Shimano XT M785 brake problems

M

melonhead

Likes Bikes
I have a set of M785 Xt brakes that I have been using for 3 or 4 years, and have been awesome up until now. Recently the lever feel was very inconsistent and playing around with the reach adjuster would usually make them feel good again. Then they got worse, so I bled them with Shimano mineral oil as per Shimano instructions and they felt good until I went to ride them a couple of days later. Sometimes the lever feels very firm with little or no travel, which causes the brakes to drag/not retract, then more commonly the lever does not return to its resting position after applying brake. I need to push the lever forward by hand. The brakes can be sitting there nicely one minute, then next they feel crap. Seems to be worse in hot weather. I thought there might be too much fluid in the master cylinder, so I removed some fluid and no difference. I have inspected the calipers and the pistons seem to be moving freely and evenly. Seems like the master cylinder piston is sticking. Has anyone encountered this, and have any suggestions?

Thanks

Lee
 
Can you see that there is a piston not retracting into the bore when pumping the brake?

Oh and you want the system to be full to the brim with oil. If it's not, there will be air where there is meant to be oil which is bad.

Edit: Actually, it's more likely something wrong with the lever rather than the caliper now I think about it. 50 bucks for a new lever from CRC.
 
Last edited:
Both pistons on both calipers are moving out to meet the rotor and retracting as I reckon they should. Forgot to mention that both the front and rear brakes are exibiting exactly the same symptoms.

When I took some oil out of the system, I opened the master cylinder port, then pushed the brake pads apart to completely retract the pistons into the bores. This pushed a slight amount of brake oil out into the funnel because the caliper pistons got pushed further into their bores than when the yellow brake block is installed during bleeding.
 
I've had a couple of M785s through work with sticky master pistons (in the lever). They were on the same bike, but weirdly at different times. Sounds like this is what you have happening; they squeeze on fine but don't retract.

I managed to get one lever working properly again, but not the other, and it's a pain in the arse job to fully strip out the guts of the lever & reservoir for only a 50:50 chance of successful surgery. Just get new levers.
 
sticking

my M785's lock up when i lay the bike flat in the back of the ute for half an hour ?? have to stand the bike up for an hour to fix ..
Hot days is worse .. bled the systems today and rear lever has a partially seized piston .. may have something to do with clearance/heat .. CRC $99 lever/caliper/hose and pads .. throw em away mate and start again ..
 
The Duckmeister said:
I've had a couple of M785s through work with sticky master pistons (in the lever). They were on the same bike, but weirdly at different times. Sounds like this is what you have happening; they squeeze on fine but don't retract.

I managed to get one lever working properly again, but not the other, and it's a pain in the arse job to fully strip out the guts of the lever & reservoir for only a 50:50 chance of successful surgery. Just get new levers.
Click to expand...

So what is the cause duck? Just age and wear sticky piston xt don't have lever return spring do they?
 
There's a return spring on the piston but not the lever itself, so if the piston seals get sticky it'll gum up the system.
 
Courtesy of SlowMick, there's a sale on M8000s:

http://www.torpedo7.com.au/products/SHBDDNN8C/title/shimano-deore-xt-br-m8000-f---r-brake-set

SHBDDNN8CAA_zoom.jpg


I bought a pair!

Yes, resin pads, but I'm going to put one set of M785s on the commuter with the resin pads & put these on the TBc with the existing metal pads. Sorted.
 
Glad you scored some before they sold out...

I never got around to it. Like i missed the saints 820s for $180 an end.
 
If considering M8000 brakes just keep in mind that if you're currently running I-Spec mounted shifters on M785s they won't fit.
 
The Duckmeister said:
I've had a couple of M785s through work with sticky master pistons (in the lever). They were on the same bike, but weirdly at different times. Sounds like this is what you have happening; they squeeze on fine but don't retract.

I managed to get one lever working properly again, but not the other, and it's a pain in the arse job to fully strip out the guts of the lever & reservoir for only a 50:50 chance of successful surgery. Just get new levers.
Click to expand...
On my second set with this same issue happened to both. Only one lever at a time. Had a mate in same time frame with same problem.
I have contacted Shimano Australia as i suspect this is widespread issue they must know about. I have pulled lever out and it is the piston the pushrod activates deep inside body of master cyclinder that gets stuck in. I noticed that there was a little spring that is a V shape with the spring section where the 2 arms join behind the lever to return it.
Bit of a bitch to get back together again

Sent from my SM-G900I using Tapatalk
 
Contacted shimano australia who claimed this was first they had heard of the problem (yeah right), and was told to return with proof of purchase.

Watch this space....

Sent from my SM-G900I using Tapatalk
 
Update....

Nice shiny new m785 brake assembly arrived in the mail today. Very happy with that outcome, especially as i owned up it was bought from crc

Sent from my SM-G900I using Tapatalk
 
You must log in or register to reply here.
Back
Top