920 cat air brakes

Hi guys need advice on no brakes on a cat 920 loader. It has air / hyd . no parking brake , no air pressure on the gage , but the machine will move and when the main service brake is applied it doesn't stop the machine ( not working ) cannot hear air pumping or leaking . Machine has disc brakes .

Thanks

What have you checked apart from looking at the gauge,have you drained the air reservoir,what came out,have you checked the compressor drive,if the air reservoir has filled with moisture there will be very little pressure,if its not that the compressor is not pumping,could be a burst line,a blocked inlet,a stuck valve or broken drive or or.
AJ

I"ve seen quite a few of those machines that had a variety of problems. To figure it all out you need to check to be sure you"ve got air pressure first and foremost. you should have around 120 to 130 PSI showing at the tank. If nothing, then remember, without air nothing will work.

If you"ve got air pressure at the tank, make sure your getting air pressure through the treadle valve/valves to the master cylinders. If you have air there then move on to the master cylinders. There should be one for the front and one for the rear.

On the master cylinders insure there is brake fluid in them, and that it"s not leaking out. Too, check for rust and other debris in the chamber that holds the brake fluid. Stuff in the chamber can block the hole that allows the fluid to go into the cylinder that does the pumping. Basically no fluid getting to the cylinder, no brakes, even if the air system is working.

If all of that is working properly then try bleeding the brakes. If they bleed but the pads don"t move in the calipers, then the calipers are frozen up and need to be replaced.

Beyond that, it"s a really simple system. The only difference in it and a standard system on a car is that there is no direct connection between the pedal and the master cylinder. Instead the treadle valve varies the amount of pressure, and volume applied to master cylinder and ultimately the amount of braking force applied.

Hope this helps. Good luck.

Thanks for the step to step check approach . I am not to familiar with air brakes , so I have a mechanic looking into it . But my biggest question is ; normally you need 60 - 70 psi to release any air brake , and in this case the air gage reads 0 and the machine will drive ( brakes released ) so has something been by-passed . I guess I will learn that when he gets into it .

The air handbrake! if fitted the air handbrake is engaged by mechanical pressure of a spring inside the actuator,when you need to release the handbrake the air pressure has to be high enough to over come the spring and push the piston back for the handbrake to release, should the air pressure drop suddenly the handbrake will come on, that's if you got it and it's connected up,have a look at the drive shaft and see if you see a drum,chances are that it has a mechanical cable operated parking brake.
AJ

As AJ states, most of those machines had a drive line brake for parking. It is spring set, and air released. That means with no air going to the release cylinder (ie button out, or no air pressure in the system) it should be set by a big spring and hold the machine. When you get air pressure built up and push the button it will send air to the cylinder, which works against the spring to release the brake.

That said the parking brakes on those machines is one of the most overlooked, under maintained things I have seen, beyond the brakes themselves. Baically when loading a truck most operators don"t use the brakes, and when parking the bucket is on the ground so a parking brake really isn"t needed to keep the machine from moving.

For that reason the parking brake is more than likely either broken/worn out, or so far out of adjustment that the spring won"t hold it enough to even notice it, whether it"s supposed to be working or not.

The way to tell on that is to simply look under the machine at the park brake setup, and see if the cylinder moves when you push in the parking brake button to put air to it, or if it goes the other way when air is released. If it doesn"t move, then you either have no air pressure, or there is another problem with the system.

Again, I hope this helps you understand what"s going on. Hopefully the mechanic looking at it can get it straightened out for you without a lot of problems.