Soft rear hydraulic brake lever on Trek Powerfly

[pagheca]

The rear hydraulic brake lever on my Trek bike (Shimano brakes) feels very soft and doesn’t build pressure. I’m guessing it might be due to low brake fluid in the system.

Any suggestions?
Can I just top it up with any brake oil, or does it need a full bleed and refill?
How to avoid air bubbles?
Or could this indicate a different problem altogether?

 

[pagheca]

What does @Greg Watts think about that?

 

[Greg Watts]

What does @Greg Watts think about that?

Soft lever that won't build pressure is almost certainly air in the system rather than low fluid. A top-up alone won't fix it - air is compressible in a way that fluid isn't, which is exactly what gives you that spongy, going-nowhere feel when you pull the lever.

You'll need a full bleed. Shimano brakes use mineral oil (not DOT fluid - important distinction, as DOT fluid is glycol-based and incompatible with Shimano systems). Don't use anything labelled DOT. Generic non-Shimano branded mineral oils are acceptable alternatives to Shimano-branded oil, so you have some flexibility there - however, Shimano's newer low viscosity oil for their latest brake models is a different spec and shouldn't be substituted without checking which your system requires. When in doubt, Shimano's own blue bottle is cheap and the right spec for most setups.

For the bleed itself: a Shimano bleed kit with the correct bleed nipple and funnel is about £15-20 and makes the job considerably less frustrating. The key to avoiding air bubbles is keeping the funnel full throughout, bleeding slowly from caliper upward, and tapping the hose gently to dislodge any stuck bubbles. It's genuinely not a difficult job once you've done it once - given your analytical approach to this stuff, you'd probably have it sorted in an hour.

One other thing worth checking before you start: inspect the caliper and lever for any visible fluid weeping. If there's a leak somewhere, you'll bleed it today and have the same problem in a month.

EDIT: Corrected the advice on mineral oil alternatives - generic non-Shimano branded mineral oils are acceptable substitutes for Shimano-branded oil, and DOT fluid is glycol-based and should never be used with Shimano brakes. Thanks @pagheca for the heads up.

 

[skizzian]

The rear hydraulic brake lever on my Trek bike (Shimano brakes) feels very soft and doesn’t build pressure. I’m guessing it might be due to low brake fluid in the system.

Any suggestions?
Can I just top it up with any brake oil, or does it need a full bleed and refill?
How to avoid air bubbles?
Or could this indicate a different problem altogether?

you could try burping it
but pushing a new lot of oil up from the bottom will give the most confidence and clear out any dirty oil.

if you're burping - you just need the oil cup screwed into the lever port (on the horizontal) with some fresh oil in.
then it's squeezing the lever until no bubbles appear.
I end up knocking the brake lines and tilting the bike at all sorts of silly angles to get the bubbles out.

Greg has it covered really

 

[pagheca]

thanks. Would you suggest it is better for me to go to a maintenance shop? I am quite pratical and experienced but I do not have much space to work here.

 

[skizzian]

thanks. Would you suggest it is better for me to go to a maintenance shop? I am quite pratical and experienced but I do not have much space to work here.

it's something you want to learn yourself really, you will be going back and forth a lot otherwise
the full bleed only really needs the space the bike takes up (and a towel underneath maybe)
the burp just needs the bike to be leaning to either side and rocking from the back to the front

 

[pagheca]

it's something you want to learn yourself really, you will be going back and forth a lot otherwise
the full bleed only really needs the space the bike takes up (and a towel underneath maybe)
the burp just needs the bike to be leaning to either side and rocking from the back to the front

just a question: should I look for EXACTLY the same oil or I may mix similar oils? It looks like the brake oil used by these brakes is not a DOT, but a Shimano one, but I am not fully sure about it (difficult to find out).

 

[skizzian]

just a question: should I look for EXACTLY the same oil or I may mix similar oils? It looks like the brake oil used by these brakes is not a DOT, but a Shimano one, but I am not fully sure about it (difficult to find out).

you can mix other mineral oils with the Shimano oil - of the same viscosity
Shimano have a new low viscosity oil for their latest and greatest - so not that stuff
some people do play around mixing different viscosities - but that's pretty advanced (and unnecessary)
might be worth getting a kit off Amazon?

 

[Pizzman]

Rebleed, the timeframe between a rebleed and your lever getting soft again will help you decide if a rebuild is necessary. Short = bad
Long timeframe = fixed

Brakes are cool, don’t neglect them.

If there’s no brake fluid loss , then air got in. Probably a seal.
Don’t mix fluids. Just rebleed the whole system with clean fluid. It will be a better result.

 

[skizzian]

Rebleed, the timeframe between a rebleed and your lever getting soft again will help you decide if a rebuild is necessary. Short = bad
Long timeframe = fixed

Brakes are cool, don’t neglect them.

If there’s no brake fluid loss , then air got in. Probably a seal.
Don’t mix fluids. Just rebleed the whole system with clean fluid. It will be a better result.

if he's just burping it - it'll be fine mixing I reckon
but - YES - for sure bleeding the system is definitely the best way to go
and isn't that much more work than burping

 

[pagheca]

you can mix other mineral oils with the Shimano oil - of the same viscosity
Shimano have a new low viscosity oil for their latest and greatest - so not that stuff
some people do play around mixing different viscosities - but that's pretty advanced (and unnecessary)
might be worth getting a kit off Amazon?
View attachment 182828

thanks. Actually I already ordered a kit. On amazon (it).

