Hi, I just found your forum although I've been a member and enjoying the sister, heavy equipment forum. I've been wrestling with this problem for a bit and need a hand. My 96 c6500 flatbed dump with the Cat 3116 has been giving me fits. A few weeks ago while I was carrying a load, the brakes suddenly lost 90% of the pedal, but fortunately there was still enough to stop. I thought it seemed like a classic case of a bad master, so I changed it and all seemed good until the next trip out when they suddenly started locking up and gradually brought me to a halt. As I stood there scratching my head, I heard them suddenly release and the truck rocked back on its e brake. I wasn't far from my shop, so I started back when the pedal went back to the 90% loss mode. Ok, I thought it must be the booster, which is a Hydro Pac. So I bought a new one and installed it and all seemed good after a pretty good test drive. I was wrong again. As I drove it today they gradually brought the truck to a halt, though the pedal did stay normal. I hopped out and released the pressure at the front right bleeder and took off until they locked again, so this time I released at the front left bleader. Both rotors were pretty hot. The next time I released the pressure at the rear with the same results. So, I'm thinking that somehow the system is building pressure with out the brake pedal ever being applied, which it wasn't on all three tests. is anyone familiar with this system and have an idea of what the heck is going on? I'd sure appreciate some thought if you are. Thanks in advance.
96 Chevy c6500 brakes locking up
- Thread starter Philosaw
- Start date
Truck Shop
Well-known member
If for some reason the brake pedal does not return to the top-there is an electric motor also on that system If I remember right. That is activated by the brake
light switch. I very well could be all wet on this but I would check that out. On a vacuum booster when the pedal slowly goes to the floor the return spring inside
the booster is broke allowing the vacuum to pull the pedal down applying the brakes
Truck shop
light switch. I very well could be all wet on this but I would check that out. On a vacuum booster when the pedal slowly goes to the floor the return spring inside
the booster is broke allowing the vacuum to pull the pedal down applying the brakes
Truck shop
BoxCarKidd
Active member
Ran into something like that on a Ford years ago. The hole in the master cylinder piston was not quite as deep. The pedal did not allow full return of the piston. It would lock down but release with time and then work for a while, or not. Yours has a bit different twist. Problem between booster and master cylinder?
Today I disconnected the electric motor that provides power to the booster in the event of engine failure. After driving about 15 minutes the brakes once again locked up, though this time right in front of my shop. I wanted to see if it was just the front brakes, so I jacked up the rear end and they were locked as well. As usual, after sitting for a while with the engine off, the pressure backed off. It's gotta be that for some reason the power steering pump is over pressurizing the booster, or a pressure relief isn't working?? Stumped again.
Truck Shop
Well-known member
I doubt it's the P/S pump. Sounds like the booster has internal pressure leak even though you already replaced that. Next time it locks up try cracking the pressure line to
the booster first to see if the brakes release.
Truck Shop
the booster first to see if the brakes release.
Truck Shop
Last edited:
That's a good idea Truck Shop, thanks. I'll try that out a little later today.
Yesterday I drove the truck until the brakes locked up (which was the exact same distance as last time!), pulled over and disconnected the high pressure line from the pump to the booster, which did nothing. I then backed off the master from the booster to see if the piston was depressing the master, and it wasn't. I then released the pressure at the bleeder and brought it back to the shop and pulled the master. Not knowing what else to do, I ordered a new master and will see if that works later today.
Truck Shop
Well-known member
Question, did you bench bleed the first master cylinder you installed or did you install and then bleed the entire system?
Truck Shop
Truck Shop
The piston in this model is so stout that bench bleeding is out of the question, so I mounted it and used tiny strokes with the lines off and plugs in. I then hooked the lines up and bled the entire system. Today I replaced that master and all is now good. The hardest repairs are the ones where a new part is defective, and I wish I could say it was rare!