Shimano brakes loosing all pressure (not a bleed issue)

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Shimano brakes loosing all pressure (not a bleed issue)
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flag stenty
Posted: Aug 16, 2019 at 19:19 Quote
Hi there
I recently brought a new set of shimano deore 4 pot mt520 brakes for the front and rear. They have been working great for the last few weeks riding every day so they have had good use with no issue. Then the rear brake pads wore out and they got so thin that the piston on one side almost popped out to the point where it was leaking brake fluid and lost all pressure. so I bled the brake and got a fresh set of pads. It was working as normal again untill near the bottom of the descents where the brake would get very hot. This is when it would loose all pressure and the lever would reach the bars with no bite at all!!! (pretty scary)
once I let it cool down it goes back to having a normal bite point where it should be and it feels good and solid with no spongy-ness so it is NOT a bleed issue or air bubbles however I have no idea what it could be as its only when it gets really hot. It also can't be that the fluid is boiling because it worked perfectly for weeks before i had to change the pads so something piston related is up i think??. I also can't find anyone else who has had the same problem online.
Can anyone help??
Thanks in advance! Smile
flag Will022505
Posted: Aug 17, 2019 at 10:14 Quote
I would try either replacing the fluid as the fluid can become airated, or maybe get a new set of pads that have the cooling fins?
flag stenty
Posted: Aug 17, 2019 at 17:08 Quote
Will022505 wrote:
I would try either replacing the fluid as the fluid can become airated, or maybe get a new set of pads that have the cooling fins?

the fluid has been super recently replaced though so its not airated and the strange thing is i had no issue with the last set of pads i had in the brakes and they weren't finned so it cant be that either haha. it's super strange
flag Will022505
Posted: Aug 17, 2019 at 21:46 Quote
If they're not finned, then try finned ones
flag chrysanthos
Posted: Aug 18, 2019 at 2:40 Quote
what about the brake rotors? maybe they get too hot
flag rikkibarrattmtb
O+
Posted: Aug 18, 2019 at 12:55 Quote
I’ve got the same brakes and have had some issues with varying pressure on the lever ... fresh bleed and after market finned pads from Uber bike sorted it all out
flag butters1996
O+
Posted: Aug 18, 2019 at 13:29 Quote
I'm sure you've seen the recent reviews on the xt brakes and bikes with the xtr levers.
Seems like shimano still hasn't fixed the wandering bite point issue that they have had for years. I think super regular bleeds is the fixed but I assume can get pretty annoying after awhile.

Pretty much every shimano brake that's rolled into the shop in the last few years has had an odd feeling lever, even in the stand the point at which they engage changes constantly.
flag stenty
Posted: Aug 20, 2019 at 18:05 Quote
Will022505 wrote:
If they're not finned, then try finned ones

it never had finned pads before though and the brakes still worked perfectly so i think its more likely to be something withing the caliper maybe??
flag stenty
Posted: Aug 20, 2019 at 18:06 Quote
KnoblyKnees wrote:
I’ve got the same brakes and have had some issues with varying pressure on the lever ... fresh bleed and after market finned pads from Uber bike sorted it all out

Thats good to hear. did it just start happening all of a sudden or out of the box and also is it only when it gets hot or at any time?
Thankls
flag rikkibarrattmtb
O+
Posted: Aug 21, 2019 at 7:52 Quote
stenty wrote:
KnoblyKnees wrote:
I’ve got the same brakes and have had some issues with varying pressure on the lever ... fresh bleed and after market finned pads from Uber bike sorted it all out

Thats good to hear. did it just start happening all of a sudden or out of the box and also is it only when it gets hot or at any time?
Thankls


All of a sudden and at totally random intervals not necessarily on a downhill
flag chrysanthos
Posted: Aug 21, 2019 at 9:49 Quote
KnoblyKnees wrote:
stenty wrote:
KnoblyKnees wrote:
I’ve got the same brakes and have had some issues with varying pressure on the lever ... fresh bleed and after market finned pads from Uber bike sorted it all out

Thats good to hear. did it just start happening all of a sudden or out of the box and also is it only when it gets hot or at any time?
Thankls


All of a sudden and at totally random intervals not necessarily on a downhill

Use the warranty and see where it takes you
flag eshew
Posted: Aug 21, 2019 at 15:07 Quote
Might want to try a funnel bleed to make sure the fluid is topped off in the lever.

The only times I've had issues with random lever on bar scenarios is when their is not enough fluid in the reservoir.

back off free stroke screw if you have one, screw in funnel, add fluid pump the lever and let it spring back. Might do the trick, might be warranty time.
flag stenty
Posted: Aug 23, 2019 at 19:25 Quote
eshew wrote:
Might want to try a funnel bleed to make sure the fluid is topped off in the lever.

The only times I've had issues with random lever on bar scenarios is when their is not enough fluid in the reservoir.

back off free stroke screw if you have one, screw in funnel, add fluid pump the lever and let it spring back. Might do the trick, might be warranty time.

yeah that sounds like a good shout Ill do that i think. cheers
flag ben-t-bishop
FL
Posted: Apr 9, 2021 at 15:57 Quote
(I know I'm late to the convo but here you go)

It could be the brake pads that you are using. For the brakes that you are using you could either get resin or metal brake pads. If you got metal then I'm not sure what the problem is but if you are using resin pads it is not uncommon for them to over heat. Resin pads are more for XC. They are very usable for trail and some times enduro riders but they will over heat.

That would be my first guess but if they are metal pads I don't know what to tell you
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