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Author Topic: Rear Brakes will not engage  (Read 3612 times)
miles2go
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« on: October 04, 2010, 09:08:54 PM »

Well thats one of my most recent problems with my GMC 4905. When I apply the emergency/parking brake I can see the slack adjuster arms on the rear brakes move when the brakes are applied, release the emergency/parking brake and the arms move again and release the brakes. When I apply the brakes by pressing on the brake pedal only the front brakes work, the rears have no movement at all. Does anyone have an idea what component to look at first? Air pressure is not being released somewhere in the rear system so the brakes will apply. There are more than enough places to look so it would really help if there was a "usual suspect" that I could start with to narrow it down. Looking underneath, there is a nightmare of lines and control valves. Any ideas? What particular part releases the air pressure so the rear brakes apply?

The second urgency is the "post" on top of the spicer transmission that holds the 1st and 2nd shift rod from the shifter. The post or shaft is metal and is protruding up out of an aluminum "tower". Over the years it has wallowed out the hole it is mounted in and now slops from side to side about 2" when shifting. I am not sure if it is pressed in, screwed in, or retained by a nut or some other type of retainer. Between the slop in the bushing on the shift arm and the sideways movement of the post, shifting is difficult. This is really loose on both the 1st/2nd post and the 3rd/4th post. The bushing I hope I can remedy with a new one, but the post is a problem. If there is not a fix that does not entail the removal of the top of the spicer to repair the posts I wonder if I can "cement" them in there with JB Weld or something better that would affix aluminum and steel? Any suggestions on that repair? There is not too much pressure on it and it is just a pivot point for the shift arm... but it has to remain stationary.
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wildbob24
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« Reply #1 on: October 04, 2010, 09:22:08 PM »

Miles,

It's possible your treadle valve is at fault, but more than likely it's the relay valve. It will be mounted on the bulkhead just in front of the rear axle. It'll have a small signal line from the treadle valve, a large supply line from the tank and two delivery lines, one to each brake chamber. It might be stuck and not responding to the signal pressure from the treadle.

I don't have a suggestion for the transmission, but the JB Weld might be worth a try.

Bob
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miles2go
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« Reply #2 on: October 05, 2010, 04:15:50 AM »

Thanks, I will try the relay valve first if I can find it. It is hard looking at suspects near the rear wheels and pushing the brake pedal at the same time, lol.
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robertglines1
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« Reply #3 on: October 05, 2010, 06:00:20 AM »

the small line is a signal line from front telling the relay to open up and apply pressure to brakes..can you access thru floor? you can take line loose at either end and apply brakes with engine off  and hear air rushing thru open line..
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Don4107
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« Reply #4 on: October 05, 2010, 05:35:26 PM »

"Air pressure is not being released somewhere in the rear system so the brakes will apply."

Your brakes are applied BY pressure, not releasing pressure. 

You are releasing the parking brakes after safely blocking the wheels before you apply the brakes, right.  If not the brakes are already applied and you will not see much movement.  The brakes are applied by the rod extending from the brake can.  The parking brakes only work on the rear axle.

Sorry if this is stuff you already know, just not sure from your post how you checked. Good that you are checking things out and learning about you brakes.

Good luck
Don 4107
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Don 4107 Eastern Washington
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miles2go
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« Reply #5 on: October 09, 2010, 04:06:45 AM »

I finally got the bus blocked up enough to look underneath. The valve at the top of the picture that is above the Relay Valve exhausts air out of the threaded port you can see there. When I block off the threaded port and have the brake pedal pushed applied the slack adjuster arms move and the brakes apply.

When the port is left as is, open, the brakes do not apply when the pedal is depressed, only when the emergency knob is pulled up. My question is that it looks as though this port has never had a plug in it from the grime buildup. should it be plugged for the brakes to work? I will try to find the name of this component and it's function today.

I just do not want to plug this port to make the brakes work but break something else if the port should not be plugged.

Andrew.
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Lin
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« Reply #6 on: October 09, 2010, 03:01:48 PM »

I believe that is the relay valve.  As I remember, air will exhaust from it if one of the brake can diaphragms is ruptured.  On a DD3 brake can, there is a separate diaphragm for the park and service brake, so it would be reasonable for the park to still work eve if the service brake diaphragm was bad.  So before you try replacing the relay valve, you would need to know if the brake can is okay.  Maybe someone else can tell you how to determine that.
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« Reply #7 on: October 09, 2010, 04:08:10 PM »

sounds like your rear relay is bad, they are inexpensive, yours looks old, replace it.   FWIW if you keep the exhaust port plugged your brakes won't release (or at least not quickly).  Sounds like you may need a qualified mechanic to help you with this and to adjust and maintain the rest of the braking system, this is very dangerous stuff you are working on and it sounds like you may not have enough knowledge/ability.  Speaking of which not repairing the shift rod pivot post until it is all wallowed out is the kind of thing that leads to more expensive repairs later.  On my spicer taking the cover off is not that hard, but yours is different.  You will probably be able to fix this affordably, some pics would help.  Cheers, Patrick.
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FloridaCliff
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« Reply #8 on: October 09, 2010, 05:19:27 PM »

Miles,

Before you buy a relay valve, try this.

