Brake bleed fail

[toerag69]

Tried bleeding my brakes and the brake fluid came out from the piston…

I pushed the pistons back.
I have put a 4 piston brake block in my 4 piston brake.

From the picture does it look like I need a thicker brake block? There is a gap between piston and block potentially causing leak?

Thanks

 

[mooby77]

I don't think this is a major problem , you need to centralise the pistons , I assume none of the pistons extended out past the seal and you lost oil ? did you check the seals on the pistons for damage.
This is a good guide , not just for Hope brakes but general

https://www.hopetech.com/_repository/1/documents/page-25-centralising.pdf

 

[Mikerb]

Tried bleeding my brakes and the brake fluid came out from the piston…

I pushed the pistons back.
I have put a 4 piston brake block in my 4 piston brake.

From the picture does it look like I need a thicker brake block? There is a gap between piston and block potentially causing leak?

Thanks

View attachment 161914

the pistons move out under normal conditions with the brake disc in place...ad that is considerably thinner than the brake block you are using. So if fluid is escaping from a piston with that brake block in place you have a piston seal problem. that may be due to dirt on one of the pistons if you did not clean them before pushing them back into the calliper. I suggest you remove that brake block and then clean the pistons by extending each in turn and cleaning with IPA. Then push each piston back into the calliper.....use your brake block and try again.

 

[Stihldog]

It’s a good idea to insert the brake/pad block during transport, if you need to remove the wheel. The lever can accidentally be depressed …thus popping out the piston (s).

There is also a block that allows only one piston to extend for cleaning. Removing the bleed screw sometimes helps. Clean with iso, a rag and Q-tips. Don’t get any mineral oil on the pads or rotor.

Once you’re satisfied that everything is clean, apply a very small amount of mineral oil around the pistons with a Q-tip. Don’t contaminate your pads or rotor.

 

[Mikerb]

It’s a good idea to insert the brake/pad block during transport, if you need to remove the wheel. The lever can accidentally be depressed …thus popping out the piston (s).

There is also a block that allows only one piston to extend for cleaning. Removing the bleed screw sometimes helps. Clean with iso, a rag and Q-tips. Don’t get any mineral oil on the pads or rotor.

Once you’re satisfied that everything is clean, apply a very small amount of mineral oil around the pistons with a Q-tip. Don’t contaminate your pads or rotor.

View attachment 161927

Yep...that's the process I use. During normal use the calliper pistons sit very close to either side of the rotor meaning a portion of each piston is exposed to dust mud etc. That applies to both front and rear brakes but more so on the rear calliper since it is most often fixed horizontally on the seat stay and the top of the calliper is open. The front calliper is better protected being mounted nearly vertical and behind the fork leg.
The dirty section of the piston does not go back into the calliper until/ unless it is pushed back in order to insert a full brake block .........and that is when that dust/mud etc can foul the piston seal. So pistons must be cleaned before doing that.
The reason why pistons should be pushed back into the calliper in order to carry out a bleed is that (unlike most cars) mtb brakes are a sealed system and the correct volume of dot/mineral oil is set with the pistons fully retracted.
For travel with the wheel removed you only need to insert the thin brake block which is roughly the same width as the rotor.

 

[Mikerb]

Ps.....the new Maven brake callipers provide much better protection for the pistons and pads......the pads are inserted/ removed from below the calliper.

 

[BushLevo]

I would suggest not putting mineral oil anywhere near pistons or caliper internals...it will destroy piston seals. Clean with alcohol and lubricate with brake fluid only before reassembly.

 

[Stihldog]

I would suggest not putting mineral oil anywhere near pistons or caliper internals...it will destroy piston seals. Clean with alcohol and lubricate with brake fluid only before reassembly.

What kind of brake fluid would you use? DOT fluid or the specific brake fluid for that particular brand of brake system? Will certain types of brake fluids damage seals and O rings in different brake systems? …even when cleaning and re-lubricating pistons that were cleaned by iso or brake cleaner?

 

[BushLevo]

What kind of brake fluid would you use? DOT fluid or the specific brake fluid for that particular brand of brake system? Will certain types of brake fluids damage seals and O rings in different brake systems?

My LEVO uses DOT 5.1 as I think most MTBs do these days. DOT 5.1 is a glycol-based fluid compatible with DOT3, and 4 (apart from temperature rating). The standout is DOT 5 which is a silicone-based fluid and must not be mixed with the others. If a DOT 5 system was thoroughly cleaned and then DOT 5.1 used, (or vice-versa) I'm not sure if seals and hoses would be o.k. My guess is not. More info here: DOT 5 vs DOT 5.1 Brake Fluid: What’s Best for Your Brakes? - AskMyAuto.com

 

[Svarun]

Majority of MTB brakes use mineral oil these days. Even SRAM is switching from DOT to mineral oil, as evidenced by their latest models like Maven and Motive.

 

[irie]

I would suggest not putting mineral oil anywhere near pistons or caliper internals...it will destroy piston seals. Clean with alcohol and lubricate with brake fluid only before reassembly.

IIRC Shimano recommends the use of mineral brake fluid oil with its 4 piston MTB brakes so are you saying that this is wrong?

 

[Stihldog]

It’s my understanding that brake fluid’s (Dot or Mineral) also have different boiling points. Dot fluids are heavily regulated …mineral fluids …not so much.

I use the recommended brake fluid. The boiling points and additives can vary between fluid brands. The warranty on your brake system could be void if you decide to mix things up…so I don’t mix things up.

Dot fluid is good but there are pros and cons. Shelf life, moisture absorption, bad for paint. I’ve used Magura Royal Blood as a facial moisturizer during a full bleed before …when I pressed too hard on the syringe and caused a mini-explosion …good times .

Anyways, I prefer not to use Dot fluid brake systems. In fact, if I could store that little bottle of Dot fluid in a secured bunker, behind a safe, 50’ below my home, I would, but a family member needs it.