Topic: Potential 1970 CB750 K0/K1 Hybrid

[MauiK3]

The 1 into 4 throttle cables were a chore, the rack system is sooooo much easier to manage.
Nice bike!

[grcamna2]

I am happy to report that my bike finally has the correct front forks and fender.  The lowers polished up rather nice, and I was surprised to see a number of casting flaws in the right one.  I wonder if they were still ironing things out this early in the production run.  In any case, the front went together well enough.  My only problem was a stud that pulled out of the bottom of one fork, but I've got a replacement on the way.

I also lucked into a VERY nice pair of HM300 pipes for the right side.  My current set is solid, but the inside of the headers are pretty ratty.  The plan was to eventually get a reproduction set, but for the price these two pipes were listed at, I couldn't say no.  I figure the left two will turn up eventually, and I can wait.

The one stud that pulled out of one fork is a pain;do the threads in the aluminum where it came out look 'good to go' ?

[Mikey G.]

Threads seemed pretty good.  The nut was FROZEN-frozen on the stud, which is what backed it out.  The new replacement stud may not be a completely seated as the others (it might be a turn out), but I didn't want to risk the lowers by pulling out threads.  And a dab of thread locker was insurance enough.

Of course now my front caliper refuses to release, so I have to look into THAT now.  Never a dull moment.

[MauiK3]

Might it be the hose?

[grcamna2]

Might it be the hose?

Yeah,I've seen old rubber brake hoses close-up inside from decades/years of sitting with strong brake fluid in them.

[BenelliSEI]

Check the caliper first. This one came off a 1970 cb750K0 that I totally restored and sold to a friend less than five years ago. He has kept the motorcycleabsolutely spotless but here is what the “pot” looked like! Cleaned it all out, and reused the same seal. Works perfectly now.

[BenelliSEI]

On my cb750K3 I had an old bottom hose (probably original) that turned itself into a check valve. The master could develope enough pressure to push fluid through the line, but it was such a gooey mess inside the pressure would take 10 minutes to release! If you need lines, try Raymond at APEX BRAKES.

[Mikey G.]

I'll have to take a look.  Chances are I just had the caliper arm set up incorrectly, because after I took the caliper off and put it back on again after adjusting it it seemed to work better, plus the piston could be pushed back in by hand pretty easily.  That said, my lower hose IS still original, and while the caliper was gone through with new piston and seal a couple years ago, Benelli's pic shows how that can go.

[BenelliSEI]

If the hoses are disintegrating it’s easy to spot. The fluid that comes out of the caliper will be absolutely black……

[MauiK3]

I would think any hose that old would be ready to retire no matter how it was used or stored.

[Mikey G.]

I went ahead a bought a replacement lower brake line.  While looking at things I realized the upper line had already been replaced by me not too long ago, so the lower line was the only old part left.  Still, I'm going to have to pull and clean the caliper- cracking the bleeder didn't seem to help the brake release, and it's almost like the brake pad is getting hung up in the caliper.  It boggles my mind that this worked before I swapped my front forks, but now it doesn't.  Ah well.  Never a dull moment.

[BenelliSEI]

That’s exactly what JP’s K0 was doing and I couldn’t believe the crud that built up in what was a spotlessly rebuilt caliper -5 years ago i had a few pouches of BOSCH silicone brake grease that I very sparingly coated the exposed part of the bore. I also  applied a small amount on the steel edges of the pad…..

[newday777]

Still, I'm going to have to pull and clean the caliper- cracking the bleeder didn't seem to help the brake release, and it's almost like the brake pad is getting hung up in the caliper.  It boggles my mind that this worked before I swapped my front forks, but now it doesn't.  Ah well.  Never a dull moment.

Also, it could be the return port in the master cylinder is plugged(the tiny hole) not allowing the piston to relax when you let go of the lever.
Use a wire from a wire brush held with needle nose pliers to poke the hardened gunk in the hole to clear it.
The test to know if it’s a plugged return port is crack open the bleeder to release the pressure. If the wheel spins freely you know you have to clear the plugged port.