bleeding Tektro m285 brakes (why is there a pin hole?)

[tk_MTB]

 

Hi there,

I have been having trouble keeping pressure in my brakes.  I will bleed them, and they will work for an hour or so then I go to ride and they are spongey.  I’ll top them off make sure all the air is out, again, and go ride.  then the bike will sit for a day or two and the brakes have no pressure at all.  I’m very confused about what is happening.

I noticed that there is a pinhole in the cover for the reservoir (picture below).  Does anyone know why it is there? I would assume that a brake system should be a closed system to work properly.  I have noticed that oil will seep out of this hole sometimes.  Did I do something wrong?

Thanks in advance for any thoughts/tips. I have been dealing with this for a few weeks and got tired of trying to troubleshoot this on my own.

[Sunspot ✓ supporter]

The pin hole is a vent for the the bladder gasket to breathe. The bladder gasket is there to separate the outer space from the fluid side in the reservoir and prevent fluid leaking out. The bladder will expand into the fluid space gradually as the brake pads and rotors wear .

What may be happening is air is leaking past a seal. While air can sometimes bypass a seal, it might not leak fluid due to its viscosity or going under pressure expanding the seal and closing the breach. If the master cylinder doesn’t work properly and causes a vacuum in the system that can pull air in through the piston seals. Again no fluid loss to indicate what is going on. Might need to overhaul the master cylinder to be sure there is no dirt or debris blocking the feed port or return port. The return port is the smaller of the two ports in the bottom of the reservoir.

[tk_MTB]

Thanks for the reply.

Never would have guessed that the bladder moved with wear to fill in the gaps.  makes sense when you say it though.

I will take a look at the master cylinder this week and see what I can find.

[Sunspot ✓ supporter]

With wear causing the caliper piston to protrude further to keep the pads in place, the fluid will drop in the master as it now fills the caliper. With that said, the bladder will follow the fluid level in the master.

[tk_MTB]

So I was about to work on the bike and reread your first comment.

I do have fluid loss when I go to ride, after bleeding.  I have topped it off right before my ride and had the brakes work fine during my ride.  I have also had spongey levers then drive with my bike on my hitch and have the brakes work better when I’m done driving.

Does it still sound like the master cylinder? I am planning to check it today anyway. Im just trying to learn as much as possible.

Thanks again!

[Sunspot ✓ supporter]

Does the spongy return later? First place to check would be the master since debris can cause intermittent issues.

[tk_MTB]

Yea the sponginess comes back after the ride, when the bike sits.

So the debris in the master cylinder can cause this?

Guess i’ll be cleaning that this evening.  would this need to be completely disassembled? or could a simple clean by pushing the cylinders out a bit. ( just wiping the cylinder with alcohol and re-lubing.)

[tk_MTB]

So. The oil is leaking from the front of the reservoir. Right under the cap. I looked over the bladder didn’t seem to be worn or ripped in anyway. I tightened the screws that hold the cap/bladder on.  I don’t understand why it would be leaking out of the front so bad.

[Sunspot ✓ supporter]

Faulty bladder/gasket and or cover warped… Also check the hose connection for leaks. Key to hydraulic lines is not to ever tighten. Solving a connection leak can sometimes be done by simply loosening and tightening the connection hardware a couple times to shape the metal parts and seat them so they no longer leak.

Worse case, replace master cylinder.

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