brake XR400 front brake master cylinder rebuild question

[Mason Randles]

Hello from Washington! i've been trying like crazy to get my '96 xr400 ride worthy again and the last thing i gotta fix is the front brakes. i got a master cylinder rebuild kit and got it installed and even got some fluid coming from the caliper bleeder but it seems kinda slow? brakes still don't work. i'm wondering how much fluid should i expect to come from the bleeder when i grab the brake lever? it took about 5 minutes for anything to come out, is that normal? or should it be shooting out with some force? 

i'm trying to figure out if my MC is toast or it i should start tearing into the caliper. 

also, before it gets said i do have a clymer manual.

[Toadster25]

I'm thinking you might have air it the master cylinder still. The reservoir is kind of small on these bike master cylinders so it is easy to accedentally suck more air into it as your trying to get it bled. 

I've had trouble with getting the fluid going if there is air in the master cylinder. The service manual says to use a vacuum pump to suck the fluid and air out through the bleeder screw. You might need to try using something like that. https://www.ebay.com/itm/142159250967 

If it was me and I didn't have a tool like that I'd fill the reservoir up and leave the cover off,  remove the bleeder screw all the way and then squeeze the brake lever and hold it, then cover the bleeder screw hole with you finger so no air gets sucked back into the caliper when you release the brake lever, then remove your finger and squeeze the lever again, cover your bleeder screw again and release again just do this over and over till you get fluid coming out.  Keep a very close eye on the fluid level so you don't empty it out. If it's empty then that means you just pumped more air into your system.  The reason I would just take the screw clear out is because air can get sucked back in around the screws threads and it will go a lot faster just using your finger instead it open and closing it every time. You do want to keep the screw close by and ready to put it back in quick because once it's bled the fluid will start flowing and you don't won't to let the reservoir run dry.  

You need to create a vacuum inside the caliper to suck the fluid into the mastercylinder and when you plug it with your finger that is what your doing when you release the brake lever.  It's not much of a vacuum but hopefully it will  be enough to get the master cylinder to suck the fluid down into itself.  It may take quite a few pumps of the lever but once it gets going it won't take much.  Remember keep an eye on your fluid level in the reservoir the whole time or you'll be starting over.  

 

[Mason Randles]

Went and got a legit brake bleed pump and put half a bottle through the whole system. No air got in. Still no brakes. I'm feeling pretty defeated. I even took the MC back apart to make sure the cups were facing the right way and to check again for pitting. There's a whole brake system on ebay for like 90 bucks. I'm real close to pulling the trigger on that because i am at my wits end with this.

[nomad221]

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Edited August 27, 2017 by nomad221

[Mason Randles]

well guys, I AM an idiot. i gave up, ordered new shit, and then went to shamefully clean my shop up. had the old MC piston in hand to throw away and noticed that i had the damn plungers mixed up. they were facing the right direction, but i had the smaller one on the outside. i pulled the MC apart while still on the bike, threw the old piston back in and BOOM. brake pressure. it's always the stupidest mistakes right? at least now i have a sweet brake bleeder. now to go see if i can cancel the order i made. thank you guys for the informative replies! of course my first interaction with this great forum is the result of a dumb blunder on my part.

[mxaniac]

Don't think of that as being an idiot.  You were just testing a potential performance modification and while it didn't work out, you've now eliminated that and can focus on other methods.