Ineffective Air Brakes

[jecarr]

Cabover Mack 1987 MR630P sanitation truck is VERY difficult to stop. Brake pedal all the way on floor to stop-OR- hold truck. New rear shoes, and all shoes adjusted. Has a heavy PAKMORE compactor body. MAX diameter on rear drums is 16.620 inches, they measure 16.5 inches. The park brake will BARELY hold the truck still, while idling in drive, and it will creep backward in reverse, with park brake applied. New rear drums have been ordered, and will be here, today. How much will the new drums help?

[Freightrain]

Has it always been bad? What is the angle on the slack adjusters to the pushrod?

http://apps.leg.wa.gov/WAC/default.aspx?cite=204-76-99004

[jecarr]

Not sure how long it has been this bad, it was previously worked on in our heavy equipment shop, but that mechanic retired, with no replacement, now all the vehicles that went there, are being brought to our bus maintenance shop. The angle of push rod to slack adjuster is close to 90*, as it sits in our wash bay, now (with park brake applied). Thanks for any advice, you can give me.

[41chevy]

Got full air to the brake system?

Could the brake foot valve be bad?

[jecarr]

Got about 130 psi on air guage, I don't have the equipment rigged up to measure pressure at each chamber hose, and I have a new foot valve, but not installed, yet, as it is under the cab floor, and the cab tilt hydraulics have issues.

[Freightrain]

The park brake will BARELY hold the truck still, while idling in drive, and it will creep backward in reverse, with park brake applied.

That is not an air pressure thing, that is a leverage thing. Or lack of.

[mrsmackpaul]

or an adjustment thing it has either being put together wrong or the slack adjuster arent locking when adjusted and are simply unwinding them selves when first used I seem to remember someone on here had similar problem with an R model and it was faulty slack adjusters after all our high tech well thought out guess so maybe adjust them up then take it for a drive the check them again see if they have moved at all

Paul

[Freightrain]

I think it was Trent that has issues with his RL700 back when he first bought it.

[jecarr]

Ok, will do that, thanks,guys, I will get that done, and report back. That does make sense.

[41chevy]

That is not an air pressure thing, that is a leverage thing. Or lack of.

Is the MR parking brakes air applied or mechanical with a pedal or lever?

[chief doug]

if it's not holding with the parking brake and the adjustment is correct, I would start by replacing the rear brake chambers. Also make sure the right length slack adjuster is used and the right size chamber ( I.E. a 24/24 can or a 30/30 can ETC). It's a leverage thing.

[gearhead204]

as chief Doug said try new cans you may very well have two broke springs in the "park" chamber of the maxi cans, thus not creating enough leverage to properly apply the brakes.

[jecarr]

In response to 41 Chevy--the park brake is air brake. In response to Chief Doug, the park brake springs are applying the levers, and it does hold, but just barely hold, after putting in gear.

[Freightrain]

So it basically has a typical Maxi brake on it. Nothing out of the ordinary?

Seeing it has poor service brakes AND not much parking brake, I would concentrate on slack adjusters and adjustments.

[gearhead204]

chock the wheels and release the brakes, then mark the push rod flush with the front of the can, then back off the slack adjusters several turns then apply the park brake. check to see that the slacks are still loose then measure from you mark to the front of the can you should have 2 or2 1/2 inches of rod showing beyond your first mark if you dont there is either a broken spring in the can or crap built up in the application chamber not allowing the pistion to move.

[Quickfarms]

I would also look at the S cam to make sure it is not binding on the bushings.

Also make sure the slack adjuster is not moving on the S cam

[chief doug]

change the cans, the springs are shot

[jecarr]

Thanks, to all of you guys! That's enough to keep me busy for a minute or two. I will verify findings, in the near future. It is on a limited time basis between other vehicles. Have a good weekend.

[TiredIron76]

Sorry, bad edit. see next post

Edited July 23, 2015 by TiredIron76

[TiredIron76]

Considering the age of the truck, you might have two problems here; weak maxi springs could account for your poor parking brakes and a bad air supply to your service brake relay (R12) valve (or a bad R12 valve) could result in poor service brake performance. If your maxis are OEM and have never been replaced, trust me, it's time to junk them. Even if the springs are in one piece, they've already cycled about a million times and they're weak.

You may have already done what follows, but if you haven't, it's worth a try:

The R12 valve supply should come direct off the primary air tank. Drain the tank, undo the line from the valve, air the tank with shop air and you should get a blast of air from the line. If you don't, or if the air flow is weak, pull the line from the tank and check for sludge, collapsed or deteriorated line or any obstruction.

Reconnect the supply line and undo the service (signal) line from the treadle valve. Have someone step on the treadle valve. The harder the treadle is pushed, the more air you should get from this line. The air pressure to your brake chambers is directly proportional (after valve crack pressure, no more than one and a half or two pounds) to the air delivered to the relay valve from the treadle.

If there's plenty of signal air, hook it back up and pull a line at one of the chamber delivery ports. A weak airflow there indicates a bad R12. Replace that, and you may have it fixed.

Assuming you've checked and corrected all the stuff mentioned in previous posts and that the basics (drums, shoes, brake hardware) are all present and correct you should have brakes on the truck. Good luck!