V4 brake burning

Hey everyone, I recently purchased a 2018 V4 with around 7,000 miles and it's completely stock. The rear brake was practically nonexistent when I bought it. After a good bleed, it felt fine, but if I ride for any length of time, it practically starts smoking. At the very least, the rear disc and caliper are scalding hot. This happens even when I ride without using the rear brake at all. I'm running 5.1 fluid and have bled the brakes multiple times now, with the bleeder angled high to get the air out. I've checked the runout on the rotor and disassembled and cleaned the calipers. Still, the issue persists. I've noticed the master cylinder is hot too, but I assume that's because of its proximity to the exhaust and motor. Any advice?

Sounds like the brake is dragging. The pistons in the caliper are not retracting when you let off the brake. Need new rear brake MC probably.

Agree on the brake dragging. I was thinking rear caliper piston was possibly sticking.

I'd take the caliper off and fully spread the pads and seat the piston. Then without using the rear brake, take it on a test ride and see if the heat issue was there.

To be clear, is this only on the rear? The OP mentioned "calipers" which makes me thing more than the rear.

bp_SFV4 said:

Agree on the brake dragging. I was thinking rear caliper piston was possibly sticking.

I'd take the caliper off and fully spread the pads and seat the piston. Then without using the rear brake, take it on a test ride and see if the heat issue was there.

To be clear, is this only on the rear? The OP mentioned "calipers" which makes me thing more than the rear.

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My bad. It's caliper (singular), just the rear. I've spread the pads and test rode it, and yes, the issue is fixed. I figure it has to be the MC as I've disassembled, cleaned, and lubed the caliper. It wouldn't bother me, but I don't feel good about getting brake smell after a short ride. Just wondering if anyone else has had this issue. I may try spreading the caliper and seeing if the MC is still hot as well.

roadrash said:

My bad. It's caliper (singular), just the rear. I've spread the pads and test rode it, and yes, the issue is fixed. I figure it has to be the MC as I've disassembled, cleaned, and lubed the caliper. It wouldn't bother me, but I don't feel good about getting brake smell after a short ride. Just wondering if anyone else has had this issue. I may try spreading the caliper and seeing if the MC is still hot as well.

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And by fixed I mean with the caliper doesn't get hot but as soon as the pistons extend it starts dragging and getting hot even if I don't use the brake.

You need a new MC. Sounds like the check valve in it has gone bad. Not worth the time to rebuild it.

roadrash said:

I may try spreading the caliper and seeing if the MC is still hot as well.

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I wouldn't think it would.

roadrash said:

but I don't feel good about getting brake smell after a short ride.

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I wouldn't either. It's a problem for sure.

If you're confident if your rebuild of the rear caliper, the only thing that's left is the master.

I recon its rearrsetcadjustment there is no free play in the rear brake lever

True that could be a problem as well

So, I pushed the pistons all the way in and took a ride. The caliper, rotor, and lines are hot but not burning. The MC is very hot. I confirmed that the pads aren't touching the disc with a small mirror after the ride.

Does anyone else have a V4 to confirm how hot their MC gets? It is mounted right next to the engine and exhaust after all. Fluid has less than 500 miles and is getting dark already.

A mate had the same issue, it got so hot that the disc started glowing and the brake line failed leading to a fire! Check lever freeplay, then the next steps are rebuild calliper and mc, its cheap