rear brakes locking up

I have a mid 70s cat 930 wheeled loader rear brakes were dragging bad. I found the rubber hose on the rear brakes was bad replaced the hose bleed the sys worked fine for about 1 hour came back started working and rear brakes are dragging again.
Thanks for any help I can get.

serial number will get you help

The serial number is 41K12779

Well it appears to be a somewhat basic air over hydraulic system with disc brakes.

First off when doing any checking make sure to have machine blocked so if something is done that releases brakes you don't get run over!

Just thinking off top of my head about a way to narrow down the cause .

I would start by checking for air pressure at the air inlet to the brake master cylinders while brakes are released but brakes are dragging.

Next if no air pressure in lines to the masters next step would be to open bleeders at master to see if that releases the brakes.

If no pressure there next open bleeders at rear calipers to see if there is pressure there causing the brakes to drag.

If opening everything from the air inlet to the masters to the bleeders on the calipers does not release the brakes then I would assume the calipers are in need of work.

If on the other hand something along the line does release the brakes I would need to know what did to come up with another idea.

One thought might be the hose to the rear brakes being bad inside and acting like a check valve. Never had it happen on a Cat machine but did have a problem with a front brake hose on a Ford truck doing just that. If that was the case opening the bleeder at one or both the rear calipers should let the brakes release.

I may have missed something along the way but I'm sure someone will correct me and fill in the blanks.

Check the brake pedals for dirt and rust build up around the pivot. Clean the area and lube thoroughly.

highwayghost said:

Check the brake pedals for dirt and rust build up around the pivot. Clean the area and lube thoroughly.

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I should have added that but guess as the original question was mentioning only the rear brakes I was leaning toward something that woud most likely effect just the one axle. But yes sticky brake peddle or crap on floor boards are too common the cause of brake problems in loaders!

Hi After checking above suggestions look at the following
first thing to check is that oil instead of brake fluid has not been added by mistake this will cause the seals to swell and will require a complete tear down and seal replacement
second thing is because these old girls use compensating port master cylinders and because of the position of the masters and carelessness dirt can get in the master cylinder and block the compensating port

Thanks everyone. Frist the rubber hose was bad after replacing it the brakes worked fine for about 20 min then the rears locked again. If I open the bleeder on the master cylinder the brakes will release. I am thinking the master cylinder the fluid was pretty black when I bleed the sys after replacing the hose.

Jim Carroll said:

Thanks everyone. Frist the rubber hose was bad after replacing it the brakes worked fine for about 20 min then the rears locked again. If I open the bleeder on the master cylinder the brakes will release. I am thinking the master cylinder the fluid was pretty black when I bleed the sys after replacing the hose.

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If they only lock up after use, it sounds like the compensation port or ports in the master cylinder are blocked. You should see a small hole in the bottom of the master cylinder reservoir. This small hole is wide open to the wheel cylinders/calipers when the brakes are not applied. As you use the machine and the fluid warms up, the fluid expands. It needs somewhere to go, so it's supposed to go back to the reservoir through that small hole. If it can't, it starts applying the brakes instead.

Two things usually cause this; Either the hole is plugged with rust and junk or, the master cylinder rod is out of adjustment and is being slightly pressed. After just a slight movement of the pedal, this hole is plugged mechanically inside the master so the brakes can be applied. If the master is being slightly pushed all the time, not enough to apply the brakes, but enough to plug the compensation hole, it will cause the problem you are having also.

Thanks I will check that

I am assuming this machine takes brake fluid. Did the fluid get contaminated at some point? If the seals swelled from the contamination it can cause the master not to return correctly.

A brake system that uses automotive brake fluid will be seriously damaged if a petroleum based fluid is added to the system. The rubber seals will swell up. All and I mean all items with rubber will need replacement including the hose(s).

after checking everything here I believe the master cyl is the problem. If the pedal was sticking it would affect both front and rear brakes. Now trying to find a master cyl is the next problem. Thanks for the help it was very useful.

Jim Carroll said:

after checking everything here I believe the master cyl is the problem. If the pedal was sticking it would affect both front and rear brakes. Now trying to find a master cyl is the next problem. Thanks for the help it was very useful.

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If you can post a picture of the master cylinder might have some idea as to where to get one or a kit to rebuild it.

franklin2 has a good point on that little vent hole. It is a common problem even on something like a motorcycle, I could tell a funny story about that!

edgephoto also has a good point on the type of fluid. Some machines use DOT# and some use Oil. And the cylinders may look identical with only the "rubber" bits inside that are of different materials, even between two different models from the same manufacture!

Jim Carroll said:

Thanks everyone. Frist the rubber hose was bad after replacing it the brakes worked fine for about 20 min then the rears locked again. If I open the bleeder on the master cylinder the brakes will release. I am thinking the master cylinder the fluid was pretty black when I bleed the sys after replacing the hose.

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Strip down and clean the master cylinders followed by a complete system bleed (F & R) until you get clear brake fluid to all the wheel ends would be my first suggestion. As Mark pointed out, and others alluded to, the correct fluid for the system is DOT 3 or 4 hydraulic brake fluid, NOT hydraulic oil.

You may find the attached parts listing of some use.

SIS is not being helpful for this machine as I can not find the maintenance manual section for this specific machine just a general Fluids Recommendations. One thing that was bothering me is that on one page they refer to "oil flow" when talking about the brake system but then part way down that page they use the term "brake fluid" not the best way to word things and I can see where someone could misinterpret what to use.

kshansen said:

SIS is not being helpful for this machine as I can not find the maintenance manual section for this specific machine just a general Fluids Recommendations. One thing that was bothering me is that on one page they refer to "oil flow" when talking about the brake system but then part way down that page they use the term "brake fluid" not the best way to word things and I can see where someone could misinterpret what to use.

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I came across the same thing but got round it by using the option "41K-5315-Up" which led me to an O&M Manual from which the extract below is taken from the Maintenance Recommendations.

I found in the manual I have it said brake fluid j1703 witch is dot 3 fluid

Nige said:

I came across the same thing but got round it by using the option "41K-5315-Up" which led me to an O&M Manual from which the extract below is taken from the Maintenance Recommendations.

View attachment 221594
View attachment 221592
View attachment 221593

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Guess I stopped trying too soon, probably wont get paid for today's work! Maybe finding the part numbers for Ed's 77 Terex 72-41 log grabber fork cylinders will off set my loss!

Cat said the numbers off the cylinder were not real numbers!