SLX rear brake sometimes goes soft

[Ricker]

Couldn't work out what to search on google to find the information I was looking for, so though I would bring it here. Basically my brakes work fine, but every now again, if I do a wheelie or a jump my rear brake goes soft in the trigger. And I have to squeeze it in another whole cm or so before it start applying pressure to the discs. After a couple of pumps it goes back to normal? Kinda disconcerting when i'm flying along and then suddenly I need to squeeze my brakes much further than i'm used to.

Does this sound like air has got in the line? I'm new to brakes, so not sure whats involved in any fixes offered.

 

[teK--]

Air in lines. It is working into line then getting pushed out to reservoir and working back into the lines. Bleed them.

 

[The Duckmeister]

How's the pad wear? Although the pistons are self-adjusting to a large degree, if the pads are getting thin they can tend to retract more, requiring pumping up.

 

[moorey]

All my 785's (and mates) do a funny thing with the bike vertical. Squeeze lever bad it goes into bar. Lower wheel and it's fine immediately again. It's not a bleed issue, it's just a quirk of the brake. Are they current model slx?

 

[Ricker]

They are on a Giant Trance 2014 27.5 1. So I reckon they would be current.

Cant say they have ever done this before when doing wheelies or jumps, seems to be only happening recently. I have had like 5 large crashes on the bike so it may be i've done some damage to a brake line through a large tug, when the handlebars have been thrashed around in a crash?

Pads are fine, not worn much.

How easy is it to bleed the brakes? Whats the best place to get the mineral oil?

 

[moorey]

They are on a Giant Trance 2014 27.5 1. So I reckon they would be current.

Cant say they have ever done this before when doing wheelies or jumps, seems to be only happening recently. I have had like 5 large crashes on the bike so it may be i've done some damage to a brake line through a large tug, when the handlebars have been thrashed around in a crash?

Pads are fine, not worn much.

How easy is it to bleed the brakes? Whats the best place to get the mineral oil?

Bleed is very easy, but you need a $5 funnel. Get it all on crc. Free delivery at the moment. 1lt of oil is about $30.

 

[ride_29er]

Yes there is air in the line, the reason why it is only sometimes is that at the top of the shimano reservoir there is often an air bubble, it is harmless sitting there when the bike is level as there is enough depth and fluid in the rest of it to effectively work the system, however when you jump or do a wheelie the air bubble moves towards where the piston is so when you apply your brakes the air bubble is pushed down the line causing th brake fade you are feeling, however when the bike returns to normal the bubble moves back out of the way.
its a problem with most shimanos, what you can to is just top up the reservoir, open up the big bolt with a torc key (i cant remember the size off the top of my head) and fill it up with some more mineral oil, make sure the reservoir is totaly level whilst doing this to allow the air to escape. do not over fill though! if the liquids heats it can cause them to jam as it expands, also unless you have massive hands the lever take up position will move further out the more fluid you put it so its a bit of a fine art to find that happy medium but it can be done and i have done so with my SLX as well.
Good luck!

also before undoing the bolt and topping them up bounce the bike a bit to make sure all the air is at the top

 

[Ricker]

So crashed and mangled my finer. Been 4 weeks now since the doctors sewed it back up, and i've been getting back on the bike. Noticed that my brakes both seem to have gone soft and not bite nearly as hard. Does something happen to the pads or rotors if they just sit there unused for a long period? Currently I have to apply heaps more pressure than I ever had to to get them to lock up. The back won't lock unless given much to much force on the trigger….

 

[moorey]

So crashed and mangled my finer. Been 4 weeks now since the doctors sewed it back up, and i've been getting back on the bike. Noticed that my brakes both seem to have gone soft and not bite nearly as hard. Does something happen to the pads or rotors if they just sit there unused for a long period? Currently I have to apply heaps more pressure than I ever had to to get them to lock up. The back won't lock unless given much to much force on the trigger….

Shouldn't. When I dragged my old DH bike with saints out of the shed after 5 years untouched, and good as ever.

 

[Ricker]

I wonder if the crash got some air into the lines or resovouir. Maybe a huge twist in the handlebars occurred during the crash. Cant be sure, just remember losing a good chunk of my finger at the time.

 

[moorey]

I wonder if the crash got some air into the lines or resovouir. Maybe a huge twist in the handlebars occurred during the crash. Cant be sure, just remember losing a good chunk of my finger at the time.

Unless you find a leak, I doubt it.
However, if the bike has been upside down while you pump the levers, shimano can get ait in the lines of reservoir isn't chocka block. Ive seen people work on bikes upside down, pull levers, and wonder why they get air in the lines. easy mistake. Avid don't suck from the bottom (they just suck :high5), and don't have same issue, as I understand.

 

[si618]

How easy is it to bleed the brakes? Whats the best place to get the mineral oil?

Pretty easy, here's a video of the process. Check out his other videos as he does a decent job.

The funnel approach is a good improvement over the old models, and you might already have the yellow bleed block lying around, if not get one as they work well. 1L of shimano mineral oil will probably last you a lifetime, price on Wiggle and CRC is about the same.

One thing I'd recommend for SLX brakes is swapping over the split pins for these as found on XT(R) models. Remember to add a little blue loctite to the thread.

 

[moorey]

Are they threaded in the calliper to accommodate the pins?

 

[Ricker]

Yeah i will try fill the reservoirs again, but I only chocked them up to get rid of the air issue a couple of weeks before the stack. up to the stack everything was perfect, triggers were bitting very early on in the movement which was perfect. I'm gonna try clean the rotors and pads with alcohol tonight see if that helps. Can never figure out what material to use when dabbing the alcohol across the pads. Every cloth under the sun seems to stick to them.

 

[moorey]

Don't clean the pads, cook them :thumb:

 

[Ricker]

in the oven? what temp? how long

 

[moorey]

in the oven? what temp? how long

Stove top or BBQ. Very hot, pads up, cook until smoke and smell stops, allow to cool naturally. Good as new. I use saucepan and stove.
Acetone, or even better, isopropyl for rotors.

 

[Ricker]

Pads up meaning the metal non-friction part against the saucepan and the pad pointing up to the exposed air?

 

[moorey]

Pads up meaning the metal non-friction part against the saucepan and the pad pointing up to the exposed air?

metal backing plate down.

 

[si618]

Are they threaded in the calliper to accommodate the pins?

Yes.

I have XT and SLX brakes on different bikes, and the only difference I can see on the calliper is the split pin vs pad axle.

Aside from them being a pain to work with, at our last dig a few weeks ago we came across a rider who had managed to break a split pin and had it pop out, at a guess the pin metal was fatigued. I have had the pad axle come loose (hence the loctite tip), but the lock ring did its job.