Noob question on sram hrde hydro service

stackie · #1 11-05-2019, 11:23 AM

Just looking for a little advice here. My first time with hydro discs.

Recently travelled with my sram hrd hydro disc setup. I dutifully put the little spacer in front wheel for the drive down. On the way back I had to take off rear wheel as well, but didn’t have the spacer for it. So I improvised with some cardboard folded in half.

When I reassembled there was a scraping noise from rear wheel which I isolated to rear disc. At least think it was. Pulled pads to reset pistons per GCN video. Pads are worn out. Nonetheless, I did use a flat tool to push the calipers in as I squeezed each lever. Seemed like they moved pretty well. Put the pads back in and ordered new ones. Now the wheel is basically locked up from the disc. I haven’t put the new pads in yet. Hope to do that soon. But wonder if I need to either do brake bleed or have my shop do. Eventually I’d like to learn to bleed brakes. It’s been about a year since bike was built. Any thoughts?

Thanks in advance for any tips.

Jon

Jaybee · #2 11-05-2019, 11:58 AM

Did you push the pistons back in before installing the new pads? Try this first.

Poor bleed usually mainfests as spongy lever feel and lack of power since air introduces compressibility into the system.

batman1425 · #3 11-05-2019, 12:09 PM

The most obvious thing I can think of is make sure the rear wheel is centered correctly in the dropouts. If your bike has a quick release rear. You can move the hub slightly in the drops get the rotor space evenly between the pads if you need to. Just make sure that it is clamping dropout meat all the way around. Even a fraction of a MM of misalignment at the hub translates into a big problem at the rotor 140/160/180mm away where the pad contacts.

If it is a through axle rear, then you probably have something else going on. Those usually line up straight. If a TA, look to the pads not retracting all the way, or a caliper mounting bolt coming loose allowing the caliper to move around.

stackie · #4 11-05-2019, 12:35 PM

They are through axle hubs.

I’ll check the caliper mounting bolts too. And make sure to push the pistons back again. I could certainly have messed it up. If I’ve pushed the pistons back, do I recenter/align by squeezing brake levers three times?
Thanks for the tips.

Jon

Blown Reek · #5 11-05-2019, 12:43 PM

Push the pistons back, insert the pads, loosen (but don't remove) the caliper bolts, place wheel back in, tighten wheel, squeeze brake lever and hold it, tighten caliper bolts, release brake.

batman1425 · #6 11-05-2019, 12:50 PM

Quote:

Originally Posted by stackie

They are through axle hubs.

I’ll check the caliper mounting bolts too. And make sure to push the pistons back again. I could certainly have messed it up. If I’ve pushed the pistons back, do I recenter/align by squeezing brake levers three times?
Thanks for the tips.

Jon

Don't pull the lever without the wheel and rotor installed. If you do, they will continue to work themselves out and will end up set too narrow for your rotor. The video you were watching might have been designed to free a stuck piston - holding one pad in place while squeezing the lever to force the other to move. You shouldn't need to touch the lever to press the pistons back in.

Remove pads, push the pistons in, reinstall pads, and wheel, then squeeze handlebar to adjust spacing and center the pads.

If this doesn't work, follow the instructions from Blown Reek to realign the calipers as part of the setup.

Jaybee · #7 11-05-2019, 12:50 PM

It certainly won't hurt to push the pistons back in again - worst case is that you are in the same spot you are in now (provided you don't push them too far and damage the seals). Pump the levers until you have pad contact when the lever is engaged. Might be 3 times, maybe less, maybe more. Here are a couple of generic how-to's

https://www.parktool.com/blog/repair...l-installation
https://www.parktool.com/blog/repair...rake-alignment

stackie · #8 11-05-2019, 03:09 PM

You guys rock!

I don’t know if levers were squeezed with the makeshift cardboard spacer. Certainly tried to keep that from happening.

Question is it normal or usual to have to loosen caliper mounting bolts when changing pads?

Thx

Jon

Blown Reek · #9 11-05-2019, 05:35 PM

Loosening the caliper mounting bolt ensures that the system is centered around the rotor. You shouldn't have to do it, though. If you've got a dragging rotor, this usually solves it.

stackie · #10 11-05-2019, 05:51 PM

Got it.

Maybe something bumped the caliper during transit.

Thx

Jon

stackie · #11 11-11-2019, 11:20 PM

OK,

So, I took the pads out and replaced with new pads. Still dragging. So, I inserted the 3.8mm spacer and squeezed the lever x 3. Still dragging, could now see caliper obviously off kilter. So, loosened bolts and recentered caliper. Now, no dragging, but also no brakes.

Oops, I was supposed to do the 3.8 spacer without the pads. Duh, So now the pistons are pushed too far in and don't really advance.

I tried squeezing the lever with the 2.4 spacer in place, but the lever now bottoms out against the bar with only minimal piston movement.

What the heck did I do? Have I now damaged the master cylinder? Shoot.

How do I fix this?

Thanks again,

Jon

batman1425 · #12 11-12-2019, 09:17 AM

I haven't seen a scenario where you can press the pistons in too far. The farthest they will go is flush with the caliper body.

If they are moving, keep pumping the lever with the pads and rotor installed. They should reset. If not, time for a trip to the LBS.

I'm still confused why you are pulling the lever with the spacer installed after resetting the pistons?

stackie · #13 11-12-2019, 10:28 AM

My initial assessment was that the pistons were not spaced out enough and was presuming the levee accidentally was squeezed with the jury rigged cardboard spacer.

SRAM caliper reset manual states to squeeze the levers hard (22lbs) with the 3.8 mm spacer in to set the pistons spacing. This was supposed to be done without pads in place. I accidentally did this with pads in place.

Now the pistons barely move with lever squeeze.

I think it may be time to go to the the LBS with tail between legs.

Jon

batman1425 · #14 11-12-2019, 10:41 AM

I didn't know Sram recommended that. Never needed to do this with the resets I've done on them. Interesting.

You may have somehow forced air into the system resulting in the failure to advance.

Jaybee · #15 11-12-2019, 10:43 AM

Quote:

Originally Posted by stackie

My initial assessment was that the pistons were not spaced out enough and was presuming the levee accidentally was squeezed with the jury rigged cardboard spacer.

SRAM caliper reset manual states to squeeze the levers hard (22lbs) with the 3.8 mm spacer in to set the pistons spacing. This was supposed to be done without pads in place. I accidentally did this with pads in place.

Now the pistons barely move with lever squeeze.

I think it may be time to go to the the LBS with tail between legs.

Jon

The only time I've seen Sram pistons pressed beyond the caliper, the seal behind the Piston was broken and the caliper had to be replaced. I was the ham-handed mechanic in that instance and I bought my friend some new brakes. Hope this isn't you, but I would go to the LBS at this point.