| Author |
Message |
Peter Ehlert (209.242.148.130)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, February 11, 2002 - 2:22 pm: | |
I purchased a ’73 GMC 4905A 2 weeks ago and have already made few trips over the Grape Vine and I plan a trip over the Sierras of Baja this weekend.
Can someone give me a quick run down on the basics of brake adjustment?
please reply to bno@sdi-baja.com |
DaveD (216.18.113.69)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, February 11, 2002 - 5:27 pm: | |
Peter,
Please see my e-mail to you. Please note that I am not a subject matter expert and I am not certified to adjust air brakes. An alternative would be to take the bus to a licensed mechanic for brake adjustment. |
a modest guy (216.188.79.81)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, February 11, 2002 - 7:55 pm: | |
Peter,
I'm not a certified to do air brake adjustments, nor am I an expert, but here is some information.
Your bus probably has type 30 airbrakes which have a maximum stroke of 2-1/2" I have a chart with figures which show that the maximum allowable slack, using what's known as the 90-100 psi.. method, is 1-3/4", (note this is about 1/4" shorter than the maximum that an inspector would look for as a safety violation) if using the hand pull method, the maximum is 1". Allowable slack for the front brakes will likely be less since the front brakes usually use smaller brake air chambers. The brake chamber size (indicates effective square inches of the service brake diaphragm) is marked on the outside of the brake chamber. Use care to avoid getting too close to the spring brake (backside) f the brake chambers, as they can cause fatal injuries if they let go. Many buses use what are called safety actuator parking brakes, rather than spring brakes but I'm not sure this is the case with GM buses.
The Hand Pull method is a method of measuring slack by pulling or prying on the slack adjuster. The slack adjuster is the arm that attaches to the end of the shaft that rotates to actuate the brakes (by means of turning an "S" cam). the motion you are checking is the movement of the pushrod which extends from the brake air chamber to a clevis which attaches to the slack adjuster. The reason the limit for Hand Pull method is lower than for 90-100 psi. method is that the hand pull method doesn't involve full application of the brakes, where the air pressure will allow result in more movement (i.e. you can't exert as much force to move the pushrod by hand as the air diaphragm does by application of the brakes).
It's important to recognize that on long grades, brake drum can expand due to heating, while shoes may compress slightly due to the applied pressure, both resulting in more stroke of the pushrod required to maintain braking. As a brake drum expands 0.020" this requires about 1/4" more movement of the pushrod. Combine 0.040" drum expansion with 0.010" (0.010" per shoe, 0.20" total) compression of the brakes shoes and this will require an additional 3/4" stroke. If brakes are sufficiently out of adjustment the push rod could bottom out, leaving the brakes ineffective. Brakes should be adjusted for contact with the drum and then backed off about 1/3 turn to ensure adequate clearance from the drum.
Manual slack adjusters have a locking sleeve that needs to be depressed so that the adjusting bolt can be turned. If you are turning the adjuster in the correct direction the cam shaft should rotate, but the slack adjuster should not move outward. If it does, you are turning the adjustment bolt the wrong direction. The cam should rotate in the direction required to apply the brakes. The angle formed between the centreline of the slack adjuster and the pushrod should not be less than 90 degrees.
I would strongly recommend an air brake course. I've taken one at my local community college and it has been very informative. There is also a good book entitled" Air Brakes From the Drivers Seat II" by Allan C. Wright. It's published in Canada, and is available from Presto Print Ltd., 1183 Voyageur Way, Richmond, BC, Canada V6X 3J4. Tel. (604) 278-2428, Fax (604) 278-8549 |
Buswarrior (Buswarrior) (64.12.102.176)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, February 12, 2002 - 11:31 pm: | |
Hello Peter
Go and take a good airbrake course, and pay good money for it!
There a few details which are not quite accurate in the previous post, but the principals are there. Go take a course, reread the post and you'll find them too!
Re-adjustment is pretty easy, but novices can make DEADLY mistakes if they don't know the whole story.
You have an ethical and a moral responsibility to the rest of us to KNOW your brakes are ok, and to know how to maintain your brakes properly if you are doing it yourself. By the time you can feel something wrong with your brakes, it is already too late. They are gone.
Everyone also has to start thinking about auto-slacks. They will be showing up in greater numbers starting now. MC8's could come equipped from the factory, so what else has 'em or has been retrofitted? They must not be treated the same, depending on the style.
The only stupid person is the one who doesn't ask, or assumes they know what to do!
