Topic: ID a 400F master cylinder?

[Mark_Weiss]

How do I ID a stock master cylinder for a '75 400F?
The one that I have looks correct but does not offer much leverage. Kind of like squeezing a brick. The caliper is rebuilt and works smoothly, nothing binding, so I'm suspecting that I have a master cylinder sized for a larger caliper piston. The master cylinder that I have has the characters  8 H 2 cast into the underside.

Mark
in Arizona

[MoMo]

If my memory serves me well, all early mid size Hondas(360,500,550,400) used the same mc, the 450 twin and 750 had a different one.  Ditto with the caliper.  What exactly is wrong? Larry

[Mark_Weiss]

No leverage. I'm squeezing for all I'm worth and getting only moderate braking effect. The caliper is OK. Clean, & piston moves freely. I can press the piston back in with my thumbs. The pads look OK. No contamination, even contact across the face. No oil or other contaminant on the disc. I even tried cleaning the disc to no effect.

When I squeeze the lever, there's a little play, then pressure comes up as you'd expect. Then the lever quickly becomes VERY solid and there's no increase in braking. I'm thinking that the master cylinder might be for a model with a larger caliper and may have too large a bore for the small piston in the 400F caliper.

Mark
in Arizona

[MoMo]

Mark,  does fluid come out at the bleeder valve.  Seems like you may have a clogged line, happened to me many times with bikes that have been sitting for awhile. A very stiff lever with no braking is symptomatic of a clogged line.  Good luck, Larry you can email me personally if you want.

[Mark_Weiss]

I've got braking, just not very good.
Fluid flow is fine and everything works smoothly. I'm really thinking that I have put on a master cylinder with too large of a bore. Probably one for the larger calipers used on 750s and such. The only reason that I am certain that this is not the original m/c is that it has a crash scar that does not correspond to any time that I crashed the bike (raced the 400 on and off for 20 years). I looked through my box of spares and do not see another master cylinder, so at this point I'm thinking that I may have given it away or something.

What I need to know is if there is a way to tell if the m/c that I have on the bike is correct (and there's another problem somewhere) or if I really do have the wrong master cylinder and just need to locate a proper replacement.

Mark

[MoMo]

Visually both looked similar but did have different diameter bores, which I don't see how that could make a difference but possibly it could.  Seems as long as the system is bled the brakes should work.

I have a bunch of mc, but the numbers are real hard to read.  I'll try tomorrow and get back.

[faux fiddy]

Not sure what  have to trade, but I need a reasonable 350f mc.

Thanks for showing up, PA post.

Send me a pm.

[Mark_Weiss]

Visually both looked similar but did have different diameter bores, which I don't see how that could make a difference but possibly it could.  Seems as long as the system is bled the brakes should work.

I have a bunch of mc, but the numbers are real hard to read.  I'll try tomorrow and get back.

The bore of the master cylinder IS what makes a difference. It is the ratio between the area of the master cylinder piston to the surface area of the brake caliper piston(s) which determines how hard you'll have to squeeze the lever for a given braking force.

A small bore pushes less fluid when you squeeze the lever. This gives you more leverage against the caliper piston, but requires that you move the lever further.
A large bore pushes more fluid, this gives you less leverage, but also reduces lever movement.

Since I'm experiencing very high lever effort with little lever travel, I believe that I may have a master cylinder for a CB750. A model that used a larger brake caliper. I'm hoping to find a way to identify the master cylinder without having to take anything apart.

If someone knows what bore was used on the 400F, that would help. I may be able to measure the bore by just removing the brake lever. I'm guessing that it's supposed to be 1/2 inch.


Mark

[scottly]

Is there any resource that lists the actual diameters of MC piston bores? All I have been able to find is that the part numbers for the pistons for the early 750s, and some of the smaller bikes is the same, but the k7 is a different part number. I have a k1 and a k7 MC and they both look the same with the levers removed; probably less than 1/2". I have a dual disc Kawi MC that is marked as 5/8"... 

[bryanj]

750 has same master cylinder as all the others, it only changes when you get to the F2 (at least bore size and seals are the same!!)

[Mark_Weiss]

750 has same master cylinder as all the others, it only changes when you get to the F2 (at least bore size and seals are the same!!)

Thanks. This is what I needed to know. This means that even though the 750 (and I think the 450) had a larger piston in the caliper, the master cylinder is the same. In this case, the braking I'm getting is as good as it's going to get without changing pads. That will be the next step. Different compound pads and a thorough cleaning of the disc.

Mark
in Arizona