13 replies to this topic

[blob-omatic]

Hello. I'm trying to rebuild the master cylinder on my lhd 1977 Mini 1000. It has the GMC160 master cylinder which is unavailable to buy new.

 

I've removed the first ring that holds the external spring and the inner circlip that comes after that. The Haynes manual says that it should come out by stroking the piston up and down but mine just goes clank when it's pulled out and it doesn't move with a reasonable amount of force.

 

Is it safe to pull the piston with unreasonable amount of force or will something inside the cylinder break before any force goes into the copper bushing that's stuck?

[croc7]

In order to remove that copper washer (“bearing”), invert the MC in a vise, remove the circlip holding the bearing (#21 in the Haynes diagram) and soak the bearing with penetrating oil for 2-3 days, tapping it with a hammer and drift to loosen the corrosion. You may have to tap the end of the MC against something hard to dislodge it. Be patient. Once you have it out, you’ll have another challenge to get the other circlip out-you’ll need circlip pliers with long jaws to reach it. There are two cylinders inside the bore that are connected with a ‘link’ (looks like a robust paper clip) that is near impossible to find a replacement should you break it. Study diagram 14.5 and it will make sense.

Edited by croc7, 01 September 2019 - 05:44 AM.

[croc7]

Blob-omatic;  Any luck??

[InnoCooperExport]

I'm fighting a 159 at the moment and can't seem to get beyond the same circlip. Guess I'll soak and tap some more and hope it does its thing...

[croc7]

Are you fighting a copper bearing or poly?

[InnoCooperExport]

Hard to say, looking at it it could be copper.

[croc7]

Keep at it, the last one that I rebuilt took 4 days of soaking and tapping to loosen the washer (bearing). Try holding it upright and whacking the pushrod end squarely against something solid.

[InnoCooperExport]

The plastic paperclip you mentioned turned out to be twisted flat metal in mine which unfortunately snapped where it bent round the roll in the second piston (which is still thoroughly stuck in the cylinder). I´m thinking of replacing it with a solid rod of the same length, that should be fine right? 

 

What I cannot work out is why half the circlips and other guides seem to be missing. Mine is from an Innocenti Cooper Export so maybe the internals are different? Funnily enough the parts book I have doesn't identify it as a GMC159 yet the casting number in the body of MC does, and it does look identical to one. Very strange

[croc7]

I don’t even know what purpose the link serves except that it connects the two pistons that somehow regulate brake fluid to the brake lines. I don’t know about replacing it with a solid rod but if you do, be careful to make it the same length installed. My advice, if serviceable parts aren’t available, is to replace the MC with a new one. Obviously the brake system needs to be 100% reliable. If you’re able to access parts and put it back together, save yourself a headache and bench bleed the MC before installing it on the car.

Edited by croc7, 17 September 2019 - 03:42 AM.

[Rorf]

Exactly, why fight the thing and possible end up with a dangerous bodge - get a new one. Obviously much corrosion in the system due to not changing brake fluid over the years so I would also have a good look at all the brake drum cylinders etc too. All these parts are available and are reasonably priced.

[Ethel]

Aren't they NLA? Even so, crashing in to something due to brake failure won't retain the originality either.

 

I might be tempted to try and seal the reservoir communication ports and plumb it up to another hydraulic cylinder to push on the pistons.

 

Have you found this: https://www.mini-for...934-Inno-GMC159 ?

[InnoCooperExport]

Unfortunately they are NLA, I´ve seen a NOS one on eBay for about 400 euros... I could fit a yellow tag double acting MC if it comes to it, but I´d like to try and save this one. But I do appreciate it´s a safety critical item, so if real bodges will be avoided. 

 

I hadn´t seen that thread on the German forum yet, many thanks!

[InnoCooperExport]

Right, so sensibility prevailed over my wallet (I may be in the Netherlands but I am a Yorkshireman at heart) and through a friend in Italy I managed to procure a NOS GMC159 for a much more reasonable price than the ones on eBay. 

 

This one came packed full of storage grease and is like new inside. Disassembling it couldn't have been easier, thanks for the tips on how to negotiate the trickier steps and my workshop manual. The part I struggled with the most was removing the spirolox ring that held on the return spring on the primary piston, compressing the relatively strong return spring while simultaneosly fiddling with a screwdriver and pliers to remove the Spirolox was a bit too much. So I whipped up a quick tool to compress the spring for me, leaving me with two hands to mess about with the sprirolox ring. 

 

It's just a plate with three holes in it, two to mount it to the MC and compress the spring, and one larger one to let the piston through while it compressed it. I could have done it with nuts and bolts but I opted for tiewraps because it was simple. Just had to make sure you pull both tight at the same time so the bracket stays level and won't mark the bore of the piston. Unfortunately I didn't take the pictures until after I had removed the Spirolox...

 

 

 

And the guts from the MC all neatly out in one piece in order.

 

Up next is splitting the two pistons so that the cups can be replaced. It might be NOS and stored nicely but they're still 40 year old rubbers... Or is this like most rubbers now that the old ones in decent condition will be better quality than new replacements?

[croc7]

Congratulations on finding a NOS unit! I rebuilt mine using a kit sourced from Mini Spares and lubricated all of the rubber bits with Castrol's Red Rubber Grease. Still works great. I highly recommend bench bleeding of the MC prior to installation. If I could figure out how to post a pic, I'd show the 'octopus' set up that worked for me. Or if you wish to send me an email, I’d be glad send a picture.

Edited by croc7, 17 October 2019 - 04:13 AM.