Air brake canister 1993 917 AF
- Thread starter theangrychicken
- Start date
where_the_heffalump_roams
Active member
It was even set far out enough where there was drag. But even at that, the truck rolls with the parking brake on.
They spent 2 hours trying to adjust adjust the brakes. I was hoping the parts I received from Mercedes had everything I needed to replace the drums inward. But the shoes that I received need to be riveted on (this shop does not do that.
I may be pulling at strings, but the shop did not replace anything.
1. Is it possible the actuator has in fact failed, if one of pistons was seized but is now moving, but it is not applying enough pressure to expand the shoes?
2. The shoes are also not worn evenly, the bottom shoes have nearly half the width as the top. Can that be the problem as the shoes aren't making enough contact?
I'm a bit frustrated as this is the 3rd shop I've been at and I don't have a solution. I'm open to ideas.
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where_the_heffalump_roams
Active member
At the risks of sounding dumb. I've been trying to wrap my mind around the parking brake issues and trying to rule out some things and brain stormed with a friend. Neither of us are mechanics or engineers. He sells propane and propane accessories and I work in health care.
1. I vented all of the compressed air and emptied the tanks. The thought was if we purged any compress air, that could rule out a valve issue. It wouldn't matter if a valve was stuck open as the system was had no pressure to withhold the spring in the air canister that compresses the wedge actuator.
2. Possible, but unlikely. A new air canister was installed a few months ago and this is not extending the rod or excepting enough pressure on the wedge unit.
3. I don't think it's a bad idea to have the wedge unit replaced as it was previously stuck. I was hoping to have all the parts on hand to replace the drums and most everything inward. But the pads I received need to be riveted on vs simply replaced. So ideally, I'd be able to take this to single place that could rivet the brake liners on, swap out all the parts, and also diagnosis the problem if if was something other than the actuator.
Please correct me if any of the above is wrong. I know of some places in Washington State and Oregon, but I'll be down in central/northern CA for the next couple of months and would love to get a solid referral to a place within those regions.
1. I vented all of the compressed air and emptied the tanks. The thought was if we purged any compress air, that could rule out a valve issue. It wouldn't matter if a valve was stuck open as the system was had no pressure to withhold the spring in the air canister that compresses the wedge actuator.
2. Possible, but unlikely. A new air canister was installed a few months ago and this is not extending the rod or excepting enough pressure on the wedge unit.
3. I don't think it's a bad idea to have the wedge unit replaced as it was previously stuck. I was hoping to have all the parts on hand to replace the drums and most everything inward. But the pads I received need to be riveted on vs simply replaced. So ideally, I'd be able to take this to single place that could rivet the brake liners on, swap out all the parts, and also diagnosis the problem if if was something other than the actuator.
Please correct me if any of the above is wrong. I know of some places in Washington State and Oregon, but I'll be down in central/northern CA for the next couple of months and would love to get a solid referral to a place within those regions.
Joe917
Explorer
With the front wheels chocked jack a rear wheel off the ground. With air in the system disengage the spring brakes and rotate the wheel. There is an adjuster you can turn with a large screwdriver. Adjust until the brake drags then back off a bit. Repeat on the other rear wheel. Test spring brakes, the truck should have trouble moving when engaged.
where_the_heffalump_roams
Active member
Thanks Joe and Neil for the tips on adjusting the pad contact points.
After my work shift tonight, I'm going to head to a friends house to work on Heffalump projects for a few days. My plan is is to clean the webasto heater, take a closer look at the bumper hydraulics, and some other miscellaneous wiring issues. I'll try to see if any further pad adjustments help. However, my confidence in pad adjustment is pretty low. There has been nearly 12 fleet/truck mechanics that have looked at the brakes at 3 different, independent shops. None were confident that the pad adjustment was the fix and 2 shops spent time seeing if that would get the parking brake to lock the brakes. 2 of the 3 shops found the right rear wedge actuator wasn't working although the last shop found that it was not completely seized.
Though I bled decompressed the system yesterday to try to rule out a valve issue. The only other thing I can think of is the parking brake often take time to release the air. Sometimes a few seconds, sometimes 1-2 minutes. When applied, the air escapes at one of these ports. This is in the left front of the trunk, under the "hood".
I'm wondering if there is anything I should be considering besides pad adjustment?
I'm still very open to referrals to capable shop.
Thanks again!
After my work shift tonight, I'm going to head to a friends house to work on Heffalump projects for a few days. My plan is is to clean the webasto heater, take a closer look at the bumper hydraulics, and some other miscellaneous wiring issues. I'll try to see if any further pad adjustments help. However, my confidence in pad adjustment is pretty low. There has been nearly 12 fleet/truck mechanics that have looked at the brakes at 3 different, independent shops. None were confident that the pad adjustment was the fix and 2 shops spent time seeing if that would get the parking brake to lock the brakes. 2 of the 3 shops found the right rear wedge actuator wasn't working although the last shop found that it was not completely seized.
Though I bled decompressed the system yesterday to try to rule out a valve issue. The only other thing I can think of is the parking brake often take time to release the air. Sometimes a few seconds, sometimes 1-2 minutes. When applied, the air escapes at one of these ports. This is in the left front of the trunk, under the "hood".
I'm wondering if there is anything I should be considering besides pad adjustment?
I'm still very open to referrals to capable shop.
Thanks again!
Joe917
Explorer
The fact that it sometimes takes minutes for the spring brake valve to release air is concerning, it should be instant. The fact that dumping all the air does not engage the spring brakes rules that out as the problem, but it needs to be looked at. Do you routinely dump air from the wet tank? The truck has an air drier, (you can add that filter to your service list) but moisture can still build up in the tanks. Routinely dumping the wet tank stops that moisture going through the system. It may have carried some crud into that valve, it should be serviced.
