a great Foretravel and now are back to living in a traditional sticks and bricks in Florida.