Brakes acting up yet again.

[Gubbins]

This has been an on and off problem all the time I have had this bike..
Today my rear brake is totally ineffective . I can pull the lever as hard as I can and all I get is groaning and it barely slows me on the flat but I go down a steep hill and it's fixed it in 20 seconds and is fine from then on.
So I take it off and clean it. Check the pads and both piston are moving. No oil about anywhere, so refit and the video shows what I get when I apply enough pressure to slow the bike.
Brakes are a mechanical thing so there must be a reason for this..
I would fit a new one but it's internal cabling so is beyond my skills (bleeding)
After this cleaning the brake now has little effect but will be ok after a fast downhill stop which is obviously the heat...
Only rear. Never a problem with the front...
So it must be me so what am I doing wrong?

 

[Fat Rat]

Have you tried different pads

 

[soundwave]

you could try a new rotor as thats easy to remove and if it is still the same then if you cant do it a new caliper

 

[Gubbins]

Yes.

Have you tried different pads

 

[Gubbins]

you could try a new rotor as thats easy to remove and if it is still the same then if you cant do it a new caliper

I did wonder but how can the disc be the problem. It's just a bit of metal.

 

[Fat Rat]

Yes.

Is the cable moving free in the liner
How does it feel
Also are you still on the first half of your calliper lever pull or have you gone past the 90degrees
Have you a side on photo showing lever under pressure

 

[Gubbins]

Is the cable moving free in the liner
How does it feel
Also are you still on the first half of your calliper lever pull or have you gone past the 90degrees
Have you a side on photo showing lever under pressure

It's hydroulic which is why I can't replace it myself

 

[Fat Rat]

It's hydroulic which is why I can't replace it myself

Ah sorry you mentioned mechanical I assumed non hydraulic

 

[Fat Rat]

It's hydroulic which is why I can't replace it myself

What make are they

 

[Gubbins]

Ah sorry I thought you mentioned cables

Yes I did.. they are all internally routed so I can't just buy a pre bled brake.. which is my preferred option..

 

[Gubbins]

What make are they

Shimano SLX I think.

 

[Fat Rat]

Yes I did.. they are all internally routed so I can't just buy a pre bled brake.. which is my preferred option..

Shimano brakes are a doddle to bleed
You just shringe them from the calliper till it comes out of the lever

 

[Gubbins]

I should take it in to LBS, but it doesn't go in my car easily without taking it to bits.

 

[soundwave]

heat warps disc rotors its why i have t2 parts ones

 

[soundwave]

 

[Gubbins]

View attachment 25400

Yes but it would either work or not work.. wouldn't it?

 

[soundwave]

heat also destroys the pads and thus no brakes at all but first thing you need to do is get the brakes bleed and go from there.

you can take of the rotor a place it on a flat surface to see if it has a warp in it as you said when it gets hot it stops doing it.

are all the rotor bolts tight ?

 

[Gubbins]

Spun the wheel and rotor is true enough.. just been on line and found a mobile bike mechanic...

 

[soundwave]

i was just about to say try a mobile mechanic lol

 

[JPGiant]

Could it be that you've lost a load of fluid at some point in time and have a big airlock in the tubing?
Like in a car with the same problem, first pump on the brake pedal - no braking just pressuring the air, moments later they work when a bit of fluid gets to the pads pistons.
You may have fluid at either end of the tubing with air in the middle?
As already suggested, give it a good bleed first.