Clamp off one of the supply hoses to the brakes, not parking brake and see if the other applies or there is a stop in air coming out of the relay valve exhaust port when you depress the brake pedal.

If it stops passing air when one side or the other is clamped, you have a bad diaphram.

The air should only come out the exhaust port when you release the brakes.

Let me know what you find.

Cliff
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Bill B /bus
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« Reply #9 on: October 09, 2010, 07:04:04 PM »

As the saying goes you could be in deep kimchee with the shifter. The cover is aluminum. My fix was a rethread and then an insert as the post is threaded into the cover hole. Or maybe weld, bore and rethread.

Now the cover won't come off the transmission in place due to the shifting fork length inside the transmission. That means that the transmission must come out in order to remove the cover. You will need a new gaskets for the transmission cover and bell housing. Make sure the transmission is in neutral when you lift the cover. And make sure you know, pictures are good, exactly where the fingers are inserted.  Otherwise you can have four reverse and one forward  gear. Yep. Got a refreshed transmission and got to put in in twice.

If you do pull the transmission the change out the bearings on the rear bulkhead shift tower. Look at the reverse light switch and make sure it works. Make sure you have a working low oil pressure switch for the transmission. Flush out the oil sump. New filter and new oil. At that point you might as well pull the clutch pack and check for wear.
If you have an external oil pump install is "easy". If you have an earlier transmission with the internal pump then follow the DD manual closely to ensure that the oil pump gear drive meshes properly with the drive shaft.

Good luck
Bill
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zubzub
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« Reply #10 on: October 09, 2010, 08:03:48 PM »

not so fast Bill, he has a GMC V drive, I doubt he needs to pull the tranny to access the G box cover it is right there accessible at the side entrance door  of the  bay.
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RJ
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« Reply #11 on: October 10, 2010, 01:11:16 AM »

Patrick -

The 4905 gearbox is NOT in the same place as the gearbox in your 4104.

Half the gearbox cover Bill's talking about is on the rear axle side of the engine bulkhead, starting with the 4106 and all later model GMC 4-speeds.

That's because GMC put the bevel gears between the clutch and transmission on these models, not after the transmission like your 4104.

FWIW & HTH. . .

 Wink
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RJ Long
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« Reply #12 on: October 10, 2010, 02:21:18 AM »

Patrick -

The 4905 gearbox is NOT in the same place as the gearbox in your 4104.

Half the gearbox cover Bill's talking about is on the rear axle side of the engine bulkhead, starting with the 4106 and all later model GMC 4-speeds.

That's because GMC put the bevel gears between the clutch and transmission on these models, not after the transmission like your 4104.

FWIW & HTH. . .

 Wink

Oh well, I knew it was a little different but I didn't think it was buried that far.  A Buffalo was living in the field next to my'04  and I've looked in there a few times.   Does that mean the trans has to be pulled to remove the cover?
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miles2go
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« Reply #13 on: October 10, 2010, 04:16:36 AM »

Bill B and RJ are right. The bulkhead is in the way of lifting the cover off of the transmission. Thanks Bill B, now I know the post hole is threaded and can try to repair it.

Cliff, that is a good diagnosis procedure, thank you. I will try that first. I will report back. Zub Zub and Lin, could you look at the picture again please? I was thought (but could be wrong) that the Relay Valve was the component in the lower part of the picture. The air is coming from the threaded port on the component you can see in the picture above the Relay Valve. That is where when I put my thumb over it the service brakes move and apply. Still, the theory of a ruptured diaphragm in the DD3 can is a good lead to follow. I am just wondering what the component actually is if the relay valve is the lower item in the picture.

I was sent a PM with this thought, that the component leaking the air from that threaded port (mounted above what I think is the relay valve) is the Inversion valve. I will try Cliffs procedure to see if there is a ruptured diaphragm.

Thanks everyone,
Andrew.
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bevans6
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« Reply #14 on: October 10, 2010, 04:18:59 AM »

If you have four or five inches of vertical clearance, you can put a helicoil or solid threaded insert in.  I've put them in ridiculous places...

I would recommend a solid insert in this case, although I usually start with a helicoil.  With helicoils you need to buy a kit, with the correct special threaded tap and insertion tools, along with the inserts.  With a solid insert you can use a standard tap, typically two sizes larger.  You may need to find a machine length drill bit, and a right angle drill, and make up a tool to thread in the insert, but maybe you can get it done in place.

Brian
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