I’ve done quite a bit of searching on this forum and found plenty of threads about the difficulty in bleeding the brakes on these trucks, but I haven’t been able to get a complete answer to my questions.

I have a customer’s dump/grain truck that’s giving me issues with bleeding the brakes, but it doesn’t seem to have a standard factory setup. I don’t know if this is due to previous owner modification or not, so here’s what it has:

1) Vacuum booster on the firewall with single piston master cylinder.
2) From MC, line runs down to frame and splits to F and R wheels. There isn’t a secondary vacuum or hydraulic booster, nor have I seen any kind of metering or proportioning valves.
3) All axles have dual wheel cylinders with orientation as follows:
-Steer axle cylinders are positioned vertically at 9 & 3 o’clock with brake hoses hooked into the rearmost cylinders, steel line connecting them and bleeders on the forward cylinders only.
-Cheater axle cylinders are positioned vertically at 9 & 3 o’clock with brake hose connecting to a tee at top of backing plate, then steel line running to each cylinder and a bleeder in each cylinder.
-Rear axle cylinders are positioned horizontally at 12 & 6 o’clock with steel line feeding the bottom cylinder, steel line connecting the bottom to the top cylinder with a bleeder screw at the top.

We replaced the vac booster and master cylinder. I’ve been attempting to pressure bleed (set at 20 psi) with little success - the pedal will feel solid with the engine off but immediately sinks/gets spongy with the vacuum boost.

In reading previous threads, I understand that air is likely trapped in the vertically mounted wheel cylinders in the steer and cheater axles, and I might have a potential fix for this problem but I need good info on how the brakes are adjusted.

I think if the brake shoes are fully retracted, it will compress the wheel cylinders enough that the air pocket will either be eliminated or minimized, but I’m having trouble figuring out how to adjust the shoes. The rear axle is a typical star wheel and I know which way those need to turn (not that it matters because they’re mounted horizontally).

The steer and cheater axles use some type of cam adjuster, but I’m not sure if it’s an S cam or not. Each wheel has a 7/16” adjuster screw for each shoe that passes through the backing plate with a spring on the outside (if it weren’t for the hex head, I would’ve thought they were anchor pins for the shoes). With the wheels off the ground, I can turn each screw fully clockwise and get the shoe to drag on the drum and stop rotation - however, I can also turn them fully counter-clockwise and get the wheel to stop rotating as well. This leaves me confused as to which way actually tightens the shoes. I would estimate the range on each screw is about 1-1.5 turns from stop to stop.

There are no inspection ports of any kind and I haven’t been able to find any diagrams either (I suck at using search engines so that’s more likely the problem). Short of removing a wheel and drum (not desirable at this point) I’m not able to know for sure how to adjust them or whether or not they’re fully retracted to see if that solves the air pocket problem.

Any info or diagram is appreciated.

Thank you.

 

A bit confused here. Does this truck have 2 or 3 axles?
You already know this is not the factory set up. A single master cylinder running directly to the front and rear was outlawed around 1967. It is not a safe or good idea.
I do not thank it needs a perportioning valve because they are all drums.
If I were in your spot I thank I would use a small grinder on the backing plate. Cut a slot big enough to see the shoes.
I prefer to adjust the shoes to lock up and then back them off a bit instead of adjusting up to drag.
Maybe use a new electric fuel pump to pump fluid into the wheel cylinder bleeders. Connect a hose off the master cylinder line back to the container you are pumping out of.
Just sharing thoughts. Open for discussion.

 

Truck has 2 primary axles (steer and drive) and then what I call a cheater axle - it’s an axle that’s positioned just in front of the drive axle. It’s primarily pulled up off the ground and supports no weight, but when needed can be lowered to provide additional support if the truck is loaded heavy (gets around DOT regulations regarding weight limits per axle).

The other few customers we have that use cheater axles have a shut off valve for the brake circuit (or it’s disconnected entirely). Unfortunately this one is active just like the rest of the brake system and our customer wants it to stay that way, so I need to figure out what it’s gonna take to bleed it properly.

 

Well, my theory seems to have paid off!

After much searching, I finally found out that the adjusters I was struggling with were an eccentric type bolt/cam and the reason they would drag in both directions is because the cam would basically rotate all the way around and press the shoe out in both directions (I marked each adjuster bolt so I could actually measure the degree of rotation - it was between 3/4 and 1 turn). So I positioned the adjusters about halfway between the full left and full right positions to retract the shoes as much as possible.

I also realized an error in my initial description - the drive axle wheel cylinders aren’t at 12 & 6 - they’re at 11 & 5 - so I decided to back them off too. Unfortunately I got a little carried away with one and backed it off too far, so when I started bleeding them, it started leaking.

I ended up replacing both cylinders on that wheel, then I was able to bleed the brakes without any further problems. I adjusted the shoes out until I could just turn the wheels by hand.

The pedal felt great both engine off and running. The brakes functioned perfectly on the test drive.

I’m gonna double check for leaks and pedal feel in the morning, but I’m certain backing off the shoes was the trick to bleeding the vertically mounted cylinders properly.

 

Thanks for letting everyone know how that worked for you.

 

Good ! glad to hear! In the spirit of uniform terminology that setup is called an Arkansas tandem

 

I didn’t have (or want, for that matter) a lot of experience with medium/heavy duty trucks prior to becoming a tech. I knew how to do a few things because I used to run line haul and many times I just didn’t want to wait for the roadside repairman to show up. But my boss/the shop owner started out working on them so he doesn’t turn away big truck service if he doesn’t have to, so I’m getting the experience anyway.

As for the “Arkansas tandem” set up, I was curious and wanted to know a bit more.... I don’t think you’re understanding what I was describing, buzzinduzzin (screenshot from when I used the google).