Brakes soft then hard

[sermis]

My brakes are possessed. When I start the truck with no air the brake peddle will go almost to the floor. The truck has some brakes but little. When the air builds up the brakes seam ok, however sometimes the peddle will go soft and sometime it will get hard, almost to the point of not being able to push it. One time the brakes locked up in the rear and would not release. I got an air pack rebuild kit. Wrong one my luck. $75.00. I went into the air pack on the truck and it is real clean, even has white grease from whoever built it. No history on the truck but looks like the air pack is new or rebuilt. Paint does not match the rest of the truck. My air pack has a rubber “O” ring seal on the large can. Any ideas why the brakes are acting this way?

For sale
Rebuild kit 2530-00-040-2190
P/N 19207-834507

 

[williamh]

i don't think that its your airpack it sounds like you have air in your lines or your master cylinder is going bad .

 

[Rocco]

Mine was doing the same thing.
i changed the master cylinder and to this day it works fine.
And I also have a airpack rebulid kit that i never used.

 

[WillWagner]

Rocco, wanna sell your rebuild kit? I have a spare air pak that needs rebuilding.

 

[sermis]

I cleaned the master. It looks new but there was a lot of junk inside which could have been causing my problem. Refilled with DOT 5 and bled the system.

I do not have a vent line on top of the master cap. It looks like someone put a petcock on top of the cap. There is a “T” fitting in the airline on the cross member. The “T” is open and has about 1” of pipe. Is this where the vent line goes to?

Unless I am blind I only have one air pack booster. Is this normal or has someone modified the truck? Looking at the manual it should be duel circuit if I am reading it correctly. I used air to blow out the brake lines to the wheel cylinders and all seam to be on the same circuit.
The master is square with the brake line coming off the rear.

 

[ken]

Steve that vent goes to the crankcase vent tube on the passenger side of the engine. There is a coulpe of ports on this tube. It's unessary unless you have the deepwater fording kit. It then keeps water out of the airpack/master.

 

[sermis]

No fording kit. Thinking about using an axel vent on the cap. The master has not been vented. That might have been cone of the problem.

 

[ken]

You can try venting it and see it that helps. Mine is plugged off also. I have a frend who put a windshield washer bottle on the back of his M54 cab and ran the tube to his master vent. So he can check the level by just looking behind the seat.

 

[stumps]

No fording kit. Thinking about using an axel vent on the cap. The master has not been vented. That might have been cone of the problem.

The military used axle vents on the master cylinder if there wasn't a fording kit installed. It is, in my opinion, the cause of most of the need for bleeding the brakes on deuces that sit unused for long periods of time. The vent is a one way valve (out), and after several good temperature cycles, it puts a vacuum on the brake system, drawing back the wheel cups. Eventually, air gets past the wheel cup seals, and you will need to bleed the system.

Far better is to put a hose barb on the cap, and run a hose up to some place clean and dry in the cab. There is a change order for deuces that have the brake vent going to the engine snorkel tube. The crankcase fumes are bad for the brake system, so they moved the vent to the air cleaner.

Having your pedal start out soft and get hard simply from building up the air pressure on your deuce is a classic symptom of an air leak in the control valve of the air assist cylinder. It needs rebuilding, or replacing.

What happens, as I understand it, is air leaks past the control valve into the assist cylinder causing it to behave like the brakes are partially on. This causes the brake pedal to rise and get hard. Eventually, your brakes will drag, and burn up... or you will try to start out and find your brakes are locked and you can't move.

-Chuck

 

[tm america]

the master cylinder needs to be vented .think of a straw put it in a glass of water put your finger over the hole in the top pick the straw out of the water and the water in the straw will still be in the straw. if the master cylinder cant bring air in it cant push fluid out.you will probably need to bleed the brakes after you fix your venting problem you may also have a problem with the air to the airpak or a problem with the airpak itself try taking the vent off the airpak and spray some oil in there then put the vent back on this should keep the airpak from sticking and giving you a problem