I have an older 980C (with a grapple) and I recently put new rear brakes on it. Now when you apply the brakes the rear's grab way more aggressively than the front's. As a result of this the rear final drives are heating up. One day it got up to 270 degrees which will cook the seals so we parked it. We took the rear brakes off, took them apart and could find nothing wrong. we put it back together and they are still running way hotter than the front. I isolated the rear brakes and ran it for a bit on the front brakes only and the machine stopped great , you could feather the brakes and the front final drives were running around 170 degrees. We add the rear brakes back into the system and run it for an hour and the rear final drives increase to 200 degrees. both " master cylinders are brand new. The CAT mechanic is very perplexed. Help . Any ideas ?
Sounds like they are adjusted up too tight. They might be riding partially on all the time.
Is the air over hyd. barke system? Have you checked the slack adjusters at the wheel? I'm assuming you have bleed the system. Can you lift the rear end off ground and try turning the rear wheels by hand? You should be able to turn them with out too much effort. If they are hard or impossible to move try opening one wheel bleeder at a time to see if that makes a difference. One question more why did you need to replace the rear brakes? What Failed?
The loader had two original problems, poor braking and leaky seals on the rear end. we pulled the rear end apart replaced the seals and inspected the discs then put it all back together. all of the brake components (discs) were actually really good so we reused them. We replaced both master cylinders because we found that hey were both faulty. The brakes are air over hydraulic and the slack adjusters aren't the adjustable kind like you see on a 966-C or a semi tractor. Its just a piston with a spring behind it, so its really technically not a slack adjuster its more there to hold the oil so it doesn't want to bleed all the way back to the master cylinder, which technically it cant do if its bled properly. I lifted the back end off the ground and can spin the back tires easily by hand. What seems to be happening is that the back brakes come on much more aggressively than the fronts now. Normally the harder you press the gas pedal the harder the brakes come on but with the rear brakes even a light application has them coming on really hard, so naturally they doing the brunt of the work and heating up too high.
The front and rear brkes use separate master cylinders, how about hooking an air gauge into each to see if the air pressure is equal and proportional to the pressure on the brake peddle? I'm assuming that both masters were the same type. One other thing comes to mind. Our 980G's need an aditive to be used when changing the oil in the axles it's Cat P/N 1U9891. If this is not used we found the brakes to be grabby and noisy. I'm familiar with the hyd. slack adjusters, been around them since the origianl 988 Loaders! all the oil that brake wants can go in but only so much can vent out. Keeping the same clearance in the brakes as they wear. Wear if treated properly is almost zero. We have a 980G with close to 26,000 hours on it and the original brake disks, just replace seals once. If I have any more ideas I'll let you know.
Is it possible to connect the hydraulic lines from the master cylinder that currently feeds the rear wheels to the front ones and vice versa..? That way you could identify if the problem is in the lines/wheel brakes from the master cylinder south or if the cylinder itself (despite it being new) is the cause of the problem. A word of warning. Don't completely discount slack adjusters even though mechanically they appear to be a fairly simple device. Example: Recently we had issues of a 789 haul truck that would not pass the park brake holding test so we changed both final drive/wheel station assemblies for rotable components we keep in stock. On disassembly the brakes in one FD were 110% shot where in the other one they were probably less than 50% worn. Both Final Drives were original with exactly the same operating hours on them. Root cause - slack adjuster not allowing service brake on one side to fully release. with oil cooled brakes the issue never showed up as high temperatures or grabbing brakes but we measured slight residual presure in one side. Replace slack adjuster (I don't believe in trying to reseal them) - zero residual pressure - job done. Agree with KSH on the additive. We use it on all ours and have never had grabbing, chatter, or any other noise issues.
How are the brake pistons connected to the air piston rods? Is there any adjustment of the rods? Just thinking back to an old 769 truck that the brakes wouldn't release on. It had two air and master cyl's, one for foot brake one for retarder. Retarder worked fine foot brake locked up. Problem was the master cyl piston on retarder was loose and covering a return oil hole half way up the cyl. Later Bob
I also meant to put in my post above that if there was any way to "T" a pressure gauge (or gauges) into the brake lines between the slack adjuster and the wheels and check what your residual brake line pressure does while the machine is working. It may start off at zero then start to climb as the machine has worked for a while. If this happens - slack adjuster would be my first bet. Not discounting the possibility that the new brake master cylinder could be dodgy though.
If your brakes are operated with a treadle valve, a sticky treadle valve could cause the problem you are describing. They pressurize your air pots and it possibly could be putting more pressure to the rear as opposed to the front. I agree with putting a t in the airline and checking pressure as the brakes are being applied.
Just to clarify. I was talking about putting a T in the hydraulic lines, probably right next to the brake calipers. However putting one in the air side as well might also be a damn good idea.
I checked all the pressures on the air side of things. we had gauges all over the place just to see if they were getting more pressure off one master cylinder versus the other. We also jacked up the back end so the tires were off the ground and immediately after the brakes were released we could spin the wheels by hand so I am fairly certain that there is no residual pressure. Just to do a test we removed the slack adjusters and the back end still heating up but is not nearly as grabby. I have not put the additive in yet so I will do that right away away and let you know how it works. Thanks so much for all the great feedback, I really appreciate it.
measure all the tires for the correct circonference, removed the rear drive shaft and run the loader and check if its still heating. already saw this...
If your unit using ABS (air brake system), this is because under master brake including front and rear get a intruding air (java language "masuk angin") you should do "bleedding air"