Topic: 1978 CB750 F rear m/C.... seen it's last day?

[Ericcb750]

Hey guys, I just took apart (or began to) my rear m/c on my 1978 CB750F SS and the piston is rusted/frozen into the master bore pretty bad.... I've done some research and will try the grease gun method that seems to work for a bunch of guys. I have all the fittings off so it's just the housing/casting and the piston/internals....the plunger looks like it went down with the Titanic but i figure it doesn't need to look too pretty...

Just wanted to see in a worst case scenario if anyone has ever been able to use a different/new rear master that bolts up to the stock 1978 CB750 F frame? I hope it can be salvaged as i already have the re-build kit (honda oem) and am wanting to do a full stock resto....even was lucky enough to locate an original (black tip) muffler for the one that was missing on the bike when i bought it...All thanks to the guys here.

Any advice/help would be awesome, all the other brake components are still in great shape so i hope I can use it. Thanks guys. Take it easy.

[flybox1]

Really depends on how wasted the bore and piston is....
I think most bores can clean up enough.  Its the piston and seal integrity that really matters.

[Geeto67]

I have rebuilt several rear masters for supersports and to be honest, the thing that works best for me is to let it soak for a couple of days in a can of carb dip before trying to remove the piston. I will then usually heat the body up and can usually yank the piston right out after that.

I know the rebuild kits are kind of expensive for these bikes if you can find them, but the 1978-79 GL1000 uses the same rebuild kit ( 43530-410-305) and the aftermarket kits are more common (I don't know why but the aftermarket kits for the 78 cb750F are usually about $45 and the same kit for the GL1000A is $26, I think only K&L uses the same kit number for both). the GL1000 internals are the same but the master body is not - as far as I know (and what the parts fiche tells me) the 77-78 rear m/c was only used on that bike and no others. I don't know of any others of different bikes or universal ones that will work without you having to rework the foot plate.

I can tell you that a rebuild kit will come with everything you need including a new piston so if you have to drill the piston out it isn't a big deal. I've probably rebuilt 5 or 6 and only drilled one and as soon as the bit took hold it pulled the piston out.

I can tell you right now I am looking into using DOHC CB foot plates on my 1978 and so far everything but the exhaust hanger looks like it lines up. If you want to fall down that rabbit hole my suggestion would be to get a set of cb1100F lattice foot plates and use the 1981-1983 DOHC cb750-1100F rear master cylinder, but I am guessing this is a lot more than you want to spend. Still - the cb1100F foot plates are pretty sexy. Also if you wanted rearsets there are factory honda rearsets sold in europe and asia called "sport kits". I just bought a sport kit to use on my 1978 from the UK for $145 but I have to supply my own master (using a rebuild DOHC - even though my 78 master works fine). This would also be a good time to upgrade to the dual piston caliper brakes used on the DOHC from 1981-1983 (there is a thread about the conversion somewhere on this site).

[Geeto67]

in case you do want to fall down the DOHC footplate/master cyl rabbit hole....here is my old 1979 cb750F with CB1100F "Lattice" foot plates:



see how good looking that is? It looks like a custom milled piece but that is actually a honda factory part. They have nicer passenger pegs too.

the foot plates use the rear engine mounting bolts to attach, just like the 1978. So far comparing my 1978 and sport kit foot plates the mounting holed for the foot plates are in the same location, thickness, and spacing. I will be bolting them up this weekend.

[Jerry Rxman Griffin aka MuthaF'er]

I saw a Chinese master on EBay for cheap that was supposed to fit. Wish I could provide the link??

[Geeto67]

I saw a Chinese master on EBay for cheap that was supposed to fit. Wish I could provide the link??

I know the one you are talking about -- this one right?:
http://www.ebay.com/itm/Rear-Brake-Master-Cylinder-Fluid-Reservoir-For-Honda-CBR-250-600-1000-CB-NSR-/262434244371?fits=Year%3A1978|Model%3ACB750F&hash=item3d1a4cc713:g:fgoAAOSw241YUlf0&vxp=mtr

it doesn't fit. They set it to "fits all bikes" so if you do a search for model specific it still comes up but it really will only fit these bikes:
Fitment:
For Honda
CBR600RR 2003-2013
CBR1000RR 2004-2013
CB250 All year
CB400 All year
CB600 2003-2012
CB900 1992 UP
CBR250 1988-2012
CBR400 1984 UP
NSR250 All year
RFV 400 All year
VFR400 1988-1992
VTR250 All year

[700504]

I saw a Chinese master on EBay for cheap that was supposed to fit. Wish I could provide the link??

Over in my thread you commented on my use of a gsxr rear master, it is similar to the one referenced below. I just made a bracket to fit the bike; still need to take pictures and draw a schematic. But no, it is not a direct fit, mounting holes on stock mc's are diagonal, all these aftermarket ones are vertical.

