Topping up / Bleeding Shimano 525 brakes

noppy

Hi,

im just servicing my new used bike.

The brakes were a bit soft (pads still 2mm) so i undid the top of the reservoir (using Shimano's PDF) on how to do it properly horizontal etc.

Anyways when i took the top off the fluid was only half to 3/4 of the way up the reservoir?

and when i compressed the brakes a few times small air bubbles came up out of the piston bit.

So i have a few questions.

1- Shall i bleed them and start from scratch?
2- Shall i top them up?
3- Can i use Cyclo or juice lubes fluid or is shimano best ?

Thanks

supersonic

Is the reservoir meant to be filled to the top? Many aren't - the fluid expands into the res when the oil heats, and naturally drops at the pads wear and auto adjusts. Push the pistons right back and see what happens.

Bubbles are never good, may indicate that a bleed is needed anyway.

Any mineral oil will work really, but Shimano will naturally say their's... LMH fluid works fine.

noppy

thanks supersonic.

in the PDF it says fill to overflowing and secure lid so im assuming it is ,

This is why i think bubbles are in there.

are bubbles the only thing that make brakes spongy ?

Thanks

supersonic

If the bubbles are in the lines then there will be problems. Spongeyness and I would certainly bleed them.

The reservoir should only be filled to the top when the pistons are fully retracted.

noppy

Thanks,

is there an easy way to retract them a trick or just a flat head screw driver and gently push?

il let you know how i get on

supersonic

Gently push Actually they can be quite stubborn, so don't be afraid to give them some force.

Cover the driver with a cloth though.

noppy

right well i topped up both and spent 10-15 mins squeezing and relaxing whilst flicking hoses.

the front brake now works a treat.

however back is squeeling and needs 3x the force of the front to actually stop it spinning completely

supersonic

Make sure the rear pads and rotor anre not contaminated.

noppy

what am i looking for ?

and how do i clean it off?

sorry for so many questions im new to all this

this is quite an upgrade from my last bike in all aspects

supersonic

Any oily residue on the pads or rotor. If this is the case, clean the rotor in alcohol (IPA) and replace the pads.

kevolution

supersonic wrote:

Is the reservoir meant to be filled to the top? Many aren't

What brake system recommends you don't fill the reservoir as air is a constituent part of the system?

How many brake systems aren't meant to be air free?

All hydraulic or DOT based open brake systems are designed to allow for expansion, that's why they have reservoirs with diaphragms to separate the air from the fluid in the system with a seal that allows for fluid expansion via a breather hole in the master cylinder reservoir cap.

Closed systems have adjusters and don't need to be fully filled if you're going to open the brake up to bleed it, but most have bleed ports these days, so you you can pressure bleed out the air for a more reliable result.

noppy

supersonic wrote:

Any oily residue on the pads or rotor. If this is the case, clean the rotor in alcohol (IPA) and replace the pads.

right well i kinda cheated i put a sheet of sandpaper in the slot and push back and forth a couple times both sides

the sand paper came out grubby and dirty oil and what not on it

tried the brakes better but not quite there, il get some IPA and give the pads another sand paper to make sure all the rubbish is off

supersonic

kevolution wrote:

supersonic wrote:

Is the reservoir meant to be filled to the top? Many aren't

What brake system recommends you don't fill the reservoir as air is a constituent part of the system?

How many brake systems aren't meant to be air free?

All hydraulic or DOT based open brake systems are designed to allow for expansion, that's why they have reservoirs with diaphragms to separate the air from the fluid in the system with a seal that allows for fluid expansion via a breather hole in the master cylinder reservoir cap.

Closed systems have adjusters and don't need to be fully filled if you're going to open the brake up to bleed it, but most have bleed ports these days, so you you can pressure bleed out the air for a more reliable result.

You have misinterpreted what I am trying to say and have not quoted the rest of that post.

If the pads are nearly worn, and you then fill the reservoir to the brim, then refit the diaphragm, what happens when you change pads and push the pistons back? The diaghragm expands with the fluid then what happens if they over heat? The levers push out.

The diaphragm displaces fluid when fitted ie spilled out the top, so the reservoir is NOT actually filled to the top when done, if we are being pedantic . Must have the air gap outside the diaphragm as you say. You can fit them if careful without much spillage without introducing air. Old maguras for example and many more. Though I think we are saying the same thing but in a different way.

Always be careful when topping reservoirs up. Always push the pistons back first.