master cylinders

Awhile back I posted on repair procedures for a bad bore on the front master cylinder and got several
good replies, When I first tore down the front I was appalled at the seeming corrosion that was present in the front master's bore and top fluid chamber.
So taking the advise given I procured a kit and started to see what could be done,I started by vapor blasting the whole assembly at low pressure
and was amazed at how it cleaned up, Going on further advise I made a split hone to hold 1000 grit wet or dry sandpaper a few passes with brake fluid
showed an acceptable bore, Painted and back on the bike after reassembly and bleeding it's working like new !! So with this triumph out of the way I went on to the rear master expecting worse than the front, Amazingly there was no corrosion and piston in fine shape!! So my long winded question is what caused the front master to corrode so badly, Yes the caps were on tight. anyone else seen this?

I'll suggest that OLD fluids will have "Soured", loosing their corrosive inhibitors. Depending on the history of the bike it's likely also that every time the cover was removed from the reservoir for top-ups, natural humidity would induce moisture to the fluid and air space, compounding the problem.

My theory: Front brakes wear faster than rear brakes. In addition, dual front system pad wear requires more replacement fluid than a single disk rear. So the front brake fluid needs to be topped up more often than the rear and it's not unusual for folks to top up with a previously open container of brake fluid which may have absorbed water - leading to increased water (and corrosion) in the front system compared to the rear.

Hi A bit off topic but I'm cleaning up my front master cylinder and am wondering if both return holes should be open. On mine one is open but I cant get anything through the second hole. Should I run a small drill to clean it out? Not sure how it could be plugged so solid. t

No drill needed, just a regular sewing needle to push crap out and clear it

Pete F
UK

strate6 wrote: No drill needed, just a regular sewing needle to push crap out and clear it

Pete F
UK

I personally wouldn't use a sewing needle - if you push too hard you can dsplace material into the piston bore which can cut the seal. My preference is to use a high E guitar string and poke around gently. If you don't unplug that hole, your brakes will lock up.

Just went through this myself as I'm installing new lines on the ZN and decided to do a complete disassembly and clean of all components of the brake system. Brakes were working fine but they're going to work even better after this.