Topic: ** PROBLEM SOLVED** 1978 CB750 K Front Brake - All new - spongy feel

[Ericcb750]

Hey guys, So I have a 1978 CB750K that i just over the past couple weeks re-built the entire front braking system on. I re-built the stock master cylinder with Honda parts, INstalled new banjo fittings/crush washers, a brand new goodrich stainless line and a re-built caliper with honda seal (no corrosion on the piston either).

THe issue i'm having is that my front brake lever seems pretty spongy.... I have the mounting arm adjusted correctly and brand new pads in it. I bled the system ( a couple times now) to get rid of all the air.... I greased the pivot pin and everything is tightened.... SInce my brake was 100% Sticking before due to crud in the caliper and the master I don't know how the brakes "should feel" on a cb750

this is my first one so I'm not sure if they should be really firm or not? My Harley master has a great firm feel to the lever. I tied the lever down overnight and it helped a bit . Lever doesn't touch the bar by any means but i'm thinking there should be more "feel" to the brake..

Let me know your experiences/opinions.... What size master cylinders do the guys run that end up changing theirs out for an aftermarket one? SIngle disk brake...

Thanks

[b52bombardier1]

Take a wire tie and clamp it tightly over the brake handle. Leave it overnight and tell us how it feels in the morning.

This should theoretically allow the trapped air in the line to reach the master cylinder and exit.

Rick

Sent from my SAMSUNG-SM-N900A using Tapatalk

[edwardmorris]

^^^

I had the exact same question when I first put it together on my K2. I used a rubber band to get the lever pulled in a little past half way or something like that, then turned the handlebars all the way left to get the master as high up as possible. Next day, it was rock solid.

[Ericcb750]

hey Rafi, Thanks buddy. I will try that. I tried tying it down but I didn't move the bars like that to the left.

I know this seems like a really rookie problem to be having but I just wasn't sure if the lever should be really stiff? The brakes grab perfectly but just not as much feel as I would prefer. I'll try that buddy. Thanks for the help!

When Adjusting the pivot arm bolt (with the spring and nut) you just want to turn it in  just until it seats down right? You don't really need to tighten it too much? Seems pretty self explainatory but I figured I would ask. Tha's what i did before bleeding the system.

Thanks again Rafi

[Johnie]

The way I do that pivot arm bolt is I lift the front tire off the ground and spin the tire. I then adjust it until the pads are off contact ever so slightly.

[Old Scrambler]

+1 with Johnie and the reason to turn the forks to the left is to get the small orifice in the MC level or tilted slightly upward so the air is always moving up.

[Ericcb750]

THanks guys.... so I did the trick by turning the bars full left (To raise the master cyl as high as possible). And I pulled in the lever (about half way full travel)..... this is where it starts to get firm.... and tied it over night...seemed to help a bit.

The brake pads are grabbing the disk but I just don't have the best feel in the lever....Its hard to describe. My Harley and my Dads Shadow 750 both have rock solid feeling brake levers..... Mine on the CB750 seems to give a bit more...now granted I can't pull it all the way to the bar....but i just wasn't sure how firm the lever should be.

I've checked rolling it around in the driveway and in the garage and at about half the brakes travel it stops the bike completely so i'm assuming it's probably normal? I'll try bleeding again to get any little bits of air out but after about 3-4 master cyl's full I got not one bubble.

I have a bigger issue now...... CHanging the oil last night and the damn Filter bolt is all buggered up..... This thing is pretty darn stuck..... I've read a bunch of threads on tricks to get it off so hopefully i can get it off.

[flybox1]

Heat and vice grips are your friends for filter bolt removals.

[Ericcb750]

Hey Thanks Fly, I appreciate it buddy.

I wasn't sure how much I could heat it up considering there is a paper filter soaked in oil on the other side..... But This darn thing has got to come off.....  I may look at going to a spin on filter but I will 100% put some antisieze on the flange of that bolt next time.

[ekpent]

 I have hammered a 12 point 11mm on the old buggered 12 mm before and got it off. A close fitting next size down SAE may work also. Actually the more messed up the better for that trick. With the oil in there don't worry about burning the paper filter with a small bottle propane torch, won't get that hot. Good luck on that nasty old bolt !

[Ericcb750]

I know this has nothing to do with my original Master cylinder/brake posting....

BUT I GOT THAT BUGGERED UP OIL FILTER BOLT OFF!!! WOHOOOO
My dad had a special socket set called "grip rites" that have these crazy teeth in them.....I hit it with a torch, gave it a shot of pb blaster (nice little fajita Sizzle).... came back in a bit, torch again for about 30-45 seconds and used the grip-rite socket with my Half inch ratchet which is about a foot or so long...Bam cam right off.

