Rear brake master cylinder...big issue

[SumpMagnet]

Really running out of ideas here, so looking for advice on things to try.

Stripping down the rear brake system on the 750F2. I ran in to some issues with corrosion, as it was in a pretty sorry state and full of rust. It's also totally solid.
Nothing moves.

So...I tried to remove the circlip to pull it aparet, and after weeks of heating, soaking and the like...I couldn;t even remove the circlip. The ends of the clip broke before the clip left hte groove. In the end, I had to drill a 1mm hole through from the outside into the back of the clip groove and punch it free. Once I had both ends lifted...I managed to walk it out. Nor...I can't remove the piston.

It will not budge a fraction. I had WD40 sitting above the piston for a week...and none soaked through. I have tried heating with a heat gun...but no luck. I tried soaking WD40 in from the other side....no luck. The front master cylinder needed a bit of tapping form the back side to shift the piston, but on this unit...there is no hole to tap the piston out from.

Can anyone suggest a method that might work?

To make matters worse, my initial efforts to find a replacement unit have failed, so I either need to rebuild this one...or find a non standard unit I can use

[K2-K6]

Can you connect a good front mastercylinder direct to the output of this unit and pump it out with hydraulic pressure?

You may be able to use the first front flexi hose and bolt banjo union to the rear to connect.

Also may have to blank bolt the reservoir input of the rear if the piston moves.

[kettle738]

Can you fit a grease nipple using a drilled and tapped bolt? (if you have one of the correct thread)....... a grease gun will shift stuck caliper pistons that have defied everything else with contemptuous ease, the pressure they put out is amazing.... and it's damage free force......that said, if it does manage to damage anything, it was never going to move anyway.

Mick.   

[SumpMagnet]

thanks for the suggestions guys ...I will give them a go this weekend.

but...as mentioned...if shifting it with pressure like this damages hte thing...it was goosed anyway!

[SumpMagnet]

Well...it's buggered.....

I couldn't get it to shift ...up....  but I did manage to get it to shift down, a few mm. Probably as far as teh spring goes, and only after a lot of effort and an hour heating at gas mark 4 ...

What I could see is some nasty pitting of the surface of the bore. Also...the piston did not free up in any meaningful way, and did not spring back. I also feel that gives me scope to get more brutal, and vent my frustrations on the thing. It may be possible to get it re-sleeved, but that being the case..I could core out the piston with a drill and Dremel most of it out to release the spring etc. and then get the last remnants out.

So..now...the hunt is really on for a replacement, and I think that could be a long job. Still....it's not like the rest of the bike is finished and this is the only bit I need...I have many more jobs to do

[Bryanj]

Do you want it to work or  be original?  If just work i fitted one of the £10ish ones on ebay to the rear of a cbx550 and it worked fine.

[Moorey]

Common as muck 3 available. You might not like the price.
https://www.ebay.co.uk/itm/Honda-CB-750-Four-F2-G-rear-brake-master-cylinder-with-oil-cup-new-Genuine-NOS/292409883667?hash=item4414fcbc13:g:eFkAAOSwlv9aXOoa

[Spitfire]

Common as muck 3 available. You might not like the price.
https://www.ebay.co.uk/itm/Honda-CB-750-Four-F2-G-rear-brake-master-cylinder-with-oil-cup-new-Genuine-NOS/292409883667?hash=item4414fcbc13:g:eFkAAOSwlv9aXOoa

That's scary !

[SumpMagnet]

@Moorey .... interesting. None of my searches turned that up. German E-Bay I guess, but still don;t know how I missed it.

You are right about the price....that's a bit of a shocker. But good to know they are out there if I got a lottery win.

Short term...I would settle for working, and worry about originality afterwards, as I would really want to be able to use the bike with a pair of brakes. But...honestly...knowing they still exist, I can make something fit, and keep an eye out for a decent usable second hard part.

Scary indeed.

But also reassuring to know

[Bryanj]

All i had to do for the cbx was make a flat mounting bracket. There ae loads of them on ebay to fit the little step throughs and they work fine.

[SumpMagnet]

Bryan...that sounds like what I will be doing. Thanks for the suggestion. If I pick up a cheap one and make it fit on a bracket, then I can just keep looking for the 'right' part and use the temp one in the meantime to get the bike rolling

[Nurse Julie]

Bryan...that sounds like what I will be doing. Thanks for the suggestion. If I pick up a cheap one and make it fit on a bracket, then I can just keep looking for the 'right' part and use the temp one in the meantime to get the bike rolling

Hopefully stopping rolling  😁😁😁😁

[Spitfire]

Bryan...that sounds like what I will be doing. Thanks for the suggestion. If I pick up a cheap one and make it fit on a bracket, then I can just keep looking for the 'right' part and use the temp one in the meantime to get the bike rolling

Hopefully stopping rolling  😁😁😁😁

Wot she said

[SumpMagnet]

Julie ,..... with the weight of this thing.... it's gonna need a boat anchor to stop it rollig!