Master cylinder piston - solved

[John_S]

Do you guys consider the front piston to be wear item? I finally got around to getting all my parts for the front and rear rebuild. Caliper rebuild kit, master rebuild kit, pads, and core moto lines. I knew it would be a pain in the butt and it was worse than I expected. I figured it would be a couple hours each and I’m still on the front after hours and hours of messing with it. The first hang up was the circlip rusted and spun so it was hard to get to. Finally got that and the washer for the piston was rusted and corroded and I couldn’t get the piston out. Finally resorted to a torch to apply a little heat to the master around where the washer sits and got up the piston out.

With everything ready to go I installed the caliper and lower banjo and ran the line up but left the master off. I kept the line held straight up and used a syringe to push fluid up until it came out the banjo. Took the dry master and connected it to the banjo and then swung the master down into place. I was hoping to keep the air in the line to a minimum. Pushed fluid up into the master and kept pulling fluid from the reservoir a couple times pushing the air up and out the master. Getting the fluid in was really easy and air was escaping.

Now time to build pressure and bleed it from the bottom. Here’s where it all went wrong. It wouldn’t build pressure on its own. I brought out my wife for about 20 minutes of (pump the lever, crack the bleeder) rounds and it’s still worse than when I started the whole project. Then I went to an empty syringe applying a heavy vacuum at the bleeder while she kept fluid in the master and it helped but not much. To finish it up last night I pumped as much pressure as I could and zip tied the lever to the grip overnight. This morning it’s now as good as it was when I bought the bike which is working but spongy with a long pull. A one finger brake application ends up hitting the second knuckle of my middle finger with the lever, just like always. A short term fix would be to put a longer adjustment screw on the lever so it starts further away but I’m really disappointed with all the effort that went into it only to have the exact same result.

The master piston and the caliper pistons were the only thing not replaced. If you pull the lever slowly it feels like the master piston action is sort of notchy. I’m not sure what to try next besides the longer lever adjustment screw.

Edit: I made sure the seals on the piston were facing the right way. The flared end of both seals facing towards the fluid, not the lever side.

[John_S]

Or install a new piston on the master. The reason I ask is because some bikes OEM rebuild kits include the piston. Ive rebuilt front brakes before just like I did this one and the lever felt like a new bike. Barely any pull and a really solid feel. It’s hard to imagine there’s still air in the lines but maybe there is.

[John_S]

I’ve known of Goodrich speed bleeders for a while but never thought to get them for the KDX. They have a listing for our bikes so I ordered some last night. So in the future I can either bleed the brakes or flush new fluid in easily as a one man job. Once I get them I’ll try bleeding the system again. I was going to do the rear brakes today and now don’t feel like messing with it after being out there Friday and Saturday night with the front brake.

[KDXGarage]

Hi. What year is the KDX?

Did you use OEM parts or aftermarket? If aftermarket, which brand?

[John_S]

2002. No I didn’t use OEM. I bought the ProX master rebuild kit. All Balls on the caliper rebuild. I looked up the OEM and it doesn’t list the piston seals separately. They come with the piston. If Kawasaki thinks you shouldn’t replace just the seals without the piston, maybe thats a sign.

[MoonStomper]

I used those kits on my rear rebuild. Cleaned the piston up by soaking it in Coke overnight then rubbing it down with steel wool. Cleaned up okay, installed and reverse bled. No issues. Haven’t done the front MC yet though, it’s still operating nicely though I did the front caliper when I replaced the hose.

Sounds like maybe you don’t have enough fluid in the MC, what’s the level look like?

[John_S]

Thank you for the replies so far. The master has has enough fluid in it probably 10 times now. Reverse bleeding I filled it about 5 times and then suctioned out (not all the way though) so I could push more fluid in the bleeder.
Then I tried the empty syringe and put a heavy vacuum on the bleeder trying to draw air out while my wife kept topping off the master.
Now with the cap on the fluid is near the top of the sight glass. After reverse bleeding and having the master nearly full the brake lever had no resistance at all.
The only thing I did while it was apart was scrub the piston and bore with green scotch bite then shot them with brake cleaner.

[Charles]

John_S,I am no expert but I had a similar problem and replaced everything (piston included) and it would not build pressure until I got out of range and crush my fingers.
Turns out it was the lever which was badly repaired and had the wrong shape, giving less pull range hence not enough pressure.

Is your lever stock and not bent?

[John_S]

The lever is in good shape. I wish I would’ve known this prior to starting. A way to bench test the master to see if it will build pressure. I took my master off this afternoon and tried this. It wouldn’t build pressure like the ones in this video. I removed the piston to make sure the seals didn’t look damaged, reinstalled and tried again with the same result. A new OEM piston and seals are on the way from RMATV. We’ll see how that goes.

[John_S]

I received the OEM piston and seals, reassembled and tested it like in the video above. Everything went how you’d expect this time and I have a nice firm brake lever. I went ahead and ordered the OEM piston and seals for the rear also along with a new hose leading to the reservoir and the plastic 90 degree fitting. I’ll tackle that next week.

The Goodridge speed bleeders part number SB8125L are nice for one person quick bleeding. I got them on Amazon for $15. There’s a check valve in there so no need to pump the brake handle, crack the bleeder open, close and repeat. You open it about a 1/2 turn, attach a hose to the bleeder into a catch can and pump away on the handle. It moves fluid WAY quicker than the old method so you have to keep your eye on and keep refilling the reservoir. Maybe 5 pumps of the lever and you need to top it off again. I pumped and topped off the reservoir a few times. That was pushing a lot of air out. Doing that plus barely tapping the brake lever quickly pushing some air out the master and it was ready to go. It’ll make changing fluid a breeze in the future.
The bleeders take away the ability to reverse bleed but you can still use a hose and empty syringe and pull the fluid down and out with vacuum if you’re starting with a dry system.

[SS109]

Glad you found the issue and got it going. Thanks for updating everyone and the heads up on the speed bleeders.

[KDXGarage]

Thanks for the follow up. Good to read you got it going.

[Trackdog]

I am having the same problem
Thanks for the video John!

[Mr.Green]

I’ve always had the best luck with reverse bleeding with a small hose(fuel cap vent hose) attached to a medicine syringe and push fluid from the caliper bleeder fitting until fluid fills the master cylinder reservoir.Then close off bleeder screw on caliper and start pumping brake lever until it starts building pressure then bleed as normal!!!
Works Everytime

[Trackdog]

I’ve always had the best luck with reverse bleeding with a small hose(fuel cap vent hose) attached to a medicine syringe and push fluid from the caliper bleeder fitting until fluid fills the master cylinder reservoir.Then close off bleeder screw on caliper and start pumping brake lever until it starts building pressure then bleed as normal!!!
Works Everytime

That's next

[Trackdog]

I was able get o air out of the master while it was on the bench. Primed it and bench test passed.
Primed again and bolted the master to the bars, and did 3 reverse bleeds on the system and its all good.

Just FYI, if you have to do a rebuild, I believe you WILL need brake caliper snap ring pliers like the ones from Motion Pro, or a facsimile there of...
They have small long jaws with just the right bend.
I have a good set of snap ring pliers but could reach down into the cavity where that clip is.

I did order a Galfer braided front brake line for the cool factor. If anyone has any comment on these lines I am all ears.
So basically I will get a chance to do this again...