Okay, where to start.. This has been happening for the past little while but was hard to reproduce. Originally, when I get up to highway speeds (upper RPM range in second and third) I'd sometimes get what I originally had considered was a hesitation/rev limit issue. I assumed it was fuel related. It just seemed like I hit a wall and the car would just break up and backfire. I couldn't get the issue to reproduce on command so gathering data was difficult. Especially, when I needed to get into the higher RPMs of second or third!

Anyway, a few weeks ago it happened again (mid of second) and this time I was ready! I had the wideband ready and was watching looking for anything to help me determine what fuel related thing was happening. Only this time there was a pop and very loud grinding noise. (I don't recall a smell but I'm sure there was something). I pulled off when it was safe to do so. The sound was independent of RPM so not fuel or engine. It sounded like it was coming from everywhere/the rear so I figured transmission, diff (lsd), wheel bearings, or calipers(recently replaced/rebuilt). As I looked around the car I couldn't find anything glaringly wrong so after a quick once over I decided to try to limp back home. When I tried to pull off I heard a pop like a pad was stuck and just broke free (like how rusted discs and pads sound). I was in reverse if it matters. The sound continued then tapered off.

I have looked at the brakes and wheel bearing on the right side as I thought it was that side. (Just replaced a bad caliper piston) But I'm not finding anything that looks bad.

Searching online suggests (among other things) that the master brake cylinder is getting stuck causing the pads to not retract sometimes. Any ideas? And how can I test/check the master cylinder? Could it be the brakes dragging, getting hot from higher speeds, then grabbing? More likely something else? I plan on pulling apart the other side too just to check.

Thank you for reading!

 

What about the brake soft line? When they age, they can cause the rubber to swell internally and restrict fluid going back to the reservoir and cause the pads to stay engaged.

 

Or, since you recently rebuilt brak3 calipers and stuff, check your rear U-joint on the Driveshaft. Grab hold and try to jerk it in all 4 directions, if you get clacking in any dimension, it could be the culprit. I went with a MazdaTrix user serviceable Driveshaft and never looked back. Happier knowing I can carry spare spiders and replace them myself on the road if necessary.

 

Sorry for the delay in updates. Rebuilt the calipers again this time using the pink grease (08887-01206), first time I only used brake fluid as lube. The difference in the feel of retracting the pistons with vs without was night and day. So I believe what was happening is the piston was getting stuck due to lack of lube and not retracting as well as it should have.

I did check the drive shaft as suggested by LongDuck, and also replaced the brake hoses and the driver's side hard line (rounded off and seized).

Thank you both for your time and suggestions!

 

So you've driven it and the noise went away? What condition were your brake pads in? Heavily worn pads, usually on one side are an indicator of a stuck piston or sliders, plus severe overheating of the pad backer. Glad you may have found the culprit,

 

Yeah, everything seems to be going much better this time around. The pads appeared glazed, maybe slightly cracked. But I'm not an expert.

I still occasionally have the hesitation/misfire on the top end WOT/full load but I assume that's just tuning at this point.

Thanks again!