Hi, Im new to the forms, I just bought a older 55b machine was told late 70's early 80's. It has no brakes, tring to figure out how the brake system works, its air over hydro. For starts i had to remove both brake pedal asms and grease them they were frozen, got them working (i Think) the front master cylinder is bad so i removed it to rebuild it. But on the rear brakes the master cylinder seems to be working. The air chamber on the back of the master cylinder is moving in and out with shop air and if i blow air in the brake chamber fuild is coming out of the bleeders. So im thinking from the master cylinder to the calipers are working, there is also a valve mounted on the frame that has 1 line going to it from the master cylinder and 1 bleeder valve, but no other lines on it. Not to shore what this is or what it does. Also there is the valve for the air system has 3 lines on it 1 line is from the air tank, the other line is from the pedals and the third line goes to the brake chamber on the master cylinder. I have air going through the line to the brake chamber all the time even without pressing the brake pedals.(should i ) really not to familar with this type of system. i Know air brakes and hydraulic both not this system. Can anyone help with this And also does anyone know were i can get a service manual for this machine. I have the parts book but i was told there is not a repair manual for it, any help would be great thanks
I'll dig out my manuals tomorrow and have a look, there may be a service bulletin on the brakes.
The system is in mostly two parts front and rear. The compressor feeds the Wet air tank which traps most of the condensation and has a drain on it some were manual other had automatic drains. from the wet tank air is supplied to the foot valve and to the 2 emerency air tanks through a check valve on each one and on to each of the ERQ valves and on to the power cluster (master Cylinder), which directs fluid to the brake calipers immediatly and applies the brakes untill the air system is charged enough to operate the machine safely and charge the emergency tanks, at that point the ERQ valves shift and block off air from the emergency tanks releasing the brakes to the operator. The emergency air tanks should maintain air pressure while the machine is running. If there is a failure in the air system from either the foot valve or the hand brake valve, the ERQ valve will shift and prevent air from the emergency tank from leaking off through the foot valve or the hand valve and send air to the power cluster to apply the brakes and stop the machine until the failure is repaired.
The valve on the side of the frame with one hose to it is a compensator valve, this valve acts like an accumulator so when you have the brakes applied for a long time you don't have to keep reapplying them, saveing wear on the cup in the master cylinder, the is one for front and rear system.
thanks for the info I pulled the lines off the foot pedals going to the brakes and they are not leaking so i take it they are good.The little hand valve for the brakes leaks out the back when i pull it out so im thinking that might be bad. im also not to shore about the hydrualic side of it so im thinking of rebuilding everything in the rear, the front is disconnected. do you know where i can get a repair manual?????
There is not a shop manual for the tractor, but there are manuals for the main components, transmission, convertor etc. The brakes are pretty straight forward, parts support through Minnpar.com Power cluster (master cyl.) kit should be available. One thing on this is to remove the residual check valve at the outlet end of the cluster, this has in come cases caused the brakes to drag and overheat. You should really repair both front and rear brakes for safety, first 2 wheel brakes are marginal for stopping the machine on grade and second if you have a failure you have no brakes except for parking brake.
good morning joe...just picked up a cat920 with similar brake issues. my front brakes seem to work normal, back brakes would stick periodically. I have a similar air over hyd system. Did a bunch of the same things you did but no luck. Tried to loossen the bleed screws but ended up rounding them off. Vice grips, small pipe wrench, no luck. Tried various penetrating sprays /heat..got one loose but couldn`t get the other one. Finally removed the wheel,took the caliper off and when it was laying flat was able to apply more heat/sprays and get a good grip with the pipe wrench and finaly it broke loose. There was a bit of caked dirt/rust etc hindering the movement of the caliper pistons. Removed the pistons and found a severe case of pitting and corrosion. Pistons were not able to move freely back and forth to apply/release the brakes. The machine had not been used for many years, hence the corrosion etc. Now am waiting for parts but am confident that this will solve my problem.
Finally got all the pieces put back together. The boys were working on the other wheel caliper and managed to break a bleed screw. Easyouts weren`t that easy. The local machine shop had to do the extraction but all is good. After a good cleaning, new pistons,seals etc everything works like it is supposed to. Welded a hook for a chain to the bucket and pulled a 1/2 mile of posts. My farm loader wouldn`t have been able to handel that job. Question, why don`t the manufactureres incorporate a couple of hooks for a chain when they build the machine in the first place? So far the machine works fine, lots of hyd capacity/power, engine starts excellent, runs nice and smooth. There are a couple of minor oil leaks but those will be addressed soon. This is my first piece of Cat equipment, just wondering how long Cat has been getting parts built in China?
free parts look up books on line www.minnpar.com