Hydro front brake not as effective as the rear.

homers_double

I have Shimano hydros, around Ultegra spec, possibly BR R785 (cant recall exactly)

The rear works ok and the pistons travel well however the front of late is quite lacking. It seems as though the pistons may not be actuating fully and possibly resulting in the pads being glazed by a constant (but slight) rub on the disc.

Usually the brake fluid itself lubricates the piston and these self centre with sufficient clearance but is there any other way to give the disc a little more space to strop the rub?

homers_double

Just found a BR "how to" on cleaning and lubricating the pistons with silicone grease rather than the brake fluid itself. Will investigate...

photonic69

homers double wrote:

Just found a BR "how to" on cleaning and lubricating the pistons with silicone grease rather than the brake fluid itself. Will investigate...

Can you post the link please as I'd be interested in this too?

homers_double

https://www.bikeradar.com/advice/worksh ... ers-video/

photonic69

homers double wrote:

https://www.bikeradar.com/advice/workshop/how-to-service-hydraulic-disc-brake-calipers-video/

Thanks muchly. I’ll have a look at it tonight

veronese68

Hopefully that will work. If not, how worn are the pads? I don't think this will be the problem but in cars, especially those with single piston calipers, as the pads wear things tend to stick a little more and can cause such problems. New pads might push the pistons back a smidge and they work better.
This could all be complete ballcocks of course.

itboffin

You might have a few air bubbles have you tried a rubber band or cable tie gently over the lever slack left overnight?

homers_double

Veronese68 wrote:

Hopefully that will work. If not, how worn are the pads?

Brand new...

Grease should arrive today courtesy of Amazon Prime, I thought I had some disc cleaner but no. I'll have to order some of that too.

itboffin wrote:

You might have a few air bubbles have you tried a rubber band or cable tie gently over the lever slack left overnight?

Which way will that push the bubbles? Sounds stupid but on MTB brakes the reservoir was quite easy to access and any bubbles would be trapped in the ribbed rubber at the lever end. Not so simple on the road version as it needs a tiny funnel to drip fluid in and any air is expelled at the caliper.

pilot_pete

Have you actually tried bleeding the front brake? If anything it is usual to find the front brake has more feel and efficiency due to the shorter hose run rather than your situation where the rear feels better.

It could be that you have a bit too much fluid in the system (I had this once which I think stems from bleeding the brakes without the pistons fully retracted). This can prevent them from retracting during subsequent use.
So, Start by removing the bleed screw at the lever and then prying the pistons apart fully using the appropriate tool or a soft lever (be careful as the pistons are easy to crack if you use a screwdriver or sharp edged blade). You may get some fluid seep out the port so be ready to mop up.

Once bled properly (expelling the air from the reservoir and the caliper) you should be able to see if the pistons are moving freely with the wheel off and the pads out - this may require another bleed afterwards if you pop a piston out too far, just take it really slowly without a spacer in between the pistons. If all is well refit the pads and wheel and give it a go.

If you haven’t already, adjust the lever for reach and biting point. Details are in the associated Dealer Manual. I am assuming you will fit new pads during this process as you say yours are worn/ glazed.

If that doesn’t sort it then you may be down to a sticking piston. The linked article describes the cleaning/ remedy for that, specifically holding the free moving piston retracted with a clamp and using hydraulic pressure to free the other one off for cleaning. I’ve done this in the past and got a piston working properly again.

PP

homers_double

My thoughts exactly, usually the front is sharper than the rear.

I bled them properly, been doing discs for ages on mountain bikes but when I couldn't get it perfect popped in to the bike shop and they had a go and couldn't get it much better.

I'll try your tip and bleed them again. Thank you.

pilot_pete

I’d remove the pads and the bleed port screw first then and try retracting the pads fully and then replace the bleed port screw. Now see if the pads are moving together freely and retracting properly. If not, clamp the free one to free off the other then follow your piston cleaning article. If that doesn’t work it could be a bit of trapped air, but if you are bleeding using correct technique (to remove air from the reservoir/ lever and the caliper) then perhaps flushing through with new fluid could help if there is a tiny blockage, but may not. Other than that it sounds like a problem with the piston/ caliper which may necessitate a new caliper, which if RS785s are about £40.

Good luck.

PP

step83

I'd go with Petes flushing with fluid. Otherwise is caliper winding correctly as in level and not at an angle? May need to slap the travel blocks in (pink skinny ones) an wind the piston back to test.
Could be debris caught around the piston, or a sticky piston which you have grease coming for. Not familiar with these brakes but its not something as simple as the pad spring or pin/screw being damaged or just not in right?

homers_double

Nothing is wrong with the assembly itself and the pistons do move, they just dont seem to retract enough.

A good clean and bleed will be the first thing to do.

Thanks for the input fellers

lesfirth

homers double wrote:

Nothing is wrong with the assembly itself and the pistons do move, they just dont seem to retract enough.

A good clean and bleed will be the first thing to do.

Thanks for the input fellers

Air in the system will only affect the travel of your brake lever and the springy feeling of the lever. As long as the brake lever does not travel so that the lever reaches your handlebars ,no amount of air in the system will impair your braking efficiency.