Brakes not building pressure

**bwringer** · 10-09-2022, 08:14 AM

I suppose this particular rider is happy now, but one other thing to mention for future reference is "bench bleeding"; there is often air trapped in the master cylinder that needs to be burped out.

On a motorcycle, this is easy to do by moving the handlebar so the MC is more or less level, removing the brake lever and brake line, holding a finger over the brake line hole (acting as sort of a fleshy one-way valve) and slowly pushing the piston ALL the way in using something blunt that won't damage the piston or bore. Go slow or you'll get a jet of brake fluid in the face. Obviously, use lots of paper towels and protect the paint and instruments.

You can also bench bleed like this by arranging a piece of tubing such that it leads from the brake line port back up to the reservoir. You can kind of screw the plastic tubing into the port with a little experimentation. Using a finger is messier but a little faster.

Sometimes when chasing that last stubborn air bubble it helps to crack the banjo bolt loose to allow air to escape; push all the way in, loosen the banjo (keeping an absorbent rag around the fitting to catch the brake fluid), then re-tighten before allowing the piston to return.

**GabrielGoes** · 10-13-2022, 12:03 AM

Back to square one, the master I originally used to build pressure didn't have a mirror perch. The pressure wasn't the greatest but it was definitely there. I switched to another one and had no luck.

Fast forward to today, I put a brand new (in the package) brembo master cylinder for a bmw 310r which i had laying around, on and it doesn't build ANY pressure whatsoever and there's no air in the lines. Is it possible the master piston is too small to push the fluid efficiently to the caliper pistons??

this is absolutely maddening. I ordered new caliper piston seals and new hoses on the way as well.

Edit: going to try bwringers bleeding method above for good measure

**Suzukian** · 10-13-2022, 10:28 PM

I had bad brake response on my '83 GS750ES once, and I found that the slider pins were dry and binding. Just taking the calipers off, cleaning the holes out with a brass wire brush, and copious amounts of high temperature brake great, and the bike stopped right on the dime again. This has happened 3 times on this bike, but at that time I drove it everyday for commuting for 7 years.

**GabrielGoes** · 10-14-2022, 09:14 PM

-Stainless lines (can breathe through them fine)
-literally brand new brembo master
-bench bled the new master as stated above method (nice tip)
-slide pins are greased!
-vacuum pumped the lines and also did it the traditional way to make sure there's no air.
-bled the master at the banjo fitting as well
-caliper pistons are shiny - no pitting and looked good last I checked...

Im certainly not a brake phenom, but this is definitely not my first rodeo either... but my goose is cooked right now. This is crazy

There is absolutely ZERO pressure on the lever at the moment. I have my caliper rebuild kits coming in soon, maybe the grease I used on the seals compromised them? Or maybe the seals are just shot in general.

I will give an update after I install the new caliper rebuild kit with new piston seals..

I also re readr everyones post and ACTUALLY went through it all again from a clean start, not just saying "I did that already"...

Any other tips much appreciated if anything comes to mind.. my money is on the piston seals being bad...

**GabrielGoes** · 10-15-2022, 01:09 AM

To my demise, I took one of the calipers off to move it around and check to see if that would bring any air bubbles, sure enough, the seal was leaking, I lifted the boot and saw it pooling. The other caliper the same. After disassembling them both while waiting on my new seals - I can confirm what the gentleman said earlier about grease not being a good idea, it left the most hideous residue behind that made it look all goopy and brown, I don't think that was the issue directly at hand but I will definitely never use grease again- going to use brake fluid like I always did.

I'm not out of the woods yet so I will follow up when I install the new seals!

**GabrielGoes** · 10-20-2022, 12:11 AM

After carefully replacing the caliper piston seals the issue has been resolved. Thank you all!

**bwringer** · 10-20-2022, 11:28 AM

Thanks for the follow-up!

Too often, we never hear the end of the story and so only one person learns something.

