Topic: weak brakes after most every upgrade.???????

[CBGBs]

I have upgraded my lines, rebuilt the master and caliper, new pads, scoured the rotor, drilled rotor, and am using a new aftermarket MC and still the brakes are weak. They feel very spongy so I have repeatedly bled them. I have pushed the fluid up from the bleed valve and filled from the top as well as using a vacuum pump at the bleeder. Brakes are still very weak and I have no apparent leaks. What gives?

[evanphi]

Any side to side free play in the wheel? Are your bearings good? Your piston pressing on the rotor, then shifting the whole wheel over with its short travel would result in soft brake.

[CBGBs]

No play in the wheel. Bearings seem fine. Piston is pressing but not with much force. With original MC the brake was at about 60 percent of potential and with the new 14mm replacement, I guess I am at half of that performance. The lever will fall all the way back to the grip "spongy". I have bleed it every way that I could imagine.

[scottly]

If the lever goes all the way to the grip, there must still be some air in the system...

[bjbuchanan]

Make sure your caliper bleed screw is sealing tightly, it will cause it to be spongy. New MC but you definitely have a clear hole for the tiny hole in the MC right? Fresh or not beat up sealing washers right?

A couple weeks ago I had trouble with a friends KZ the bleed screw would just NOT seal. Had to grab a new one and try a couple tighten, loosen, tighten, loosen to get the surfaces to mate up nice. The surfaces seal by deforming a little bit to eachother

[CBGBs]

 I havent upgraded the bleed valve yet and i do need a new one. This is the best place for a potential leak in my opinion but when using a vacuum on the bleed valve the system seemed to be holding vacuum quite well. I cleaning the steel line as well when working on it today.

The new MC does have two small and very clean holes at the bottom of the bowl. The sealing washers all looked nice and I don't think there are any leaks in my connections. Things look very dry but I will check it again tomorrow.

[RAFster122s]

I don't recall the stock MC bore size. 
If you increased the diameter then wouldn't your brake will feel more wooden rather than spongy, so like Scott has said, spongy is air in the system.


There is a vintage brake supplier that has some good information on his site. I forget the name off hand. He has master cylinder sizing charts showing the relationship of the piston in the caliper to the master cylinder size.

David

[lucky]

The problem is often the master cylinder cannot fill with fluid because the hole is clogged up in the bottom of the master cylinder and it just will not bleed properly.
I will post a photo.

The hole can be so clogged that you will have to use a guitar string or jewelers drill bit to clear the hole.
Also getting a stainless steel front brake line will really help too.

[Sparked]

How is the bike positioned when you are bleeding the brake? Not all aftermarket master cylinders sit with the reservoir at the same angle so in order to keep the brake fluid from pouring over the side of the master cylinder the front wheel is turned to the left. At that orientation the brake hose may not be the highest point in the caliper and there will always be air trapped in the system.  At least that was the problem I found when I was unable to bleed a brake system after installing an aftermarket master cylinder.  I just removed the master cylinder from the handle bars and made sure everything was orientated in such a way that there would be no air pockets and Voila, fixed.

[MoMo]

On bikes in which I've had trouble getting that final bit of air out I use this trick-pull the lever in and wrap a cable tie around it.  Leave it overnight and it is usually good to go the following day....Larry

[70CB750]

I had the same problem after going dual disc - it just needed bleeding. Takes some effort to get it right.

[vames]

On bikes in which I've had trouble getting that final bit of air out I use this trick-pull the lever in and wrap a cable tie around it.  Leave it overnight and it is usually good to go the following day....Larry

+1 -- zip ties, a rag, a big tight rubber band. whatever will hold that lever firm for 24 hours.

[RAFster122s]

FWIW, You don't want to hold it to the bar, just 16mm away is what I've read that the repair books recommend. Going all the way pulls the piston on the MC past a point it can leak at the MC piston opening the lever is moving, this causes the seal to degrade and can lead to the finish failing at that portion of the caliper, dirt collects, etc.
A small block of wood will allow you to clamp it tight with a rubber band and not pass the recommended value.
David

[phil71]

I farm all my overnight brake lever squeezing out to the chinese. Also, some pads just suck, I've found.

[Powderman]

I thought of cheap pads but didn't know if he was getting spongy feel sitting or actually riding it. If it's spongy while sitting it is not the cheap pads. He also said he replaced the brake lines but didn't say if they were SS braid or not. Even new rubber lines expand and can feel spongy. If you replace the lines with SS and have good pads and still feel spongy look for more air in the system.

[Black 750K8]

I have a feeling you have a slight uphill run that you have not notice and the air will not move out of it.

[jackseattle]

+1 everyone's comments about the hidden air pockets.  FWIW, I switched to a smaller bore MC piston (stock is 14mm, I went down to 11mm) and I got much braking better force on my stock single piston caliper.

[crazypj]

I have a feeling you have a slight uphill run that you have not notice and the air will not move out of it.

 That's my thought.
It's not uncommon for me to remove master cyl from bars and bolt it to a short length of 7/8" bar so I can tilt it around to allow all the air bubbles to 'float' up and out
With modern sport bikes it's often necessary to remove one caliper to get all the air out (if it has curved link line over top of front mudguard)

[CBGBs]

I do have SS lines. I will make another attempt at manipulating the lines to get bubbles to pass up easier. I have played around with that theory and it didn't get me anywhere but I'll try again. If this doesn't work I'll check the caliper and install a new bleeder valve. The brake has always been poor (12 yrs) and it is worse now with my new MC. I have clubmans and installed a tilted MC which now sits level on the slanted bars.

[crazypj]

Doesn't matter if the cap is level, you need to look at where the piston is running.
 I've also had problem where banjo bolt only had 1 hole, when it was 'at the bottom' air could stay in the recess of fitting.
I now drill a 1/16" hole in the 'blank' side andor fit different thickness washers
Also, I've been using Harley Davidson brake washers for several years, they have a rubber seal as well as crush
Bolt it to the old handlebars so you can move it around.
Don't pull lever all the way in, 10mm from bars (put a finger behind lever so it won't come in all the way)

[phil71]

Not trying to hijack the thread, but for those talking about letting the bubbles out, does this also apply when using a power bleeder?

[crazypj]

Yes. I find 'power bleeders' extremely wasteful though so don't bother with them

[evanphi]

Even bleeding from the bottom with a syringe I have to do the brake lever trick over night.

I've also had problem where banjo bolt only had 1 hole, when it was 'at the bottom' air could stay in the recess of fitting.
I now drill a 1/16" hole in the 'blank' side andor fit different thickness washers

This is a really great point! I might try this on my next brake rebuild.

[lucky]

If that bleed hole is clogged it will always be spongy and will never work correctly.

[Cqyqte]

I have upgraded my lines, rebuilt the master and caliper, new pads, scoured the rotor, drilled rotor, and am using a new aftermarket MC and still the brakes are weak. They feel very spongy so I have repeatedly bled them. I have pushed the fluid up from the bleed valve and filled from the top as well as using a vacuum pump at the bleeder. Brakes are still very weak and I have no apparent leaks. What gives?

Is the new aftermarket MC the same sized bore as the OEM MC?  If not there's your problem, it maybe a hydraulic disadvantage.  Others more familiar with cylinder bores and the calculation may jump in.