Cat 775F brakes overheating and locking

I greet you all. We have Cat 775F truck with a brakes issue. This truck is sometimes locking brakes after being driven for just 20 minutes but without any warning on the dash. We checked the brake pressures that is the service brakes and retarder pressures through the slack adjuster. We also checked park brake release pressures on each side and all these were within specs. We sent it to work and worked for 2 hours and started locking the brakes but this time with brakes overheating warning. I don’t know what else to check that might be causing this. The other functions like hoist and steering are working good.

How many total operating hours on the truck..?

This truck has 13300 hours

I would suggest replacing the slack adjusters on both front and rear brakes.

We replaced the rear slack adjuster with another one from another machine but still has the same problem.Does not seem to have a front slack adjusrter.

I wouldn't trust a used slack adjuster TBH, even if the other machine was apparently working OK.
Is it all four wheel brakes that are locking up or just the rears..?
If just the rears as a first step can you tee a pressure gauge into the brake lines between the slack adjuster and one of the rear brake packs (doesn't matter which side) and run a line up to a gauge in the cab. See what happens to the gauge as the machine starts to operate from cold, residual pressure should be zero or close to it.

Thank you Nige I will try this on Tuesday when I go back to work. This machine has a switch in the cab for turning on/ off the front brakes and they normally operate them in the off position using the rear brakes only. So it’s only the rear brakes that are locking.

OK, I know the system. We had the same thing on 773's in a previous life. I'm not sure but maybe your 775 has brake pressure test points with quick couplers on the rear brake packs. If it does you could probably just plug a transmission test line into that quick connector. Just make sure to zip tie it to the brake lines so that it can't catch on anything.

If the truck has dry disc brakes on the front it's a deffo good idea to run with the front brakes switched off otherwise they get extremely hot going downhill loaded when you're using the retarder.

Once you see what pressure you have in the brakes on one side as per above, switch the fittings to the other side and repeat the test. If you have enough equipment hook something up to both sides at the same time to save you grief. Report back with your findings.

Probably nothing to do with this trucks problem but back when we got the then new 769D we had problems with front dry disk brakes getting hot and this was in an application where there was no down hill running loaded.

Basically the problem was a relay air valve that was not completely venting air pressure and keeping the front brakes on full time just at a very low pressure like single digits PSI.

If anyone is interested I might have notes on that someplace..

Have you had brake cooler issues?

I have seen brake packs swell up when the coolers fail as the coolant gets into the hyd oil and swells the brake material

Slidey said:

Have you had brake cooler issues?

I have seen brake packs swell up when the coolers fail as the coolant gets into the hyd oil and swells the brake material

Click to expand...

Good point. If the friction material is the cellulose paper variety it does not like being exposed to contact with coolant. Fail a brake oil cooler and you might just as well pull the brake packs and replace the friction discs at the same time. I just had a quick look and found a 9W7017 Part Number. If that is correct for the OP's particular truck it's definitely a disc with cellulose paper friction material.
The test for residual pressure that I suggested above will tell whether or not there is a problem with the hydraulic side not releasing. If there is no residual pressure when the brakes heat up/lock on then it's pull the brake packs apart time ...........