What do you think was the root of the issue? I believe you were suspicious of the grease, and I'd have to agree that anything but brake fluid in the system is a big no-no.

**alke46** · 10-20-2022, 05:54 PM

Originally posted by bwringer

Thanks for the follow-up!

Too often, we never hear the end of the story and so only one person learns something.

What do you think was the root of the issue? I believe you were suspicious of the grease, and I'd have to agree that anything but brake fluid in the system is a big no-no.

I totally agree with this statement, but when I had to replace some caliper parts from a Brembo supplier, he stated I should use the "Brembo" paste that goes with the rubber seals.
I told him I would stick with the proven brake fluid.

**Suzukian** · 10-22-2022, 04:04 PM

Great resolution!

**GabrielGoes** · 10-23-2022, 12:21 AM

Originally posted by bwringer

Thanks for the follow-up!

Too often, we never hear the end of the story and so only one person learns something.

What do you think was the root of the issue? I believe you were suspicious of the grease, and I'd have to agree that anything but brake fluid in the system is a big no-no.

-the piston seals being compromised
-I went in so many circles with this darn job i have to say there were probably many factors to attribute to the issue, master cylinder and seals probably the biggest.

And yes, the grease solidified and became really nasty goopy solidified brown stuff possibly compromising the hydraulic seal.

**Runningdog** · 10-23-2022, 11:56 AM

This is a good tutorial for anybody doing brakes......good read.

**KiwiAlfa156** · 10-26-2022, 04:57 AM

Originally posted by GabrielGoes

-the piston seals being compromised
-I went in so many circles with this darn job i have to say there were probably many factors to attribute to the issue, master cylinder and seals probably the biggest.

And yes, the grease solidified and became really nasty goopy solidified brown stuff possibly compromising the hydraulic seal.

Red rubber grease (which is non-mineral based) doesn't damage caliper seals and is compatible with DOT3/4 and 5.1 brake fluid. And is readily available.
It took me ages to discover that sticking calipers seals caused excessive lever travel, because we all 'know' that spongy brakes 'must be' air in the lines. Irrespective of the fact I knew I had bled the brakes correctly. It was actually reading a post on another forum that helped me solve the problem.

**GabrielGoes** · 10-26-2022, 08:45 PM

Originally posted by KiwiAlfa156

Red rubber grease (which is non-mineral based) doesn't damage caliper seals and is compatible with DOT3/4 and 5.1 brake fluid. And is readily available.
It took me ages to discover that sticking calipers seals caused excessive lever travel, because we all 'know' that spongy brakes 'must be' air in the lines. Irrespective of the fact I knew I had bled the brakes correctly. It was actually reading a post on another forum that helped me solve the problem.

Yeah I saw that red rubber grease offered in some brake kits. I'll probably still stay away considering I'm already going to get messy with dot fluid anyways and that's what all the old timers told me to use lol.

I will have to experiment how that red grease reacts to brake fluid in a controlled environment - If it immediately dissipates than maybe i can trust it...

Also as another follow up my lever got soft again lol, this time i noticed it looking wet around some of the banjo crush washers.... of course...

I replaced every single one with a new copper one. the old ones were silver colored - i am assuming aluminum.

Zip tied my lever overnight - its been 2 days and so far it is remaining very nice !

I also noticed the left caliper had a mini pool of brake fluid fountain sitting in the tip of the bleeder screw, I'm going to replace that thing too - luckily they sell those bleeders at autozone

**KiwiAlfa156** · 10-26-2022, 09:41 PM

This is vid may be helpful. As it shows how the piston has to be able to slide through the seal in order to rest at the correct (retracted) distance from the pad. Excessive lever is caused by the piston sticking in the square seal, not slipping through the piston and being retracted too far from the pad. Brake binding is caused by binding pistons.

**GabrielGoes** · 10-26-2022, 10:02 PM

That is a good video refresher!

Just for clarity purposes - In this case the NO lever was caused due to a compromised seal that failed to hold hydraulic pressure

Brakes not building pressure

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