XT M785 brake reservoir leak when bleeding and possible piston seal leak? Help appreciated.

PB Forum :: Mechanics' Lounge
XT M785 brake reservoir leak when bleeding and possible piston seal leak? Help appreciated.
  • Previous Page
  • Next Page
Author Message
flag ElTaco
Posted: Oct 14, 2013 at 14:08 Quote
Hey guys, I really need some help here. I just recently finished a new bike build, and I'm having problems with my Shimano XT M785 brakes. I bought them online from TBS Bike Parts in early August. During installation I had to shorten the brake hoses, and I made sure to install new olives and barbs. I proceeded to bleed the brakes per Shimano's instructions (using the Shimano funnel), but I immediately noticed a very significant brake reservoir leak under pressure (when squeezing the syringe from the calliper side) along the long edge of the master cylinder cover. The strange thing is that this happened to both the front and rear brakes. I quadruple checked that I'm doing everything right. I found that the only way to bleed them was to remove the stopper from the funnel completely. Anyway, I did finally manage to bleed them, but it was a pain in the ass.

Now the second issue. I took the bike out for it's first trail ride, and overall things were alright, but I found the rear brake to be a bit spongy. I then took the bike out for another ride this past weekend, the rear brake was really weak and it squealed like 'crazy'! I figured that I'd try to bleed it again. I did a really thorough job (once again I had issues with the reservoir's master cylinder leak), and after taking out the bleed block, I noticed that it had oil on it. I proceeded to check the pistons, and I found oil and grime all over the place. Could all of this then be due to a leaky piston seal, and what about the leaky reservoirs? Is it possible that I just got massive set of dud brakes? I was originally going to buy the XTRs, but after really thinking about it, saving an extra couple of grams just wasn't worth the money. Now I'm thinking that maybe the XTRs would have been a better idea after all.

Anyway, I'm just wondering if anyone with any experience can chime in? Thanks in advance for reading my essay, and any sort of help.

Cheers!
flag Ploutre
Posted: Oct 14, 2013 at 14:42 Quote
The leak at the master cylinder is because you're putting too much pressure on the oil inside the reservoir over the seal. The top cap is just a piece of plastic that bends under heavy load. Had that once, surprised me and then saw I had closed the seringe in use in place of the funnel Big Grin

A spongy lever feel definitely means something has gone wrong in the bleeding procedure...

As for the 2nd one, the squealing probably comes from contaminated pads, and if it's leaking from the caliper piston well ... Can you send the brakes out to Shimano for warranty ?
flag ElTaco
Posted: Oct 14, 2013 at 14:57 Quote
Ploutre wrote:
The leak at the master cylinder is because you're putting too much pressure on the oil inside the reservoir over the seal. The top cap is just a piece of plastic that bends under heavy load. Had that once, surprised me and then saw I had closed the seringe in use in place of the funnel Big Grin

A spongy lever feel definitely means something has gone wrong in the bleeding procedure...

As for the 2nd one, the squealing probably comes from contaminated pads, and if it's leaking from the caliper piston well ... Can you send the brakes out to Shimano for warranty ?

Thanks for the help Ploutre. Yah the reservoir leak does seem to happen under pressure, it's just strange because the pressure seems to be cause by the actual 'pin' plug that comes with the funnel. The only way to do a proper bleed is to take it out completely. That's the part that has be confused, but whatever, in the end, they seem to bleed fine, except for the rear, which is most likely something wrong with the piston seal I guess. I suppose it could be pulling air in through the seal as well, explaining the spongy feel? I'm going to email the store that I bought the brakes from and see what they say. I might end up having to go through Shimano though. This really sucks as I was really hoping to get some more riding in on the new bike before the winter.

Anyway, thanks again for your help friend, I appreciate it Beer
flag ElTaco
Posted: Oct 14, 2013 at 17:51 Quote
Alright, so I cleaned and sanded the rear brake pads. Took the bike out for a ride around the block. The rear brakes do feel much better now, but definitely not as nice as the front. Also, I noticed that when I pump the rear brake lever, a minuscule bead of brake fluid becomes visible around one of the pistons. I have also done some more reading and apparently this is a common problem. Not happy at all Frown
flag Chuvak
Posted: Oct 14, 2013 at 22:46 Quote
when the brake is fully assembled and you press the lever with much power, u really can see a thin oil ring around a piston. it's okay, the same thing was on my 09'saint and m775. the oil must not make drops and leak out, only minor rings around pistons allowed. they must disappear when you release the lever.