Now the nut is pretty banged up but the rest of the bolt and threads in the case and everything are perfect.....

Would you guys recommend getting an actual OEM Honda filter bolt or can i use an Aftermarket Emgo one? I know they have a special pressure relief valve and i'm not sure what EMGO's quality control standards are like.... IF anyone has used them and been fine then i'll grab one of those but if I should get an OEM one , please let me know your advice/opinions.

Thanks again guys, this is the best forum!!!

ALSO got my drilled/re-surfaced brake disk from Godffery last night.....so sweet!!!!

[ekpent]

 Bill Benton just started a new table top sale and has 3 oil filter bolts on the table. If given a choice I would go OEM but re-pops can be had with 17mm bolt heads for around $10.93 with free shipping on E-bay.  http://www.ebay.com/itm/Honda-Oil-Filter-Bolt-15420-300-020-Replacement-CB400F-CB500-4-CB550-CB750-CBX-/201771544321?hash=item2efa856f01:g:S60AAOSwo4pYa9Lm&vxp=mtr

[Ericcb750]

thanks for the info!! I'll talk to Bill and see if I can't snag one.... I just wasn't sure if anyone has had  problems with the pressure relief valve on the emgo ones.... I do like the idea of a larger hex on the end.

I'll see if can get one from Bill and i'll just put some anti-seize under the flange and not tighten it too much. Really appreciate the help!

[edwardmorris]

Back to your original issue, spongy feeling brake is NOT good on the road. I'll try and dig up my old thread on this, but I recall that tilted handlebars eventually fixed it. I also had to use the mityvac pump to get the lines to bleed properly, the old fashion lever pump wasn't cutting it. I also recall someone suggesting pulling the lever away from the handlebars as far as it'll go, in case there is air behind the piston?? Not sure how that works, but I did do that as well.

[edwardmorris]

My thread from when I had to deal with this http://forums.sohc4.net/index.php?topic=148265.0

[Ericcb750]

Hey Rafi, Thanks for the help buddy. Although i have that massive project going on for the motor and everything for this bike, I really am trying to get it ready to actually ride this season....I've finally got the timing spot on and carbs all done up right and the brakes were something else that needed attention.

I'll keep doing some digging. I'll also read through your thread, thanks for attaching that.  I'm kind of confused because the entire braking system is brand new so i figured it would have a nice positive feel but it's a bit sketchy....

Here's what i did....New master cylinder re-build kit (honda kit), New banjo bolts and crush washers, new stainless braided line, Completely disassembled caliper and put in new HOnda seal, cleaned up the piston (had no pitting/corrosion), new bleeder screw and new pads, greased pivot arm, new adjuster bolts.......

So I was hoping to have a nice positive feel...i'll keep trying at it and researching.

[edwardmorris]

My frustration with it was exactly that, everything on my K2 was brand new yet it refused to stiffen up right away.

[scottly]

Remove the adjuster screw and spring, and with the forks positioned so the brake line fitting is the highest point on the caliper, compress the piston into the caliper by pressing it against the rotor with both thumbs on the caliper body and fingers against the rotor. This will force any air uphill into the master cylinder, instead of trying to force the air downhill with conventional bleeding. Holding the brake lever down will not allow air to escape through the tiny relief hole in the MC, as the MC piston blocks it off when the lever is pulled. 

[Ericcb750]

Hey Guys, I took the 78K out yesterday for a nice spin as I was feeling better about the brake...However, it's performance on the road was NOT Good...the drum did much more for me than the disk....

I did some digging and I think I may have found the issue....when I re-built the master cylinder which is a stock 78 cb750K M/C I think I used the wrong kit..... I made sure it had all the seals that came out of the original one and measured the diameter (14MM) and I installed them except the little valve at the end (towards the banjo bolt).... I installed a kit from an earlier CB750 M/C.....

After doing some digging I think the earlier piston is a bit shorter than the later one....Can anyone confirm this information? I Can't seem to figure out what else it could be.... Thanks guys.

[Ericcb750]

So I know this is kind a dumb thread but if my mistake helps someone else in the future, It will be worth it...

I fixed my brakes last night and have everything working 100% like it's supposed to.... I'll post up some photos of the internal differences between the pistons & Springs on the earlier Master cyl re-build kit vs the later master cyl re-build kit......

Looking at them separately you would not know but if you measure them with a caliper or hold them up next to eachother...the difference becomes very clear..... I used the new seals and popped in the old piston & spring which were still very clean (except the portion of the piston that contacts the lever, I cleaned that as much as possible)... and my brakes are amazing now....

Thanks to the fellas on the forum, Problem solved!!!!!

[Ericcb750]

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