Lost the cap an internals of the rear master cylinder

Was at a race yesterday an noticed for some odd reason the bolts had came off the cap of the rear master cylinder. I had lost the cap,two bolts,Plate Diaphragm,Diaphragm reservoir,Separator assy and circlip.

#'s 5-10 on the parts display for 2011 YZ450F rear brake master cylinder


This was at a sand track so I suspect sand particles got down in the master cylinder. When I clean out this master cylinder what would be the best way to make sure all sand is out? Would I just flush the line with brake a bottle of brake fluid? or will I need to even take the brake caliper apart also?

I'm thinking just flushing out the master cylinder an line will do to make sure all sand is out. What do yall think?

Edited by CaptainKnobby

I just went ahead and took the Master Cylinder off to inspect every thing. Good thing i did,because there was sand all on the inside.

I had to take the insides out and clean it all out. Here is some pics that I took while I done it.

Here is the Cylinder kit Assembly. Notice all the sand grit on it.


There was sand down in this hole where the cylinder kit goes. I had to flush it out with brake parts cleaner to get it all out.


This is the rod that pushes the cylinder kit up via the brake pedal


There was sand also laying inside the window that I had to get out.


Now the cylinder kit is back in place and is going up and down through the cylinder smoothly. If no one has taken this out before......if you notice there is a circlip you take out to remove the assembly.




Here I have the assembly kit cleaned and installed back


I think I have this rubber grommet installed back right. I cant remember if it covered that nut or stayed behind it like I have it now. If someone could look on theirs maybe you could correct me.


Here is looking down the reservoir where you put the fluid. If you can tell at the very bottom.....is that the separator assembly? If so I will not have to get one of those. I believe I just lost the cap,Two screws,Plate Diaphragm and Diaphragm Reservoir.


In this parts pic of the Master Cylinder .....Look at part #9 ....This would be the separator assembly. Is that it in the pic above down at the bottom of the Reservoir? If it isnt then it came out with the rest of the stuff and i will have to get one.


Edited by CaptainKnobby

I took a closer look down the reservoir of the master cylinder. I noticed that black spot at the bottom was sorta shiny. I shined a light through the window on the cylinder and seen what appeared to be the separator assembly. I noticed it had 5 holes in it so I took a paper clip (big one) and straightened it out and put a hook at the end and pulled the assembly out. Here it is


I thought there may be some grit under it but they werent.I put it back in and cleaned the brake caliper out by taking the piston out and then blowed the brake line out. There didnt appear to be any sand through the brake line or in the brake caliper

I will be amazed if the MC's bore is still good.

I just had mine all apart. Note to self, don't blow master cylinder out with air unless you cover the reservoir. If you don't cover the reservoir the air pressure will blow the seperator assembly out like a shotgun sending it across the shop to be a place that can never be found, LOL. I ordered my new seperator assembly last week.

You will know if the rubber peice is in the wrong place when you try to adjust the rear brake pedal. I had to put mine down below the nut otherwise it would bind up and twist when I tried to adjust the pedal. I was surprised how easy the system was to work on. Not that complicated. I ordered a rebuild kit for mine because I couldn't get the brakes to bleed. I thought it was the rubber bushings on the push rod but it turned out that the seperator assembly was clogged, which I didn't even know was in the master cylinder until it shot out of it, LOL.

If it were me I would disassemble the entire system including banjo bolts, brake line and caliper and clean the entire system before reassembly. You will have to bleed it now regardless, might as well get the entire system cleaned.

I did ........I went ahead and took the Brake caliper off and took the Piston out of it and cleaned through the passage hole real good. However the passage hole in the caliper wasnt dirty.It was clean anyways and no sand or grit made it through to the caliper that I could tell.

I took the brake line off and blew the Banjo bolts out with air and then took one of those red straws (That you get with a can of carb cleaner) and trimmed the end down a little and stuck it in the hole on my air nozzle blower. Then I stuck the other end of the straw in the hole on one end of the brake line an blew it out real good. I didnt see any grit or sand here also.

I will be amazed if the MC's bore is still good.

The bore is just fine! As slick and smooth as a Baby's butt

Ya so was mine...

I'm just going to add a line to this thread and wrap it up. I got the parts today that had came out of the Master Cylinder and was lost. The parts were..........The reservoir Diaphragm,The (Plastic piece)between Diaphragm and top cap and the Cap.......Total of 3 pieces.

I Took a syringe and filled it with Brake Fluid and then attached a plastic line onto the syringe nipple.I then opened the Bleeder nipple on the Caliper. I pushed the syringe stem until Brake Fluid came out of the plastic line. I then attached the plastic line from the syringe to the Brake caliper Bleeder nipple.

At this point I slowly pushed the syringe stem and fed Brake fluid through the caliper until fluid started rising up through the Master Cylinder. When the Fluid just passed the site window on the Master Cylinder Reservoir I shut off the Rear caliper Bleeder Nipple.

I then poured a bit more Fluid into Master Cylinder Reservoir until it was at the top. At this point I unhooked the Plastic line from the rear caliper bleeder nipple and pumped the rear brake pedal 3x's and kept the pedal held down and opened the bleeder nipple and let some fluid out and the with the pedal still held down I closed off the Rear caliper bleeder nipple and let up the brake pedal. At this time you could see the Brake fluid in the Master Cylinder reservoir slowly lower.

I repeated this same process for a total of about 20 times. Each time that I bled the rear brake like this there would be Brake fluid coming out and no air. Thing is I still didnt have any pressure on my brake pedal. In other words......I still didnt have any brakes. I knew i didnt have any air in the the brake system cause i reversed bled the system.

Only thing I could think to do was to take the rear wheel off and pump the brake pedal until the piston came out so far of the the caliper.

I done this and then pushed the caliper piston back in and put the wheel back on. I then bled the rear brake like I did before and on the 3 time I had plent of pressure on the pedal and had good brakes.

I guess there was a small pocket of air in the caliper behind the piston that wasnt letting the brake have pressure and push the piston out. So when I took the wheel off and pumped the brake pedal the fluid forced the piston out fully and then when I pushed the piston back in all the way it forced the air pocket into the fluid and then when I bled the brake few more times all the air escaped out the bleeder nipple and the brake system was fully bled of air.

So if any one has any trouble bleeding air from the brakes.....Try removing the wheel and pumping the piston out fully and then pushing the piston back in fully and putting the whell back on and bleed a couple more times. This should get all the air out.

You don't have a vacuum bleeder?

Quick question, the diagrams for my 08 rear master cylinder doesn't show the circlip. I'm assuming the 08's didn't have the circlip. Also the diamgramn apprears that the seperator goes in with the rounded side up. JUst want to be sure. I'm going to put it all back together tomorrow.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now