GS550 rear master cylinder

[Guest]

I recently stripped the rear master cylinder of my GS550. The whole bike has been stripped and this was simply the next thing I fancied doing.... I have taken it apart and checked everything and all appears fine. All the parts were given an ultra sonic bath. I was supposed to give it a coat of brake oil before reassembling but as I didn't have any to hand I used 2 stroke oil. Reasssembly was fine but when I placed pressure on the push rod the piston etc moved up but didn't return to its starting position. Obviously this is hand held as it wont be going back on the bike for a while.


Is this normal? Does it have to be under pressure for it to return ie have brake fluid in it or should the spring return it on its own?

Not sure this makes sense!!!! Will post photos when I can later.

[hannibal]

I'm pretty sure it should return on its own. The bottom of my rear MC is covered in rust and the pushrod still returns to normal after manually compressing it.

Also I don't think you're supposed to use motor oil in the brake system. You may need to clean it with brake cleaner then re-lube with brake fluid when assembling. Hopefully someone more knowledgeable than I will chime in.

[Nessism]

Jordan is correct. You should pull the master back apart now and clean everything in soap and water. Lube with brake fluid when putting it back together.

[Steve0]

Thanks guys, I'll start again with the clean and brake fluid. Too keen to put it back together!

Hannibal, so your MC returns without any fluid in and not attached to the bike?

I just want to check as I'm thinking the fluid and the pressure from springs and load will all play a part in the return....

So should I simply put back together (with brake fluid) and then leave alone until I can fit it to the bike and then fault find f necessary?

[hannibal]

The spring is designed to work opposite of the fluid. You push to pedal to move the fluid and there's nothing to return to cylinder to normal. I guess there's a spring on the pedal as well as in the MC to return it to normal, but this should happen with or without fluid. It should be more difficult to spring back with fluid.

Yes my rusty MC returns to normal after compressing without fluid. I noticed because it failed to spring back once and I keep pushing and pulling until it did. After cleaning and reassembling, I imagine it should spring back quite easily.

I'm planning to soak the whole MC in vinegar because repeated use of PB Blaster has had little to no effect on the rust. If I can get the circlip off, I'll pull it all apart and clean with brake cleaner then blow out with compressed air (a can of keyboard duster). Then assemble with the rebuild kit and brake fluid to lube and hope for smooth operation.

[Steve0]

I was hoping to avoid buying a rebuild kit but sounds as though I should, I'm going to clean it again and do as you suggest and if it doesn't work I'll get the kit, cheers for the reply mate

[hannibal]

I ordered the kit, but it makes sense to clean and try the old parts. If it works smoothly, I'll just keep the kit on the shelf.