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Squishy front brake

Kilroy

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Ok y'all, this is seriously the LAST thing my CB350F needs. I rebuilt the master cylinder on it, replaced the lines, the caliper, the piston and everything. Literally the only thing that is stock is the housing for the master, which I cleaned out. I cannot get pressure. A buddy with a speed bleeder couldn't get it, so obviously I messed up somewhere. Who can I buy dinner/a six pack/just give money to to help me?
 
Break it apart and try to pressure each separate piece. Sometimes, the master won't pressure. Put some compressed air to the wheel caliper and check that. Sometimes, the piston seal won't slide in the groove if it was put together dry.
But checking each component separately is where to start.


Hard to properly rebuild an old master. There are cheap new ones on Ebay.
I'm here in Austin.
 
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If the master cylinder bore was honed out to clean it, it may now be oversized where the replacement seals are not able to seal well enough to make solid pressure. If oversize seals are not available, then a replacement m/c is needed.

It is possible the system still has entrapped air. Replacing all the components can introduce alot of air. It is possible (though unlikely) that even using a pressure bleeder there is still a pocket of air in there. I can see a vacuum bleeder more readily leaving a pocket of air, but even those are usually very thorough.
 
Break it apart and try to pressure each separate piece. Sometimes, the master won't pressure. Put some compressed air to the wheel caliper and check that. Sometimes, the piston seal won't slide in the groove if it was put together dry.
But checking each component separately is where to start.


Hard to properly rebuild an old master. There are cheap new ones on Ebay.
I'm here in Austin.

Alrighty, I took apart the system and blew air through everything. Looks like all of my component parts are airtight, so I must've goofed up a connection or something, or my newly-rebuilt master is faulty. When the kid goes down for a nap I'll try re-assembly.
 
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I would say you have air trapped in the system still, especially since you replaced so many components. A speed bleeder won't do anything to release an air bubble or air pocket. It just makes routine bleeding more convenient.

Something to try. Do a reverse bleed forcing fluid up from the caliper to the master. You can use a large syringe with a piece of tubing that fits tight around the caliper bleed screw. Make sure the master is full, then open the bleed screw and force fresh fluid up and through the line until it runs out of the master cylinder res. It you have a second set of hands have your helper steadily tap on the line all up and down it. This often helps release a bubble.

If you are getting partial pressure but not a firm feel it is almost always because there is still air in the system. You can bleed for hours the conventional style and never get the bubble out.
 
Alrighty, I'm willing to try it. I take it I can find the syringe et al at my local auto parts store?
 

Liteitup has shown exactly what you need on the syringe. You will still need some tubing to connect the tip of the syringe to the bleed nipple. Use clear tubing. You want to be able to see that you are forcing brake fluid up and no air bubbles .

Once you get it set up and connected, it is quick and easy to do. I am not a fan of using a vacuum bleeder in this situation--mostly personal preference based on my empirical success rates.

Edit: Your disc is good, right? No bends, warps, etc? You won't ever get a firm lever if you have disc that has an issue.
 
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I would say you have air trapped in the system still, especially since you replaced so many components. A speed bleeder won't do anything to release an air bubble or air pocket. It just makes routine bleeding more convenient.

Something to try. Do a reverse bleed forcing fluid up from the caliper to the master. You can use a large syringe with a piece of tubing that fits tight around the caliper bleed screw. Make sure the master is full, then open the bleed screw and force fresh fluid up and through the line until it runs out of the master cylinder res. It you have a second set of hands have your helper steadily tap on the line all up and down it. This often helps release a bubble.

If you are getting partial pressure but not a firm feel it is almost always because there is still air in the system. You can bleed for hours the conventional style and never get the bubble out.


^^^^^^ THIS

Use two syringes... one like Johnf3 suggests and another to suck out accumulating fluid from the reservoir. There's an art and science to getting all the bubbles out of the pressuring syringe while slipping the clear hose over the bleed nipple while turning a small box end wrench just the right amount to allow flow while reciting Jabberwocky and then quickly turning said wrench again - shutting off the flow while not allowing any air to sneak by. I can make one jolly good mess doing all this but it's never failed me yet.
 
After syringe method, if still soft, try tying the handle down to the grip overnight, sometimes that will force last little air out through the return port.
 
One time I put the wrong/old crush washers on, and got spongy brakes I couldn't bleed with speed bleeders, cracking the banjos, or reverse syringing. I can't remember exactly what sin I made...I think I did both at the same time actually....

Triumphs had an interesting problem, where the pistons were sticking to the piston seal, and the piston seal would haul back the pistons after braking. It caused long travel in the lever, which feels "spongy". That's probably not your problem though.
 
I bought a syringe kit, which did make bleeding a nice and easy process. THat said, even after injecting upwards, I'm still getting a spongy brake so I'm getting air somewhere. One thing I noticed is that the piston seal isn't exactly tight around the piston. There's a half a centimeter of gap. Any chance that'd be messing me up?
 
One thing I noticed is that the piston seal isn't exactly tight around the piston. There's a half a centimeter of gap. Any chance that'd be messing me up?

Might be, the seal has to, .....well it has to seal both the piston and bore. The seal is normally a pretty tight fit part over the piston lip... it should fight you a bit, needed some brake fluid or assembly lube to get in place..
its possible the seal is not the correct size or has otherwise been damaged by solvents.

Now, don't shake your head just yet, read all the way though... I too thought this was BS Internet lore, till I tired it.
Rubber parts swell or stretch when they get petroleum products on them, most times thats solvent of some sort.
Many times that petroleum product can be removed from the rubber and the part will return to its designed shape/ size.. WHO you ask...???
Put the rubber part in boiling water and let it stew for 5-7 min, remove, cool in cold water, check fit, if still over sized, boil for 10 min, cool try again.

Ive done this with rubber carb gaskets, O rings and a few other petroleum product soaked rubber parts and it just plain works.

That said, if its the wrong seal and just not the right size this will not work.

Pro Marriage Tip: Do not use your spouses favorite pan to do this in...use something old..if none on hand, go hit the dollar store or salvation army/ good will store.
 
Thanks Erik,

I gave that a try, still a little loose. I gave up and took it to the local UJM specialist, who put on the correct seal and now it stops like a dream.

He was also kind enough to inform me that whoever did the carb rebuild "didn't know what he was doing at all" so now my pride is broken along with my bike.
 
Update on the carb situation, I apparently managed to put one of the slides in backwards, which would explain that problem. Runs like a top now.
 
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