Topic: CB 750 Rear Master Cylinder Question

[dramsell]

I am having trouble with this rebuild.  I have included a pic with the areas marked.

Question 1.  The rubber item marked A-where does this go?
Question 2.  The pushrod marked B-are the 2 nuts supposed to be able to be removed (I think so but I thought I would ask)?
Question 3.  The piston marked C-In one manual it said there should be a secondary cup here (it did not come in the rebuild kit?

If the pushrod needs to replaced-anyone have one or know where I might be able to get one?

Dave

[scondon]

Here's a parts diagram showing assembly for a '76 750F. The two nuts are for adjusting travel and can be removed.

http://www.cmsnl.com/honda-cb750f-750-super-sport-76-us_model490/partslist/F++09.html

[dramsell]

Thanks.  Mine is a 78 though-don't know if there is a difference.

Dave

Any other help will be greatly appreciated!

[scondon]

http://www.cmsnl.com/honda-cb750f-750-super-sport-78-us_model7240/partslist/F++10.html

[dramsell]

Thanks!  Any idea about the other parts of the question?

Dave

[scondon]

 The diagram should show where "A" goes, but can't quite tell from the pic whether its the rubber cup that sits between spring and plunger, or a fat o-ring. It's been a while since I rebuilt mine, but I don't recall any o-rings.

Adjuster nuts can be removed

If you need to replace the plunger then hopefully someone here has one they don't need. Often it is easier to find a used master cylinder for parts. #13 on the above parts list is the Honda rebuild kit which includes the plunger. If they are still available they probably run around $60-$70. The part number is given in the listing and you can use it to shop around.

Hope this helps

[scondon]

Actually, I do recall breaking one of the tangs on my plunger while slipping on a rubber seal that fits in its groove

[super pasty white guy]

part A slips over part C

[BeSeeingYou]

Part A is the secondary cup and it slips over the piston and sits behind the little raised lip and against the socket end of the piston.  If you look at the cup one end is flared out and wider around and this end faces toward the end of the piston with the flower shaped pressure valve.  Just lube it with a bit of brake fluid and it will slip over the socket end of the piston.  The nut on the pushrod does not come apart and the boot just slips over it and sits in the recessed area between the "two nuts". It is not adjustable.  The loose nut is for locking down the adjustable fork that connects up to the brake lever.  The pushrod availability is kind of strange.  It comes with the 75/76F kit and is not available separately.  It is listed as not available for the 77/78F but it is the same as the 75/76F.  The pushrod looks ok just try and clean it up best you can.  I partially filled my boot with waterproof grease to keep the corrosion at bay.  It seems that the boot is a place where brake fluid, dirt, and water eventually collect and form a nice corrosive mud.

If you are removing an original piston they may be slightly different with the secondary cup sitting over the raised lip and against the socket end.  On the rebuild kits the cup is not as wide but they work fine.

[Radam]

Hey, I'm bringing up this old thread because I'm having problems getting my rear master on my 78 750f put back together. I've got everything lined up the right way, but I can't seem to get it all to fit back into the body. When I pulled mine apart (it's never worked since I've had it) it didn't have a spring or the piston (part C in the pic). I hope that I'm just missing something easy. You can't take the body apart more than it is in the pic can you?