M818 brake problems

I am having trouble with the brakes on my M818.

A little history. The truck has been modified to one rear axle and the glad hands removed/ capped off.

I was having trouble with the brakes wherein I had good power brakes but as I sat with my foot on the brake pedal, the pedal would rise and there was no power brake assist; only manual brakes. I attributed this to a faulty air piston seal in the brake booster and replaced the brake booster. I also had, on rare occasion, a situation where the brakes would lock up.

After replacing the brake booster, I had no power brakes with 120psi system pressure. No response at all from the booster. Bleeding air off down to ~100psi allowed the booster to operate. I reset the air governor to 90psi and retested. The amount of brake assist is highly variable and I had one occasion wherein the brakes locked up hard and I had to bleed air off to get it free.

1) Is this issue related to one of the three air shuttle valves?
2) I will not tow a trailer with air brakes nor be towed with a vehicle having air brakes. Can I remove the two frame mounted shuttle valves and simplify the air booster plumbing to something like the deuce: hydraulic in, hydraulic out, air in, vent out the top, pressure out to only the brake switch, and cap off the line at the rear of the booster.

Other suggestions?

I reconnected the airpack: hydraulic in from master cylinder, hydraulic out to wheel cylinders, air in from the tank, service air out to the brake light switch, kept the air vent in the same spot, and left the shuttle valve at the rear of the airpack open (didn't work plugged). Brakes worked fine; for awhile. While in a short drive, the pedal travel got shorter and shorter and the pfft from the airpack vent got smaller and smaller, the the assist got weaker and weaker. I have NO pedal assist and NO pfft venting from the airpack with 120psi. Thoughts?

Sent the first replacement airpack back to "vendor A." Realizing that there will probably be a two week turnaround and really wanting to get this truck ready for a July 4th parade, I ordered a rebuilt airpack from "vendor B" and installed it today. Good news is that it now has great brakes. Bad news is that there is a massive air leak out the vent tube on the top of the airpack when pressing hard on the brakes. A lighter pressure on the brake pedal results in a proportionally smaller air leak. I am assuming that the air is leaking past the piston on the air side; going to have to return this one, too. Vendor B says that it takes five cores to make one good airpack. I wonder if we are simply at the end of the line for good parts.

Being impatient and a little discouraged with both NOS and rebuilt, I took a chance and ordered an airpack from Aliexpress that looks identical. The cost for this airpack is less than a rebuild kit. We will see...

Airpacks don't like having little 'compression' space in the circuit leading to the rear gladhands.

Putting a plug in the pack air output will give you trouble.

For the second airpack, leaving the shuttle valve on the back open or plugged made no difference. No air comes out if it. The port on the front valve, I believe service out, only runs with a short line to the brake light switch. I did all of this to eliminate potential issues with the two other shuttle valves.

I am open to suggestions on ways that I could be creating this issue, but all three airpacks (including the original) all have very different symptoms.

certainly some weird stuff with all 3 packs.. Are the gladhands still at the front of the truck and still plumbed? Shot in the dark but the one on the passenger side of bumper is supposed to be a vented cap and sometimes the vent gets clogged .. This can cause brake lock

Airpack 1: This is the original airpack when the previous owner bought the truck probably 15 years ago. It is likely just an age/ lack of maintenance related failure.

Airpack 2: This was a new airpack. When received, this airpack was soaked in brake fluid and there was thread damage to some of the NPT ports. I assumed the thread damage was due to the paper pipe plugs which had been wet for very long time and rotted the threads. I didn't know what to think of the brake fluid that had leaked out. So, I repainted the airpack, did my best to clean up the threads, used a lot of thread tape, and installed it. In hindsight, I should have sent it right back to the vendor but I was impatient and wanted to get the truck back on the road. This airpack never did work correctly.

Airpack 3: This was a rebuilt airpack. After initial installation, it worked fine and had great brakes, but very shortly thereafter developed the air leak out the vent tube.

Well, installed the second replacement unit from vendor "B" and brakes worked great but...

I did get a "working" brake booster installed in time to have the truck in the July 4 parade.

The problem is that while these boosters are new, they are also very old stock having spent their life with poor storage conditions. The below picture is looking into the air inlet at the front of the booster now on the truck. The spiral you see is the spring just inside air inlet. Pretty crusty.

The latest NOS booster didn't work out of the box. It had pedal kickback and it leaked fluid between the hydraulic cylinder/ cast iron fitting. A little driving and the kickback went away. I tightened up the fitting. I didn't think the vendor would accept yet another return.

The first vendor ended up sending me a new production booster to replace the defective unit. I am keeping it as a spare but this is what I would have ordered if I could do it over.

I had a bit of a panic stop wherein I had to press hard on the brake pedal. The booster was not the same afterwards. It was inconsistent in braking effort from day 1 but now pressing hard on the pedal causes the pedal to kick back losing all power boost. So, in went the new manufacture booster.

JasonS said:

I had a bit of a panic stop wherein I had to press hard on the brake pedal. The booster was not the same afterwards. It was inconsistent in braking effort from day 1 but now pressing hard on the pedal causes the pedal to kick back losing all power boost. So, in went the new manufacture booster.

Click to expand...

Gonna do an autopsy and report back?