M817 New Air Pack Installed Brakes Lock Up

[Construction]

Installed a new air pack. Bled the brakes (engine off, no air pressure in tanks). Drove truck, brakes lock up almost right away.

Released air pressure from the service line (lots of built up pressure) so I could move the truck.

I have missed something. Do I need to bleed the top bleed screw with air pressure in the truck?

 

[m816]

You need to pull the wheels and take one of the wheel cylinders apart. You will find a great deal of sludge and rust at the bottom of each cylinder. I believe that is because of the seperation of condensation in the system from using the Dot 5 (silicone brake fluid) The fluid is lighter than water so it settles to the top, and the water causes rust at the bottom of each cylinder. The Airpac only provides boost to the hydraulic system. The pressure is so great that it forces the cylinders apart and stopping the truck. The return springs are not strong enough to overcome the pressure in the cylinders and they lock up. Bleeding alone will not flush the crud out of the cylinders. These truck sit for monts and sometimes years befoe GL sells them to the public Every five ton and deuce I have had in the shop needs brake cylinders or at least rebuilds. Heavy work but once done the brakes will give trouble free service.

 

[Construction]

Already did that.

 

[nk14zp]

Make sure the glad hand cover on front is vented.

 

[Construction]

The service line? Drivers side?

 

[nk14zp]

Yes it should have a vent or a 1/8" hole in the cover.

 

[nk14zp]

http://www.steelsoldiers.com/5-ton-up/56248-air-pack-problem-2.html

 

[Construction]

The front service line on the passenger side has a plug in it. There is no vent, no valve, no glad hand.

 

[nk14zp]

Drill a small hole in the plug.

 

[Construction]

I did take the plug out to see if it would help and it did not. I have the rear service valve cracked open right now so I can move the truck. As soon as I close it and push on the brakes - LOCK. Crack the rear valve again, the wheels unlock.

I also took out the rear plug of the air pack and added air tool oil. Still no luck.

 

[73m819]

Is the fluid comming back to the M/C, if the piston is two far back covering the hole, the pressurs can not dump and in turn the air pack will not release, that is why dumping the air will release the brakes.

 

[Construction]

How do I check to see if the fluid is coming back to the master cylinder?

 

[Robo McDuff]

Is this specific for the 800 and later series or do the M51s have the same problem? I am asking because a full brake system overhaul and maybe installing a double system is second on my list (steering problem is first).

 

[73m819]

both the 39/809 the same system

open M/C and look inside, if you can see the fluid come back as a small geyser as you release the pedal, things are good

 

[Construction]

If it does not, does that mean the new air pack I just installed could be bad?

 

[m816]

Got me! I woulda swore it was the brake cylinders. Since you did all that to it and it still locks up. and the glad hand on the right front fender is vented? and you are still locking up . As the guy before ssaid if the MC shaft is not going all the way back to alow the fluid to go back in the reservoir ( check by looking in it as you work the pedel with your hand.) The only thing I can think of is either a collapsed line or the airpac you put in is no good. Lookin foward to hear what you find.

 

[nk14zp]

Do you have any free travel in the brake pedal linkage?

 

[bigmike]

If it does not, does that mean the new air pack I just installed could be bad?

If the fluid doesn't return, I would think it's either not enough pedal free play (1/4") or the small return hole which allows the fluid to flow back into the master is clogged. You can clean that hole with a piece of wire. I had this problem in my Duece and that was the issue.

 

[Floridianson]

If we go by the tm then we just add oil to the piston and exhaust part of the air pack. I like to remove the main air feed copper line by the front side of the tool box then we get oil on the incomming valves, the piston and exhaust valves on the airpack. Sometimes we think something is new but rust can form on the piston in 30 years or someone can put a nice paint job on something and call it new. Also when bleeding master first then rear bleed on airpack then front bleed air pack then wheel cyl's. As said needs some free play or check return spring is pulling the master rod all the way clear or to correct free play and as said when looking in the master it shoulid show signs of fuid returning to master after peddel is depressed. Some say a peice if wire in rear most hole or a turkey baster and suck any crud the might be blocking return of the fluid.

 

[Robo McDuff]

Any ending to this problem with a solution or diagnosis what was causing the problem?