about bleeding. do it the next way:
1) remove the pads, insert the bleed spacer
2)unscrew the cap of bleed port, insert the funnel. put the oil stopper away, you're not need it now.
3)attach the syringe (with 15-20ml of oil) to the calliper, turn the bleed screw 1/2-3/4 way counter-clockwise
4)push the piston of syringe gently, slowly fill the funnel. doing this, lightly tap your hose and lever. when there is 5-7 ml of oil resting in the syringe, tilt the lever 30 degrees down and press the syringe to push half of the oil. make the same with lever, tilted 30 deg up.
5) when you use all your oil in the syringe, detach it from the tube and let the oil slowly leak through the tube in the plastic bag or little glass. be careful, watch the oil level. don't let the lever suck some air. tap your calliper gently.
6)close the bleed screw at the calliper, tighten it, detach the tube. insert the oil stopper in the funnel (with rubber ring down), detach the funnel, close the lever bleed port with cap. done.

if you experience loosing lever after it, that can mean the brake defectiveness, so contact the support for warranty.
flag ElTaco
Posted: Oct 15, 2013 at 8:01 Quote
Chuvak wrote:
when the brake is fully assembled and you press the lever with much power, u really can see a thin oil ring around a piston. it's okay, the same thing was on my 09'saint and m775. the oil must not make drops and leak out, only minor rings around pistons allowed. they must disappear when you release the lever.

about bleeding. do it the next way:
1) remove the pads, insert the bleed spacer
2)unscrew the cap of bleed port, insert the funnel. put the oil stopper away, you're not need it now.
3)attach the syringe (with 15-20ml of oil) to the calliper, turn the bleed screw 1/2-3/4 way counter-clockwise
4)push the piston of syringe gently, slowly fill the funnel. doing this, lightly tap your hose and lever. when there is 5-7 ml of oil resting in the syringe, tilt the lever 30 degrees down and press the syringe to push half of the oil. make the same with lever, tilted 30 deg up.
5) when you use all your oil in the syringe, detach it from the tube and let the oil slowly leak through the tube in the plastic bag or little glass. be careful, watch the oil level. don't let the lever suck some air. tap your calliper gently.
6)close the bleed screw at the calliper, tighten it, detach the tube. insert the oil stopper in the funnel (with rubber ring down), detach the funnel, close the lever bleed port with cap. done.

if you experience loosing lever after it, that can mean the brake defectiveness, so contact the support for warranty.

Thanks Chuvak. I think I got the bleed done right now. I turned the bike upside down and left it like that for about an hour. I then checked to make sure that the lever doesn't drop any lower / isn't any softer. Seems like it's all ok. As for the piston, it's exactly as you describe. It's a 'very' thin ring of oil, and it pretty much disappears once the lever has been released. I even put some rubber bands on the lever and left it depressed for a while to make sure that more oil doesn't make it out over time, and it didn't. The point is that the pads did somehow become contaminated. For now I boiled them (they're resin pads), sanded them down, and cleaned them up with alcohol. They seem better, but I'll keep on eye on them.

The crappy thing is, after removing and reinstalling the pads so many times, the thread on the calliper where the brake pad holding pin goes, somehow stripped mid way. It still holds well enough and doesn't fall out, but that's still no excuse for the thread to go this quickly Frown

Anyway, thanks again Beer . Hopefully I don't have any more problems with them.
flag Chuvak
Posted: Oct 15, 2013 at 16:01 Quote
you're welcome.

don't forget to clean pistons once in a 2-3 weeks, if you ride in dirty conditions: remove the pads, press the lever 2-3 tomes to make pistons go out a bit. then clean their side surface, lubricate it with a little quantity of oil and insert the pads back. gently push the pistons back, inserting ans screwing the knife between the pads, if u needed (never use the screwdriver!).

PS. when you have an hour free, still bleed the brakes once to make sure that these loosens will not appear again. and don't tighten pad screw too much, there's no big load on it)
flag ElTaco
Posted: Oct 16, 2013 at 17:36 Quote
Chuvak wrote:
you're welcome.

don't forget to clean pistons once in a 2-3 weeks, if you ride in dirty conditions: remove the pads, press the lever 2-3 tomes to make pistons go out a bit. then clean their side surface, lubricate it with a little quantity of oil and insert the pads back. gently push the pistons back, inserting ans screwing the knife between the pads, if u needed (never use the screwdriver!).

PS. when you have an hour free, still bleed the brakes once to make sure that these loosens will not appear again. and don't tighten pad screw too much, there's no big load on it)

Thanks again for those tips! They are very helpful. For pushing pistons back into position I use a tire lever. Just insert it in the caliper and gently wedge the piston back in. Thank you again.
  • Previous Page
  • Next Page

 


Copyright © 2000 - 2024. Pinkbike.com. All rights reserved.
dv65 0.009743
Mobile Version of Website