[martin99]

If you do manage to salvage your m/c, make sure the return hole is clear before you put it back into service. My rear wheel locked solid on me the other day, and by the time I could safely pull in there was smoke pouring from the disc and calliper. It melted the plastic dust cover that goes over the top of the disc. When I took it apart sure enough the return hole was plugged. The disc had warped to the extent that it was rubbing hard against the calliper bracket, so the disc had to be replaced too.

[1978CB750SS]

If you do manage to salvage your m/c, make sure the return hole is clear before you put it back into service. My rear wheel locked solid on me the other day, and by the time I could safely pull in there was smoke pouring from the disc and calliper. It melted the plastic dust cover that goes over the top of the disc. When I took it apart sure enough the return hole was plugged. The disc had warped to the extent that it was rubbing hard against the calliper bracket, so the disc had to be replaced too.

I should have lent you my rubber mallet to tap the caliper back a hair! lol
It really does work on stuck calipers.I'm going to take my rear caliper apart Saturday
It's my next day off.I'm sure it will be as crusty as the fronts were.

[Ericcb750]

THanks guys for the help, i know it's been a while since my original post. I had the master cylinder (without the reservoir and all the linkage/hoses removed) soaking for a while and still can't seem to get it out. I will keep going at it and may just drill the darn thing out. I had already picked up a complete OEM Honda re-build kit which includes all the internals with a new piston and rubbers . I would like to re-use this one if I can as I want to put the bike back to stock 100%..... took me almost 1.5 years to find the original "black tip" muffler which i was finally able to run down a little while back.

THanks for the tips and i'll keep working at it, hopefully i can get it out, clean up the bore a little and make sure the return/feed holes are nice and clean and slap it all back together.... Appreciate all the help, take it easy.

[Jerry Rxman Griffin aka MuthaF'er]

Any way you can rig up a grease gun to it like we do for the caliper pistons?

[TwoTired]

Or..
Once you have removed the snap ring piston retainer, hook up another master, bleed with brake fluid, and use that good master to pressure to pump out the stuck piston.

Cheers,

[Ericcb750]

Hey Jerry and Two Tired, That's a great idea about using another master cylinder....i could just use a spare front one right? I was able to get the linkage and snap ring/ boot off fine and everything else is dis-assembled fromt it...IT can 100% be re-used if Im able to get that piston out.

I do have a brand new piston, seals (new honda re-build kit) for it so if i can just get the darn thing out it will be able to function perfectly....so If i hook up a working master to the end of the stuck one (where the line usually goes to the caliper) and plug up the return/feed hole from the stuck master where the reservoir hooks up, The pressure should probably pump the stuck one out backwards? Am I understanding this correctly?

Thanks for the help!

[TwoTired]

Yes on the spare master to use to pump out the stuck one.
That's what I did to extract the rear master piston on my 78 750F rescue.
Yes, connect the two master cylinder outputs together, bleed and pump.
I don't recall doing anything with the bleed hole in the stuck master.  But, mine was stuck in the compressed position.  IIRC, working the piston back and forth a few times allowed the internal return spring to push the master's piston out enough to grab on to.
Should work for you, too.

Cheers,

[scottly]

I don't know of an effective way to block the bleed hole; I recently pumped out an MC piston using a front MC, and the only thing I can recommend is to wrap a rag around the body to catch the fluid that will jet out of the hole. By the time you bleed the system and clean up the mess, the grease gun would win the race, not to mention the grease gun can produce 10x the pressure of a front MC. Nothing worth saving can resist the gun.
The end of the tube on my gun, after removing the zerk adapter, will just barely engage the threads where the banjo bolt goes. If you are careful and don't put side loads on the joint, it holds well enough to pump out the most stubborn piston I've encountered, and the grease is thick enough that it doesn't spray out of the bleed hole, it oozes.

[Jerry Rxman Griffin aka MuthaF'er]

+1 on Scottly's mention of removing the zerk fitting from a grease gun. I do NOT know about the threads on your master though. As he says the threads are usually NOT identical on the front caliper but they grab enough without forcing the issue to get the grease gun hose in/kinda arrached to pump the piston out. Can you make it kinda fit where the banjo bolt goes in or however that master attaches IF it has threads? Sorry I'm just not familiar with the 77/78 F.

[seanbarney41]

GL1000 mc bolts right up and functions perfectly ...but you will need an adapter for the brake line...the gl mc has a reverse flare fitting instead of the 750f's banjo

[Ericcb750]

Thanks guys for the help, I'll post up some photos and figure out which way to do this, I did get a line on a new one from a forum member but this one is 100% salvageable if I can just get the piston "which is in the compressed position" out of the bore. The kit has all the new goodies i'll need. 

I'll send a photo of what i'm working with and see if you guys agree. THanks again

[Geeto67]

you've been on this since April, just drill the f$#%ker already