M818 brakes mystery

[ClintA]

So the ranch has a M818 that we use to haul cattle, dozer, backhoe and anything else that needs a good hauling around. I live in rough country where everything is up and down, so that being said we use the brakes. Sometimes and only sometimes the truck will have full peddle and zero brakes, kinda scary coming off the hills to say the least. The brake pedal will even push back at you no matter how hard you push it down, you can let off pressure and still it will have zero brakes and full pedal. More times than not they work as should. I’ve learned the hard way to come off hills in low and low, it ran off with my 19 year old son loaded with cattle, he did a great job buy driving across country and turning it up hill. Any advice would be greatly appreciated on how to fix this problem. Before anyone asks, no we are not on public highway the “Green Mean Machine “never sees payment. I’ve put new brake shoes on not long ago and they are adjusted. Anyway hopefully someone can point me in a direction right or wrong. Hope everyone is having a great future weekend, Clint

 

[NDT]

I would start with overhauling the brake booster.

 

[msgjd]

Have had that situation on rare occasion with my M817 ... When I bought it, it had excessive (half-stroke) free travel on initial stroke but excellent braking so I left it alone.. The first time the hard-pedal happened i thought the problem may have been because I had to pump the pedal a couple times to get decent travel and the booster did not like being pumped .. I recall being warned "back in the day" not to pump them much, nor should we jam our foot into them hard..

So I shortened the master cylinder stroke / pedal free travel per TM spec, only to find it was apparently a hair too much because then the brakes didn't want to release upon application ... So I backed off the MC adjustment a bit and things were fine awhile until the next time the pedal went hard again, no brakes .. The truck was relegated to off-road service since then, but has not acted up in over a year .. I exercise the brakes on all the trucks every time they are fired up

I have not yet delved into the system to find the gremlin.. The only thing I can think of is perhaps the booster or a check valve is/was sticking ...

Perhaps a former 63-Charlie, 63-Sierra, or 91-Bravo can chime in with a cause to our shared problem .. Of course anyone else who has seen the gremlin and eliminated it is welcome too !!

 

[WillWagner]

Check pedal free play, you should be able to push the pedal with your hand about an inch then feel the pushrod contact the piston. If no freeplay, adjust the pushrod, while you are there, pull the boot and see if the snap ring is still there and no fluid under the boot. Remove the MC cap and look down into the fluid. Depress the pedal and release it. There should be a little "geyser" of brake fliid pushing up from the bottom of the MC Reservoir. If you don't see it, the vent hole could be plugged. You can suck the fluid out and try to unplug the vent hole...the small one, there are two in there, one a pinhole the other apx 1/4 inch....with a small strand of wire.