INVEST in a training course! It's like INSURANCE, you just pay it because you need it!
happy coaching!
buswarrior |
DaveD (206.47.98.151)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, February 13, 2002 - 12:25 am: | |
The angle referred to between the slack adjuster and the pushrod is with the service brakes applied. The angle check is for quick reference but may not meet regulatory requirements as a method of checking brake adjustment under newer rules.
Also for safety, wheels should be chocked. |
DaveD (206.47.98.151)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, February 13, 2002 - 12:34 am: | |
Total increase in required brake movement in the example in the post should be 0.060", resulting in about 3/4" increased push rod movement required. That's not very well stated in the post and it's abit of over simplification.
The real point is that marginally adjusted brakes can become ineffective under sustained braking conditions where fade occurs. |
dougthebonifiedbusnut (24.218.115.126)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, February 15, 2002 - 8:11 pm: | |
i think getting caught up in all those numbers can be a litle misleading having been in heavy trucking for over 20 years i hope i can make the process a little clearer at some point in time the dot came to the conclusion that only certified brake people could do brake adjustments in some cases this was a good thing i am not but having done it many times on the road and in the shop i can tell you how i have done it if you are on the road and you think you need to adjust your brakes find a level place chock your vehicle and release the spring brakes than take you usualy 9/16 wrench and a hammer push the locking collar into the slack adjuster and turn the adjusting bolt usualy clockwise untill you cant turn it any more dont force it you just want to get it so it wont turn any more this is when the shoes have contacted the drums at this point back of the same screw about 1/4 of a turn do the same to all of the adjusters if you are in the shop do all the same steps but now you want to jack up each wheel and slowly turn the tire till you can just begin to hear the brake shoes touch the drum at this point back off the adjusting bolt just enough so the shoes are not touching the drums and again do the same to all the wheels buswarrior and a modest guy are right on the money when they stress not doing this and getting some schooling if you are not clear about what i just tried to convey to you if your brakes are mechanicaly in good working and dont have any badly worn internal parts your bus will stop in good order if you do as i have explained if it doesnt you need to get proffesional help it is very definatly a moral obligation hope this helps |
Mike Eades (Mike14905) (206.62.201.168)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, February 19, 2002 - 10:27 am: | |
I ahve a 4905 and i want to put auto adjusters on and I can't find anyone who knows if they will fit. There is not a lot of room between the backing plate and the frame. Anyone done it and what adjusters did they use. Mike |
Jim Wilke (152.163.204.192)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, February 26, 2002 - 6:35 am: | |
There are a couple of reasons NOT to put self adjusters on your bus, too. If you don't have to crawl under there every 1500 miles or so to adjust, you probably won't grease the chassis or do a general visual inspection either. While adjusting my brakes yesterday, I noticed that one of the air brake hoses was rubbing on one of my 8 brand new airbags & they were rubbing holes in each other. Another 4-5000 miles and one or both would likely have failed. I was also able to notice, while lying under there, that one of the brake cans is leaking a little while the parking brake is off. I would NEVER have heard that if I weren't adjusting 'cause the parking is always on otherwise if I am underneath. Now I can plan to fix that in my driveway instead of on the road side.
I, too hate to adjust the rears because I don't have a pit & I kinda get "the willies" way under there messing with the brakes, but I run the rear up on (3) 2 x 12s per side, arranged like broad steps. These are the same ones I carry on the bus for levelling in badly graded campgrounds. After running up on the blocks, I block the tires with 4 x 4 blocks in front & behind both of the rear tires. THEN I block the body up so it can't come down on me while underneath. (Remember the air bag rubbing the hose?) I won't crawl under it if the body isn't blocked and no one else should either!
Bottom line is I sure am glad that I got under there to adjust, grease & INSPECT. The inspection alone saved me an on-the-road air bag blowout & maybe a brake failure who knows where which would have cost AT LEAST several hundred dollars & maybe a ruined vacation! Maybe worse!
Jim-Bob
PD4104-4039 |
FAST FRED (209.26.87.20)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, February 26, 2002 - 4:02 pm: | |
DA Book requires a good lube job every so often.
If you do a good job and block and lift the coach as above , it takes only seconds to adjust the brakes too.
For the best lube job , be sure to lift both front tires at the same time and swing the wheels back and forth while pumping into the kingpins.
Unloaded is the ONLY way to get lots of grease everywhere.
FAST FRED |
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