Why are the front brakes locking while driving?

Tiny
89MONTY
  • MEMBER
  • 1989 MITSUBISHI MONTERO
  • 3.0L
  • V6
  • 4WD
  • AUTOMATIC
  • 218,000 MILES
Hi, I owned the car for only a couple months. My brakes completely locked up (at least in the front) when driving at 45 MPH. I had to get all new pads, rotors, calipers, and hoses. The problem remained. As I'm driving after about 10 minutes, they slowly start to lock up again. I replaced the master cylinder and am having the same issue. I looked to see if I could adjust the push rod and to me it doesn't seem adjustable. When I remove the vacuum hose from the booster it does release the brakes and I can drive again until it starts happening after another 10min then I repeat the process and it releases the brakes again. I'm not sure if I need a new booster (which I can't find online) or if there's another issue going on. Any suggestions?
Monday, August 15th, 2022 AT 10:28 AM

8 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 51,628 POSTS
Yes, you need adjust the brake booster rod because the valve inside the master cylinder is not returning fully so it holds the brakes on. Here is how you can adjust the rod in the images below. Check out the diagrams (below). Please let us know if you need anything else to get the problem fixed.
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Monday, August 15th, 2022 AT 4:11 PM
Tiny
89MONTY
  • MEMBER
  • 5 POSTS
Hi, I really appreciate your response and I feel that I'm on the right track, but I hope right now I've not done more harm than good. When I went to adjust the pushrod prior to seeing this diagram the pushrod came loose and was sort free floating inside the booster and I could nearly pull it out if it wasn't for a nut or something on the opposite side preventing it from being fully removed. I was able to get it sitting snug again just by pushing it into place, but I'm not sure that was supposed to happen. I wasn't able to break the rounded nut and spline looking thing on the pushrod with pliers. I can try again though. Is there any possibility it's on the brake pedal side of the booster? Is it more likely an adjustment than a booster failure or is there any possibility the issue is between the MC and brake lines? I installed the MC myself. Seemed to go in fine etc..I'm no mechanic and even the mechanic I typically use suggested changing the booster since the MC didn't fix it (it seemed like all guess work for him) and didn't even suggest any adjustments. Is there any way the proportions are off between the rear drums and front disc's? Sorry to ask all of this but I'm trying to rule everything else out I can think of. All signs point to the initial suggestion though.
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Monday, August 15th, 2022 AT 6:37 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 51,628 POSTS
You are on the right track, nothing other than what we are working on can cause the issue except for the brake pedal being held down somehow, can you reach down and pull the pedal upward? Also, the push rod inside the booster does come loose like that and can be reinserted into the booster which is okay. So, was this problem the same with both master cylinders?
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Wednesday, August 17th, 2022 AT 10:51 AM
Tiny
89MONTY
  • MEMBER
  • 5 POSTS
Hi,

Yes, basically it locked up the first time really bad when I first took it on the highway. I thought maybe it was from plasti dipping my rims white and not covering the calipers and rotors, figured it got all gunked up. I had new rotors, calipers, pads, hoses put on in the front after that. I installed the MC myself after bench bleeding because the problem remained.I didn't bleed at the calipers because the brakes felt tight still. After doing the MC it had no effect. I drive for about 10min and the they start tightening up and getting hot. I can't find a replacement booster either. I tried loosening that rounded nut at the tip of the pushrod again with no luck. The only thing that works so far is when I remove the vacuum hose and reinstall it, but that only lasts another few minutes until the brakes start locking again.
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Wednesday, August 17th, 2022 AT 11:36 AM
Tiny
KEN L
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You might need to remove the booster and have it rebuilt. It sounds like it is the problem.
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Thursday, August 18th, 2022 AT 2:01 PM
Tiny
89MONTY
  • MEMBER
  • 5 POSTS
Hi, thanks for the reply again. This was going to be my next step. Finding one online is very hard for some reason. I think I will send it out to be rebuilt then. I appreciate you taking the time to address my questions!
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Thursday, August 18th, 2022 AT 3:34 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 51,628 POSTS
Use 2CarPros anytime, we are here to help. Please tell a friend.
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Friday, August 19th, 2022 AT 11:08 AM
Tiny
89MONTY
  • MEMBER
  • 5 POSTS
Definitely will!
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Friday, August 19th, 2022 AT 11:39 AM

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