Topic: Front brakes have pressure "step" after rebuild

[NOLA750]

Hi guys! I have a question about my braking problem.

I recently rebuilt both front calipers on my 78 CB750 with new seals and sanded/polished pistons. The master cylinder appeared fine. Now, a few weeks later, when I pull the brake lever, there is resistance. After applying more pressure, the breaks will suddenly engage, braking hard. What may be the cause of this? Could there be a bur in one of the pistons that is catching on a seal?

[ACK777]

Yea definitely worth checking the pads and pistons as it sounds like something is getting hooked up on something else but on rare occasions some smaller air bubbles in the lines could have merged into one larger bubble which could give you that feeling of nothing, nothing, nothing, then, bam, ALL THE BREAKING!

Your master might just be compressing the air in the line up to a certain pressure before it has enough power to hydraulically move the calipers with force against the rotors and sometimes those air bubbles start out tiny and then after riding over time with bumps and vibrations can merge.  Normally this would just result in a squishy brake but it is possible.

Have you tried the old trick of strapping your break shut for the night after a solid bleed session? 

[NOLA750]

I'm going to check my pads and bleed my lines. Thanks for the pointers guys! Ill get back with the results.

[Bodi]

Probably the caliper pad is binding, this is very common. Had to grind or file down quite a few.

I have tried this lever compressing thing, didn't do squat. And after some thought I can't see any way it could. You compress the bubbles smaller - so what? - and close the relief hole in the m/c. How does that do any good?? Reverse bleeding does get the air out.

[ACK777]

Probably the caliper pad is binding, this is very common. Had to grind or file down quite a few.

I have tried this lever compressing thing, didn't do squat. And after some thought I can't see any way it could. You compress the bubbles smaller - so what? - and close the relief hole in the m/c. How does that do any good?? Reverse bleeding does get the air out.

It is more of a last ditch effort or a lucky shot at a quick fix.  I do it after bleeding my brakes (similar to knocking on wood) because it is supposed to be the action of pressurizing the system and the flow of fluid that disturbs the bubbles, dislodging them and letting them float up so that when you let the brake lever go you should see your master level drop a little if there were indeed some bubbles in there.  I have had to top off my master a little after doing this so I can confirm that it is a thing and sometimes useful (unless your a brake bleeding badass ).  I have heard of some people leaving their brake strapped to the handlebar for over a week and I cannot imagine that would be good or healthy for the hydraulic system and seals.

[NOLA750]

So the problem has only gotten stranger. It's not the pads. Maybe I'm just not so good at bleeding my lines?...

I took off the brake pads, leaving the calipers hooked up to the hydraulic lines, and pulled on the brake lever. Both pistons showed the same jagged movement as before. So I squeezed them out, inspected them, pushed them back in (and they went in smooth), bled the whole system, and viola, the brakes were smooth as silk.... for about 5 minutes. The jagged movement slowly crept back in. Bled them again, ran fine again, and the same thing happened.

Is air leaking into the system? Is something else going on I'm not thinking about?

[RAFster122s]

What grade sandpaper did you use to sand and polish the pistons?  Did you clean out the groove for the caliper seal so it was spotless? Brass brush in dremel works very well for this but keep speeds below 15k rpm I believe it is or you are going to be spitting wire out of the brush and they hurt when they hit you or stick in the skin. Hopefully you wear eye protection every time you use a dremel. Not doing so with wire brushes can cause severe eye damage.

[NOLA750]

What grade sandpaper did you use to sand and polish the pistons?  Did you clean out the groove for the caliper seal so it was spotless? Brass brush in dremel works very well for this but keep speeds below 15k rpm I believe it is or you are going to be spitting wire out of the brush and they hurt when they hit you or stick in the skin. Hopefully you wear eye protection every time you use a dremel. Not doing so with wire brushes can cause severe eye damage.

It's possible the groove in the caliper seal has some dirt, I'll check on that. But if that was causing this, why would bleeding the brakes again eliminate the problem for a little while?

[ekpent]

 Is the vent hole in the master cylinder completely open and working correctly. Do you have a stock master ?  If a little back pressure is building up it can cause the pads to not retract. Opening the bleeder can relieve that pressure which is a good test when one is completely blocked. Internal collapsing break lines can cause the same symptoms also.

[NOLA750]

Is the vent hole in the master cylinder completely open and working correctly. Do you have a stock master ?  If a little back pressure is building up it can cause the pads to not retract. Opening the bleeder can relieve that pressure which is a good test when one is completely blocked. Internal collapsing break lines can cause the same symptoms also.

The brakes retract perfectly which made me believe the vent hole is fine. It is a stock master cylinder. It's just that the brake pressure is jagged, but only after continuously building pressure.

[Don R]

 Let us know what you find.
   I saw a photo tour of the Yoshimura shop, there was a brake system assembled with a rubber band on the lever. Good enough for them, good enough for me. Compressed bubbles lose their grip on the sides of the hose and float up, the next time you cycle the m/c they come out of the return hole. 

[seanbarney41]

Is the vent hole in the master cylinder completely open and working correctly. Do you have a stock master ?  If a little back pressure is building up it can cause the pads to not retract. Opening the bleeder can relieve that pressure which is a good test when one is completely blocked. Internal collapsing break lines can cause the same symptoms also.

Try to find some sort of tiny pin and jab it in the return hole...oh yeah, here is a good diagnosis.  Remove your mc cap, VERY gently squeeze and release the lever.  You should see some fluid movement from the return hole.  Given a full squeeze, it should squirt you in the eye.  If you see no movement, the return hole is still plugged.

The brakes retract perfectly which made me believe the vent hole is fine. It is a stock master cylinder. It's just that the brake pressure is jagged, but only after continuously building pressure.

[RAFster122s]

Is the hose on the nike a rubber hose? Or, have you switched to a braided teflon stainless steel and plastic covered line? How long have you had the bike? How old are the hoses if rubber? Have you ever replaced the rubber lines?
Wanna know what auto makers recommend on replacement interval for rubber hoses? 5-7 years max.
The braided teflon lines are much better, no bulbing and better brake lever feel and no wasted hydraulic pressure.

[Alan F.]

One easy way to tell if a rubber brake hose is trash is to spray brake cleaner through it before installing it.

They come apart internally blocking flow, but look fine on the surface.

[BenelliSEI]

Is the hose on the nike a rubber hose? Or, have you switched to a braided teflon stainless steel and plastic covered line? How long have you had the bike? How old are the hoses if rubber? Have you ever replaced the rubber lines?
Wanna know what auto makers recommend on replacement interval for rubber hoses? 5-7 years max.
The braided teflon lines are much better, no bulbing and better brake lever feel and no wasted hydraulic pressure.

+1 to all that’s been said about hoses. I’ve had a set that actually acted as a check valve. Master cylinder pressure would force fluid down, but the brakes would not release! That “step feeling” could be the pressure building and elevating as you force fluid through plugged hoses......