Rear brake lost all pressure

[Albike]

The SRAM db8 rear brake on my Fezzari has been slowly losing hydraulic pressure over the past few weeks and today there was no pressure at all. Lever goes right to the handlebar with no brake force at all.
I realize that this would indicate a loss of fluid and air now in the system.
I see no sign of fluid leakage and no obvious external damage to the components.
Any advice on what and how to check the system.
I have a sram bleeder kit on the way but will have to discover the source of the problem first to be sure that it won’t continue to happen.

 

[fooferdoggie]

The SRAM db8 rear brake on my Fezzari has been slowly losing hydraulic pressure over the past few weeks and today there was no pressure at all. Lever goes right to the handlebar with no brake force at all.
I realize that this would indicate a loss of fluid and air now in the system.
I see no sign of fluid leakage and no obvious external damage to the components.
Any advice on what and how to check the system.
I have a sram bleeder kit on the way but will have to discover the source of the problem first to be sure that it won’t continue to happen.

it could just be contamination too. overheating will cause it too. Sometimes flipping the bike over will too.

 

[il Munduato]

The SRAM db8 rear brake on my Fezzari has been slowly losing hydraulic pressure over the past few weeks and today there was no pressure at all. Lever goes right to the handlebar with no brake force at all.
I realize that this would indicate a loss of fluid and air now in the system.
I see no sign of fluid leakage and no obvious external damage to the components.
Any advice on what and how to check the system.
I have a sram bleeder kit on the way but will have to discover the source of the problem first to be sure that it won’t continue to happen.

Somewhat common on the rear brake, especially if not bled properly at install.
I would do a "Lever Bleed" first as it most likely just needs to be topped off.

Here's another video.. pretty standard process across manufacture.

Raising the front of the bike and tapping the brake line can help for stubborn bubbles. As well as removing the caliper and letting it dangle low.
Just imagine an easy path for air bubbles to escape

 

[Albike]

If I pump the lever quickly and many times rear brake does come back but loses the pressure if I stop pumping

 

[gromike]

If you haven't already bled the brake (correctly) it could be the hydraulic circuit is compromised, i.e. the piston seals are bad, etc.

I bought my son a new Adventon Pace 500 years ago. Could never get the rear brake to work right. One day I had the bike upside down and noticed the inboard brake pad was missing. That was on a new DTC bike.

 

[PCeBiker]

If I pump the lever quickly and many times rear brake does come back but loses the pressure if I stop pumping

If you pump up your lever and hold it on, and the lever slowly loses pressure then hits the handlebar, that would suggest leaking seals to me.

Rapidly pumping the brake lever can compress a bubble, but it will hold the pressure unless there is a leak in the system. (You'll have a spongy lever though)
A leaking piston seal on the brake lever will leak the fluid back into the reservoir.
A leaking caliper piston seal will leak fluid out of the caliper.

 

[Albike]

As stated, I see no fluid leaking at the caliper so that would indicate leaking from the lever.
Are the db8’s rebuildable? I’m still waiting for the bleed kit to arrive.

 

[il Munduato]

As stated, I see no fluid leaking at the caliper so that would indicate leaking from the lever.
Are the db8’s rebuildable? I’m still waiting for the bleed kit to arrive.

How old is the system?

 

[Albike]

The bike is a 2022 with about 150 miles on it. I got it used Witt 27 miles on it and I ride it regularly so it doesn’t sit unridden for very long

 

[il Munduato]

2022 isn't brand new but 150 mi isn't much at all.
I think I would still start with a lever bleed since you aren't showing any signs of a leak.
It's 30 minutes and a few ml of fluid at worst.

Eliminate the common and easy first.

 

[Albike]

I’m unfamiliar with a lever bleed.
I have done a complete system bleed on my old MTB with shimano gear.
Could anyone point me to a how to for it?

 

[PCeBiker]

There's 2 links to YouTube videos in post #3

A lever bleed is done the same basic way for all manufacturers.

 

[Albike]

Got it, thanks.