Joe917
Explorer
The spring brakes should engage/release somewhere around 3-4 Bar. The red air warning light comes on just before the spring brakes engage. When driving you should be constantly scanning that gauge. When you air up with the parking lever released you should hear the rear springs being compressed suddenly at that 3-4 Bar point.
where_the_heffalump_roams
Active member
I do dump the tanks every so often.
I haven’t replaced the drier filters, but just received 3 of them from Hans a few days ago. That’s a part of the service plan this week.
I haven’t checked to see do the spring brake activates at low bar, but they are activating when the parking brake is engaged. That last time the brakes were adjusted (Friday). The tech had them at full contact and then backed them off just enough to clear. The fact that the wedge actuator was not functioning correctly before makes we really wonder if that is the culprit. But the uneven shoe wear has be concerned too. The top shoe has double the pad thickness as the lowers. I have new shoe pads and drums, but the shoe pads need to be riveted on. So I need to get to yet another place that will do that and service the brakes. I’m wondering if replacing the drums, pads, and that actuator will resolve this issue.
Sent from my iPhone using Tapatalk
I haven’t replaced the drier filters, but just received 3 of them from Hans a few days ago. That’s a part of the service plan this week.
I haven’t checked to see do the spring brake activates at low bar, but they are activating when the parking brake is engaged. That last time the brakes were adjusted (Friday). The tech had them at full contact and then backed them off just enough to clear. The fact that the wedge actuator was not functioning correctly before makes we really wonder if that is the culprit. But the uneven shoe wear has be concerned too. The top shoe has double the pad thickness as the lowers. I have new shoe pads and drums, but the shoe pads need to be riveted on. So I need to get to yet another place that will do that and service the brakes. I’m wondering if replacing the drums, pads, and that actuator will resolve this issue.
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VerMonsterRV
Gotta Be Nuts
Ok, here is my 2 cents (and not being a mechanic it may be worth just that). Our truck had very uneven brake lining wear. From my crude understanding that can really only happen if the wedge units are failing (or the wedge units were originally very poorly adjusted). The air canister pushes a rod towards the wedge units. On the end of that rod is a wedge. When the air canister pushes the rod towards the wedge unit, the wedge on the end pushes on the 2 pistons, which then spread the brake shoes. The rear cylinders are combination cylinders. An air portion for your normal brakes and a powerful spring for your parking brake. When all air is vented from the canister the spring then pushes the rod into the wedge units locking the brakes. This can happen if you put on the parking brake or while if you have a major air leak while driving.
So if you vent all the air from the rear brakes and you still have poor parking brakes then it can be weak springs in the canisters, faulty wedge units or very worn/misadjusted brake shoes. If you're not sure if all the air is being released then removing the air lines on the canisters will make sure all is out. Just make sure to bleed the air out of the system first.
where_the_heffalump_roams
Active member
Thanks, that is actualy helpful.. I didn't think to disconnect the pair lines to see if that activated the canister. That is also a possible failure point though it is newer.
nick disjunkt
Adventurer
I haven't read through the full thread, but your problem sound similar to something that I had. When I disabled the parking brake it would sometimes take a long time to actually release (indicated by a light on the dash), or sometimes not at all. It was caused by air leaking between the foot brake and parking brake portions of one of the rear air chambers, through a perished diaphragm. When it happened, air would leak out of the load proportioning valve on the rear axle.
I could temporarily get the parking brake to release by disengaging it, and then putting my foot hard on the foot brake, this would effectively close the foot brake line and force air to the parking brake spring. The problem was sorted properly by replacing the chamber with a replacement ordered from a UK Mercedes truck dealer, and sent over by a relative.
I could temporarily get the parking brake to release by disengaging it, and then putting my foot hard on the foot brake, this would effectively close the foot brake line and force air to the parking brake spring. The problem was sorted properly by replacing the chamber with a replacement ordered from a UK Mercedes truck dealer, and sent over by a relative.
where_the_heffalump_roams
Active member
My problem is actually the opposite where the parking brake is not locking the wheels.
I'm getting movement at my wedge actuators and contact within the drums/pads. But not enough to keep the truck from moving.
Do you happen to have a diagram of the system and a part # to the valve you replaced?
where_the_heffalump_roams
Active member
Anyone have a diagram of the air brake system?
I'm trying to see what valves are involved in the systems and potentially get a parking brake valve ordered.
I'm trying to see what valves are involved in the systems and potentially get a parking brake valve ordered.
VerMonsterRV
Gotta Be Nuts
Here's my 2 cents, and it may be worth just that as I am no mechanic. If you are seeing shoe movement and you can hear air being released when the parking brake is being applied then I would think the valves are working. Your parking brake works when the air is totally evacuated from the canister. When you release the parking brake air is put into the canister to compress the parking brake spring, this air pressure is always there compressing the spring while you drive. But to answer your question, I haven't come across a diagram, and I would like to have one.
So I would check and see if the canisters have air. So drain all air from the system from the air tank drain valves. Unscrew the 2 air fittings on the canisters. This will make sure that you are using the parking/emergency brake part of the system. Test the parking brake. If the parking brake doesn't work now then you know it is 1 of 3 things. Faulty air canisters (30 year old internal parking brake spring has lost its "spring"), faulty wedge units (worn surfaces so when the wedge pushes in the shoes don't move as much), or worn/misadjusted (they touch the drums but not with enough force) brake shoes.