Mountain Bike Reviews Forum banner
1 - 12 of 12 Posts

·
Boom bap ambassador
Joined
·
923 Posts
Discussion Starter · #1 ·
I have a new pair of Formula Cura 4 brakes and after shortening the hose and doing a bleed I noticed mineral oil (cadence is a drip, drip, drip and not a blast or jet) coming out of the silver circle in the photo below (courtesy of https://nsmb.com/articles/formula-cura-brakes-teardown/)

Yellow Bicycle accessory Bicycle part White Steering part


This only happens if I pushed or pulled mineral oil up from the caliper at a certain pressure (not hulk smash pressure but from time to time) and also when i pushed the pistons in the caliper back to seat the pads to center the caliper.

I have had Shimano Zee and several sets of Magura brakes and since this is my first Formula set I'm not sure if this is normal/expected or not.

The silver disk has a hole in it so I would assume that this is going to be the case to keep some idiot like me from blowing the master cylinder seal.

Finger Nail Thumb Revolver Circle


Any Formula gurus that can weigh in on this?
 

·
Registered
Joined
·
97 Posts
I don't have 1st hand experience with Curas specifically. But from my general understanding of how hydraulic disc brakes work there is no such thing as a blow off valve. But they do use rubber diaphragms that can change shape under pressure to allow the total volume of the closed hydraulic system to increase slightly to accommodate fluid expansion caused by heat build up.

Some googling has revealed that Formula prefer to keep detailed service manuals to themselves. The only remotely useful info on the internals I can find is the NSMB article you linked to above (specifically the pic I reproduced below), and an exploded diagram of the MC that some online shops seem to use instead of an actual photo, also included below. The diagram did not include a legend for the various parts shown.

But I think even this limited info confirms what I said above. Part (18) in the exploded diagram is the black long part in the photo. This looks like the rubber diaphragm I mentioned above. The silver disc (17) just screws in to hold the diaphragm in place and seal off the system. I believe the pinhole in the disc is to simply allow the diaphragm to expand and contract without creating air pressure build up or vacuum between the silver cap and the "outer" surface of the diaphragm ("outer" with respect to the inner hydraulic circuit that is).

So, given all this, I can think of 3 reasons why you are seeing oil drip through the silver cap pinhole:

1. The silver cap is not fully tightened, so when you create pressure with the caliper syringe, with the lever in non-depressed position, this pressure reaches the diaphragm and pushes the oil around it and out, through the silver cap pinhole. If the cap appears to be fully tightened, I would still take it out, make sure the diaphragm was positioned correctly and free of any debris that might have compromised the seal, then reinstall the cap.

2. You have a tear or a hole in the diaphragm, so any oil that pushes against it from the inside leaks out. You would need to unscrew the cap, and take out the diaphragm to inspect it / replace it.

3. During the bleed, you dripped some oil onto the silver cap. The oil then drained through the pinhole and collected just under the cap and sat on top of the "outer" surface of the diaphragm. Then when you pressurised the caliper syringe, the diaphragm compressed and squeezed that "external" oil out. If this is the case - no big deal, everything is fine. However, if the dripping is constant and never ending as long as you are pushing on the caliper syringe, then #1 or #2 are the more likely scenarios.

Hope this helps. Please let me know how you go with this - keen to know if any of my theories turned out to be true.

Line Line art Artwork Black-and-white Drawing


=

Metal Silver Tool Hand tool Household hardware


=
 

·
Boom bap ambassador
Joined
·
923 Posts
Discussion Starter · #3 ·
I suspect 1 as I can't come up with a reason why the diaphragm would have a hole other than a manufacturing defect.

For 3 I did a check of everything as my first thought was that I did not have the hose connector tightened down properly.

It was good, the bleed fitting was good, and I checked the tightness of the bleed fitting on the other side of the lever (flip flop lever design so bleed port on both sides).

Will update with what I find out, I appreciate the response for sure.
 

·
Registered
Joined
·
9,754 Posts
I recall hearing about some Shimano brakes that had relief valves in the reservoir in that same area. You could force fluid out through the valve when bleeding, and they'd be fine normally, but sometimes would start leaking and you'd need to replace the levers. I don't know if any current brakes do. The rationale would be that if something overheats or expands, it'd let the fluid out and prevent the brake from locking up??? IDK.

In that parts diagram, 18 is the reservoir bladder and 17 is the cap, which needs to have a hole to let air out as the bladder is compressed by fluid in the reservoir. There doesn't appear to be any mechanism that would serve as a pressure relief valve, though it is conceivable something could be molded into the bladder itself.

If the bladder was leaking for some reason, that hole is where the fluid would come.
 

·
Boom bap ambassador
Joined
·
923 Posts
Discussion Starter · #7 ·
I just got back from the garage.

I drained off the brake to keep mess to a minimum and pulled the bladder cap and bladder from the problem brake.

The bladder cap seemed loose compared to what I would expect - not so loose that you would say 'oh, problem solved' but not as tight as I would expect either.

What I found is fluid inside the bladder itself so I drained it, dried it, and did a physical inspection followed by a leak test which all turned out fine.

The bladder has a lip on the top edge where it meets the cap and there is a lip inside of the master cylinder itself where the bladder sits.

To make sure I wasn't tripping on a jelly bean high I looked at the other brake and the silver cap was more difficult to loosen compared to the problem brake and the bladder was properly seated which confirmed what I was seeing I think.

In looking at everything when I pulled the cap off the bladder looked to me to be pushed down past this lip a bit which would cause fluid to get past the bladder and cause the issue.

The bladder shape itself is like a pyramid in shape (think three fins) and I made sure to orientate the bladder so that the fins were not in the way of the bleed screw ports on the lever body.

I sealed everything up and performed a bleed and no issues as far as having liquid come out of the silver cap.

I suspect here that the bladder wasn't seated properly at the factory somehow and the issue wouldn't really manifest itself until you did a bleed as I couldn't get liquid to come out with normal squeezing of the lever - just during a bleed or pushing the pistons back.

I would say right now that case closed and I really appreciate the feedback and insight to help diagnose this issue.
 

·
Registered
Joined
·
95 Posts
Tonight I installed my Cura 4 and after cutting the lines I started bleeding. Same issue occurred. No matter how gently I pushed the syringe, it still sipped. Once I pushed all the bubbles out,I just closed the bleeding port and left it as it is. Surprisingly, the lever felt firm. I'm afraid that I'll have to take it apart as well.

Sent from my Pixel 3 using Tapatalk
 

·
Registered
Joined
·
9,754 Posts
Hmmm. Sounds like a leaking bladder. Oil may gradually leak out and and air into the reservoir. It shouldn't cause an issue until the air gets beyond the reservoir.
 

·
Registered
Joined
·
95 Posts
Hmmm. Sounds like a leaking bladder. Oil may gradually leak out and and air into the reservoir. It shouldn't cause an issue until the air gets beyond the reservoir.
Can confirm that the bladder was torn near the lip and that caused bleeding issues. Will see how good their customer service is and will report back here

Sent from my Pixel 3 using Tapatalk
 
1 - 12 of 12 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top