Hello,

I just picked up an 1973 L Series Dump Truck that I am trying to get road(ish) worthy. It has a CAT 3208 and an Auto Trans.

I am trying to diagnose an air brake issue. I got the truck fired up, it had been sitting since 1995, at least thats the last time the tag had been renewed. I was able to release the parking brake (small level valve on the bottom side of the left panel), and air released. I moved the truck 20yds and set the park brake. I tried to release the park brake and nothing happens. I drained the pressure and removed the air lines from the brake valve, and started the truck and no air is coming out of either line, so the issue must be within the firewall attached thingymabob… Manifold, Distribution Block, whatever its called. (photo attached)

Any advice or should I remove the manifold and give it a thorough cleaning?

Also, the trucks Serial # is U814VSS05315

 

U814VSS05315

U81 = LT800 diesel

4 = Caterpillar V-200 573 CID V8 diesel engine aka Cat 1150, which became the 3208. Is yours original or a replacement?

V = Kentucky Truck Plant

Your VIN is showing 12 characters--only supposed to be 11, so trying it with this number:

S05315 = 1973 numerical series, possibly June 1973 production.

Recheck and I'll make corrections.

As to the air brake question, removal and cleaning of the manifold sounds logical.

There are others here who will chime in.

 

Thank you! The valve cover has 3208 on it. There is a Cat Remanufactured Parts booklet in the cab, so entirely possible it’s been replaced.

 

Nothing is impossible but I’ve driven and worked on L models since the late 70’s and have never had the dash manifold apart
Are both needles in the gauge coming up indicating air pressure on both sides of the system
The air compressor on that engine is belt driven, is the belt still on?
If the air pressure is only increasing 10-50 psi I’d replace the air governor
If the air pressure isn’t coming up at all I’d check the service valve on the frame back near the rear axle, if it failed air from the compressor will leak out as fast as the compressor can pump
It takes a minimum of 60 psi to release the brakes

 

Green needle goes to 115ish.
White needle is at like 20 i think.

 

That white needle was originally red but they fade to white over time
until that needle comes up the brakes are not going to release
If the green needle went up to 115-120 lbs and stopped it sounds like the air governor is working, it is plumbed into the primary system to protect the system from over pressure
Find the drains on the tanks and open them to drain any moisture and also see if both sides are equalizing, there’s a air tank under the cab doors on each side
When you build air pressure and shut the engine off does the air pressure remain or does one or both needles drop fairly fast

 

Air pressure slowly drops, there is a SLIGHT air leak along passenger frame rail under cab.

The parking brake released and then set, so that's the only reason I thought the manifold may have some trash stuck in it. I will check both tanks.

 

Just got back to mess with this… Only one tank on the truck. Pulled the drain, nasssstayyy!

EDIT: I AM BLIND…. drained both! Driver side was plum full of water. Green needle pressurizes way faster than the red.

Is there a easy way to pressurize the system with an external compressor to check for leaks?

 

Yes, there are usually a pipe plug or 2 on the air tanks and folks remove the plug, add a vale and a fitting to plug in your air hose

 

That black valve screwed into the tank is a automatic moisture drain but it’s probably stopped up with oil and crude
You can screw it out of the tank to dissemble and clean, it may work again or you could replace them with a simple manual drain. A fitting with a schrader valve for a tire chuck can be installed were you removed them with plug
There’s a safety check valve between the tanks that prevents both tanks for losing air should there be a leak in the system, the primary tank the green needle is connected to will build 40-60 psi fore the check valve opens to begin air up the secondary tank
Install the schrader valve fitting into the primary tank as the safety check valve will not allow air to back feed from the secondary to the primary tank
To find which is the primary with the plugs out of both tank and the engine running air will be blowing out of the primary tank

 

Got it sorted out and drove the truck around. Feels like maybe a brake is dragging or transmission is slipping. Did not want to pull a hill.

There is a air lever under the red air brake switch on the left dash panel. it doesnt have a label on it, any idea what that lever does? i hear air when i flip it, but nothing really changes.

 

Got a new windshield put in, old one had a decent crack on the passenger side and was pretty hazy from sitting outside.

Anyone know where I can get new cab lights?
Also, the headlights don't seem to turn on. Rear running lights and brake lights/signal lights work as they should. Haven't checked wiring or fuses so could be simple. Just looking for a source just in case.

 

Post o photo of the lever on the dash

All of the light wires for the front attach to thermal blocks on the fire wall


Turn the lights on and probe the terminals for power, if no power check the dimmer switch and headlight switch
If you have power at the firewall terminals there’s another on the inside of the hood on the left side near the grill, I take the grill out, tilt the hood and stand in the grill opening to get easy access to the terminal block
Check each function as you work your way out to the lights

 

Fantastic, simple enough! I don't have a pic of my dash, but I found one online that's similar. I put an orange rectangle around it.

 

What does it say on the panel? Looks like a diff lock, or PTO