 

[Pizzman]

if he's just burping it - it'll be fine mixing I reckon
but - YES - for sure bleeding the system is definitely the best way to go
and isn't that much more work than burping

I’d have no problem riding mixed fluid to get by vs not ride.
But I’d get fresh stuff in ASAP.
If the bike is ridden a lot (high miles). A seal may be slightly worn. Don’t waste a bunch of money rebleeding. One rebleed and if the lever softens again. Rebuild.

 

[skizzian]

thanks. Actually I already ordered a kit. On amazon (it).

in that case it might be worth going for the full bleed as Pizzman suggests - get into a good habit now
that way you will push out all the old stuff, it will likely be more than just bubbles - dirty and cooked oil too.

https://volatacycles.com/changing-bicycle-brake-fluid/

edit: one thing to note - the hose that goes on the bleed nipple wants clamping, unless it's super tight.

 

[pagheca]

in that case it might be worth going for the full bleed as Pizzman suggests - get into a good habit now
that way you will push out all the old stuff, it will likely be more than just bubbles - dirty and cooked oil too.

https://volatacycles.com/changing-bicycle-brake-fluid/

edit: one thing to note - the hose that goes on the bleed nipple wants clamping, unless it's super tight.

actually that link is extremely interesting also because it says "Never use DOT fluid — destroys Shimano seals"... I will order Shimano Mineral oil then...

 

[skizzian]

actually that link is extremely interesting also because it says "Never use DOT fluid — destroys Shimano seals"... I will order Shimano Mineral oil then...

DOT will also destroy your paint job!

 

[Sophie777]

it's something you want to learn yourself really, you will be going back and forth a lot otherwise
the full bleed only really needs the space the bike takes up (and a towel underneath maybe)
the burp just needs the bike to be leaning to either side and rocking from the back to the front

Last year, the right-sided brake lever on my Trek Powerfly FS7 2019 compressed all the way to the bar. Too afraid to do it myself, I brought the bike to my local bike shop to "burp" the brake. That seemed to fix the brake.

Now, this year, new season, both the right and left brakes compress all the way to the bars. I thought this time I would try to "burp" both brakes on my own, but based on the video, I suspect that would be a wasted effort and a waste of time.

So back to the bike shop, but this time for a full bleed.

 

[skizzian]

actually that link is extremely interesting also because it says "Never use DOT fluid — destroys Shimano seals"... I will order Shimano Mineral oil then...

as Greg initially said - don't go near DOT - it's not a mineral oil - it's a glycol based brake fluid
non Shimano branded Generic oils for Shimano brakes will be fine
as you'll be replacing all the oil in the system if you're not burping and doing a full bleed instead
buy Shimano's oil if it gives you peace of mind, but if you've already bought some I'd say save your pennies

 

[pagheca]

as Greg initially said - don't go near DOT - it's not a mineral oil - it's a glycol based brake fluid
non Shimano branded Generic oils for Shimano brakes will be fine
as you'll be replacing all the oil in the system if you're not burping and doing a full bleed instead
buy Shimano's oil if it gives you peace of mind, but if you've already bought some I'd say save your pennies

Yeah, I am quickly learning about this. Thanks you all for the suggestions!

 

[pagheca]

Last year, the right-sided brake lever on my Trek Powerfly FS7 2019

Exactly the same emtb model here... and same problem.

 

[pagheca]

at the end I withdraw a previous order and ordered this one instead with metallic adapters and mineral oil:

I hope I got the right product...

 

[RustyIron]

thanks. Actually I already ordered a kit. On amazon (it).

Good. You'll want to do a FULL BLEED on both your brakes. Anything less is pure nonsense. When your car gets low on oil, do you just keep adding more oil? No. Periodically, you completely replace all the oil. It's the same thing with your brakes. The fluid gets dirty over time and the quality degrades.

 

[pagheca]

Good. You'll want to do a FULL BLEED on both your brakes. Anything less is pure nonsense. When your car gets low on oil, do you just keep adding more oil? No. Periodically, you completely replace all the oil. It's the same thing with your brakes. The fluid gets dirty over time and the quality degrades.

I think I am going for a "full bleed", but the comparison with the car engine oils is not appropriate. In that case there are multiple rotating parts and huge thermal shocks. Also a vehicle brakes system oil is different, because they are subject to a much higher amount of stress and mileage than a typical ebike/bike.

I will go for a full bleed on the rear brake just because I do not want to mix two different types of brake oils and because I want to check if there is anything wrong in the circuit.

 

[Pizzman]

I think I am going for a "full bleed", but the comparison with the car engine oils is not appropriate. In that case there are multiple rotating parts and huge thermal shocks. Also a vehicle brakes system oil is different, because they are subject to a much higher amount of stress and mileage than a typical ebike/bike.

I will go for a full bleed on the rear brake just because I do not want to mix two different types of brake oils and because I want to check if there is anything wrong in the circuit.

Expect the first bleed to make a mess.
By the 2nd-3rd time you'll have it.

Dont do the bleed 15min before your friends want to leave to ride.

Watch a few vids.
Give ample time to make a few mistakes.
Have lots of Isopropyl (99% is best, safe and no residue) and use it freely
Little brushes to get spilled fluid out of tiny recesses in cylinder/caliper.
Leave no fluid residue, even in the tiniest crevices, will collects dirts, and leads to seal failures.
Pretend the isopropyl is free. Keep your cigs unlit until complete.

Good luck.

 

[RustyIron]

but the comparison with the car engine oils is not appropriate.

In my defense, your initial question was pretty basic. My advice was created in a way that I thought would be understood by someone asking simple questions. I'm sorry I failed you.