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Discussion Starter #1
This is on a 2018 Ranger 150 that I picked up a couple weeks ago. The previous owner warned me that the front right brake would sometimes lock up and he's just crack the bleeder fitting and let his kid keep on riding.

When I got it home, I noticed the pads all the way around were nearly gone, so I replaced them. In the process I noticed the caliper bracket on the steering spindle was not straight (previous owner admitted his 8yo hit a tree but he bent everything straight). I realigned it as best I could and bled the front and rear brake lines (removing the opposing master cylinder push rods in the process). In the process I rounded out one of the hex-headed pad retaining bolts, so I purchased new after-market calipers made for the 150. They were installed about a week ago, along with the pads that came with them.

Everything was clean and there was minimal drag when the wheels were turned by hand.

Today, going down a rather long (1+ mile) steep gravel back road, I kept the speed/RPM up enough to let the engine do most of the braking. Still, by the time I got to the halfway point, I kept hearing a moaning from the front end. When I stopped to check, both front tires were nearly skidding as I just barely touched the brake pedal. The rotors were smoking hot (like an idiot, I have the blister to prove it). And the buggy stopped on the decline as though I had the parking brake on. Restarting the trek, both front wheels skidded in the gravel. I was stuck. Wife came with a wrench so I could bleed pressure off of each caliper - and I was good to drive home from there with the kids.

By the time she got to me, the front had cooled enough that I could drive, but I could still feel the drag.

In any case, since both of the calipers did this, I'm wondering if it could be anything other than the master cylinder.

TL;DR
  1. Both front brake rotors locked up rendering movement impossible after a long down-hill run.
  2. Calipers and pads are new (replaced a week ago)
  3. Brake system was bled (full master cylinder reservoir was pushed through all calipers).
  4. Original right caliper had history of locking up (according to previous owner)
  5. After brakes cooled, forward movement was possible (but with drag until bleeder valves were cycled).
  6. Caliper bracket alignment appeared correct on both sides (right side required adjustment after wreck).
What could cause these problems?
 

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Discussion Starter #2
I think I may be answering this myself... After posting this request for help, I did some more research, and it would seem that having an over-filled master cylinder reservoir can cause my symptoms. I'm quite certain that is the case. I will check tomorrow, and report.
 

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Discussion Starter #3
Well, I guess that might not have been my problem. While I did overfill the reservoir when I changed calipers, putting the rubber funnel-like black spacer between the fluid and the cap pushed out the excess. The spacer forces an air pocket that lines up perfectly with the max fill line. So, I'm thinking that is not my problem. Once weather improves, I will make another trip down the steep drive to see if the problem returns. If so, I may pull the master cylinder and inspect/dissect it.
 

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Well, I guess that might not have been my problem. While I did overfill the reservoir when I changed calipers, putting the rubber funnel-like black spacer between the fluid and the cap pushed out the excess. The spacer forces an air pocket that lines up perfectly with the max fill line. So, I'm thinking that is not my problem. Once weather improves, I will make another trip down the steep drive to see if the problem returns. If so, I may pull the master cylinder and inspect/dissect it.
Were having the exact same issue. Any luck figuring out why the brakes seize up?
 

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Sounds like a collapsed brake line to me. It would be the flexible hose but it is odd both are doing it unless a single hose feeds both front brakes.
 

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Discussion Starter #6
Were having the exact same issue. Any luck figuring out why the brakes seize up?
Sure did. The style of master cylinder was foreign to me. There are two plungers - one for the front two calipers, and the other for the rear brake. There is a rod that goes from the brake pedal to the master cylinder, and just before it touches it, under a rubber boot, is a "fork" device so that the rod pushes both plungers at the same time. The design keeps the front and rear brakes independent of one another, but makes it impossible to bleed them without separating the plunger activation.

In order to properly bleed all the air from the brake lines, you have to remove the rear pin from that master cylinder fork so that only the front plunger is being pushed. It's a bit of a pain (access from underneath), but once all the brake pressure goes to the front, and the rear pressure doesn't interfere with the bleeding, you'll be able to fully purge the air from the front.

Once I did this, I had zero seizures and all is working like new.

It sounds complicated but, it's actually pretty straight forward once you grasp the concept.
 
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